The American Astronomical Society. All rights reserved. Printed in ... Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229, Japan. S. GUNJI.
THE ASTROPHYSICAL JOURNAL, 481 : 821È831, 1997 June 1 ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.
HARD X-RAY EMISSION FROM THE GALACTIC RIDGE N. Y. YAMASAKI, T. OHASHI, AND F. TAKAHARA Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan ; yamasaki=phys.metro-u.ac.jp
S. YAMAUCHI Faculty of Humanities and Social Sciences, Iwate University, Morioka, Iwate 020, Japan
K. KOYAMA Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
T. KAMAE, H. KANEDA, K. MAKISHIMA, Y. SEKIMOTO, AND M. HIRAYAMA Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
T. TAKAHASHI, AND T. YAMAGAMI Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229, Japan
S. GUNJI Department of Physics, Yamagata University, Yamagata-shi, Yamagata 990, Japan
T. TAMURA Kanagawa University, Yokohama, Kanagawa 221, Japan
S. MIYAZAKI National Astronomical Observatory, Mitaka, Tokyo 181, Japan
AND M. NOMACHI RCNP, Osaka University, Ibaraki, Osaka 560, Japan Received 1996 October 29 ; accepted 1996 December 26
ABSTRACT Hard X-ray and c-ray emissions from the Galactic ridge were studied with the large area proportional counter (LAC) on the Ginga satellite and a balloon-borne detector Welcome-1. In the scanning observations with the LAC, di†use hard X-rays were detected along the Galactic plane between l \ [20¡ and l \ 40¡. The measured spectrum shows that a hard component exists in the Galactic ridge emission above 10 keV, in addition to the hot plasma component. The estimated luminosity of the Galactic ridge emission is around 2 ] 1038 ergs s~1 in the 3È16 keV band. Welcome-1 observed the c-ray ridge emission at around l D 345¡ between 50 and 600 keV. These two results and a recent COMPTEL study suggest that the spectrum of the di†use Galactic ridge emission extends over the keVÈMeV range. From the observed spectral slope, bremsstrahlung by electrons is shown to be the dominant emission mechanism. This implies that low-energy electrons must be supplied continuously to sustain emission in the hard X-ray band. We propose a scenario in which the thermal electrons in the hot plasma responsible for the strong Fe K X-ray emission are shock-accelerated continuously in supernova remnants (SNRs), producing the observed hard X-ray and c-ray emissions from the Galactic ridge. Subject heading : X-rays : ISM 1.
INTRODUCTION
(Koyama et al. 1989b ; Yamauchi et al. 1990 ; Yamauchi 1991), and its spatial distribution was found to be well represented by the sum of an exponential disk and a spiral arm component. The disk component has a scale height of 100 pc and a radius of 4 kpc, and the spiral arm component is located at l \ ^30¡ (Yamauchi & Koyama 1993). The di†use c-ray emission from the Galactic ridge was studied by COS-B between 70 MeV and 5 GeV. The emission was found to have a scale height of about 130 pc (Mayer-Hasselwander et al. 1982). Recently COMPTEL mapped the Galactic plane between 1 and 30 MeV (Strong et al. 1994). The spectrum lies on a power-law (photon index D2) extension of the COS-B observation. EGRET also measured the Galactic di†use emission between 30 MeV and 30 GeV (Strong & Mattox 1996). The Ñux below 300 MeV is consistent with that of COMPTEL. The observed X-ray emission from the Galactic ridge has been interpreted in two di†erent ways. One interpretation is that the ridge emission consists of unresolved point sources.
Intense di†use emission from the Galactic ridge has been observed from the radio band to the GeV c-ray band. The emission below 100 MeV is thought to be produced by high-energy electrons via synchrotron, bremsstrahlung, and inverse Compton scattering. However, the source and acceleration mechanism of the high-energy electrons is still uncertain. In this report we study this emission in the hard X-ray/soft c-ray band. In the X-ray band, di†use emission from the Galactic ridge was discovered by the HEAO-1 satellite (Worrall et al. 1982). EXOSAT found that the emission extended along the Galactic plane to l \ 40¡ and that the total luminosity between 2 and 6 keV from the Galaxy was about 2 ] 1038 ergs s~1 (Warwick et al. 1985). The T enma satellite discovered strong 6.7 keV line emission from He-like irons along the Galactic ridge, which indicated the existence of thermal plasma of kT \ 5È10 keV and about 1 solar abundance of iron (Koyama et al. 1986a, 1989a). The intensity proÐle of the 6.7 keV line was mapped by the Ginga satellite 821
822
YAMASAKI ET AL.
The most probable candidates are RS CVn binaries and cataclysmic variables (CVs) (Worrall et al. 1982). The other is that it is of di†use origin. As for the di†use emission, the shock-heated hot plasma in supernova remnants (SNRs) has been suggested to be responsible for the X-ray ridge emission (Koyama et al. 1986 ; Koyama, Ikeuchi, & Tomisaka 1986b). However, the heating process of such hot plasma has not been clariÐed, and the thermal velocity exceeds the escape velocity from the Galactic plane. The c-ray emission below 100 MeV is considered to come mainly from high-energy electrons through bremsstrahlung and in small part through inverse Compton scattering in the interstellar medium (Kni†en & Fichtel 1981). Above 300 MeV, n0 decays produced by interaction of cosmic-ray nuclei with interstellar matter become the dominant process (Strong & Mattox 1996). In the hard X-ray and soft c-ray band between 10 keV and 1 MeV, observational data are scarce and statistical and systematic errors are large. We present in this report the results from the scanning observations of the Galactic ridge in the hard X-ray band with the large area proportional counter (LAC) on board the Ginga satellite and the soft c-ray observations carried out with a balloon-borne detector, Welcome-1. These new results on the Galactic ridge emission allow us to link the X-ray and the c-ray bands and draw a picture about the acceleration mechanism of the high-energy electrons needed for the emission mechanism. 2.
OBSERVATION BY GINGA
2.1. Ginga Scanning Observation The LAC detected hard X-rays in the energy band between 1 and 37 keV with an energy resolution of about 18% (FWHM) at 6 keV (Turner et al. 1989). In the MPC-1 mode, events were accumulated for the top and the middle layers of the eight detectors separately. Each energy spectrum consisted of 48 energy channels. The e†ective area and the Ðeld of view (FOV) were 4000 cm2 and 1¡.08 ] 2¡.0 (FWHM), respectively. The Ginga satellite was capable of scanning at a speed less than 14¡ per minute. The Galactic ridge region was scanned parallel to the Galactic plane 12 times between 1987 August and 1991 May, for a total exposure time of 2.4 Ms. In this paper we analyzed the MPC-1 mode data taken in six of the 12 scans. They included four scans passing near the Galactic center, one scan in the plane o†set from the Galactic center (7¡ \ l \ 40¡), and another scan at high latitude (b D [10¡). The six scan paths are shown in Figure 1. 2.2. Analysis Method The raw scan data contain not only the di†use emission from the Galactic ridge, but also the emission from discrete X-ray sources, the cosmic X-ray background (CXB), and the nonÈX-ray background. Discrete X-ray sources were removed by a Ðtting method described later. The intensity of the CXB between 2 and 10 keV, as measured by the LAC, is about 5.9 counts s~1, which is 20% of that of the ridge emission (Hayashida 1989 ; Yamauchi 1991), and the beamto-beam Ñuctuation of CXB in the total Ðeld of view of the LAC is 0.33 counts s~1, or 4.1% of the total CXB (Hayashida 1989). We subtracted the averaged CXB spectrum obtained from high Galactic latitude sky of 142 ks exposure from the scan data. This gives an oversubtraction
Vol. 481
FIG. 1.ÈScan paths of the Galactic ridge observations that are analyzed here.
of CXB in the low-energy band for this analysis, because the interstellar medium in the Galactic plane absorbs the CXB. The characteristics of the nonÈX-ray background in the LAC have been studied using the dark sky and the dark Earth data. Based on this study, the nonÈX-ray background can be estimated by using the monitor counts with an accuracy of 0.2 counts s~1 keV~1 in any observation taken by the MPC-1 mode (Yamauchi 1991). To remove the discrete source contribution and to extract the di†use component from the scan data, we applied the following procedure. We summed counts in 28 energy bins covering the range between 1 and 37 keV at 0¡.2 intervals in Galactic longitude to make energy-sliced scan proÐles. After subtracting the nonÈX-ray background and the CXB, we approximated the observed scan proÐles as a sum of point sources and di†use emission as follows : n I(E, l) \ A(E)[1 ] B(E)(l [ l )] ] ; S [l ; p , c (E)] . (1) 0 i i i i/1 Here I(E, l) is the scan proÐle in the energy bin E, l is the Galactic longitude of the line of sight, and A(E) is the energy spectrum of the di†use component. The slope parameter B(E) and l (Ðxed to 30¡) have been introduced for scan 6, 0 where the intensity of the di†use component shows longitudinal dependence. In the other scan paths, B(E) and l were 0 with Ðxed to zero. The point-source contribution convolved the LAC angular response is represented by S [l ; p , c (E)] ; i i p and c (E) represent respectively the positioni perpendicui i lar to the scan path and the count rate of the source at an energy E. Since the sensitivity of the LAC was better than those of past detectors in this energy band, and there could be transient X-ray sources in the Galactic plane, we treated the number of point sources n as a free parameter. First, we Ðtted the whole scan proÐle to Ðx the number of sources n and their positions p . The number of point sources n was i increased until a statistically acceptable Ðt was obtained. The number of point sources turned out to be 4È9 for each scan path. Then we Ðtted each energy-sliced scan proÐle separately and determined the energy spectrum A(E), c (E), i and B(E). We detected point sources with Ñux greater than 0.3 counts s~1, or 0.3 mcrab. This limit is close to the detection limit set by beam-to-beam Ñuctuations of the CXB for the LAC observation (Hayashida 1989).
No. 2, 1997
HARD X-RAY EMISSION FROM GALACTIC RIDGE 3.
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OBSERVATION BY WELCOME-1
3.1. W elcome-1 Detector Welcome-1 is a low-background, hard X-ray/c-ray telescope based on GSO(Ce)/CsI(Tl) well-type phoswich counters (Kamae et al. 1992 ; Takahashi et al. 1993). This well-type phoswich counter uses a GSO (Gd SiO doped 2 5 with 0.5% Ce) scintillator of size 3.4 ] 3.4 ] 1 cm3 for the detection part. The detection scintillator is placed at the bottom of the CsI(Tl) well, which works both as an active collimator and an active anticoincidence counter. In Welcome-1, 64 GSO(Ce)/CsI(Tl) well-type phoswich counters and 36 CsI(Tl) guard anticounters are assembled in a 10 ] 10 matrix conÐguration. The total e†ective area is 740 cm2 at 122 keV and 220 cm2 at 511 keV. The energy resolution is 28% (FWHM) at 122 keV and 12% (FWHM) at 511 keV. The Ðeld of view is determined mainly by the depth of the CsI(Tl) well. It is 15¡.4 (FWHM) at 122 keV and 18¡.2 at 511 keV. 3.2. Observation and Analysis Welcome-1 was launched at UT 06 : 57 on 1991 December 3 from the balloon base of the Instituto Nacional de Pesquisas Espaciais (INPE) at Cachoeira Paulista, State of Sa8 o Paulo, Brazil (S22¡39@44A, W45¡00@44A), with a 250,000 m3 hydrogen balloon. The detector reached at its ceiling altitude of 36.9 km about 2 hours after launch. The detector was cut from the balloon at UT 17 : 01 of the same day and was recovered 250 km west of the ground station. During the level Ñight, the balloon altitude was kept at 4.43 ^ 0.05 g cm~2 (Gunji et al. 1994 ; Miyazaki et al. 1996). The telescopeÏs elevation angle was Ðxed at 69¡.0 ^ 0¡.05 due to a technical problem in the control system that occurred at UT 11 : 00. The azimuthal control worked as designed. The pointing direction of the telescope was monitored by a geomagnetic sensor and a Sun camera with 0¡.2 accuracy. The Galactic plane region at about l \ 345¡ was observed from UT 14 : 35 to UT 16 : 05 by alternately pointing at the source (on-source observation) and at the blank sky (o†-source observation). The telescopeÏs FOV scanned the sky regions as shown in Figure 2. In the on-source observation, a black hole candidate GX 339[4 was in the Ðeld of view. The total observation time was 42 minutes for
FIG. 3a
FIG. 2.ÈField of view of Welcome-1 during the Galactic plane observation, and the position of GX 339[4.
the on-source observation and 48 minutes for the o†-source observation. After the gain correction and the event selection, we obtained the pulse-height spectra for the on-source and o†source observations. The o†-source spectrum consisted of the atmospheric c-ray events and the internal background events. Then the spectrum of the o†-source observation was subtracted from the on-source spectrum. 4.
RESULTS FROM THE GINGA SCAN FITTING
4.1. Energy Spectrum of the Galactic Ridge Emission The data obtained in each of the six scans were Ðtted individually, and 39 point sources were detected with intensity above 0.3 mcrab. Among the 39 sources, 34 were identiÐed with known X-ray sources. The energy spectra of the di†use emission were obtained in the Ðve scans up to 16 keV. In scan 4 at Galactic latitude b D [10¡, the di†use component was consistent with the CXB. 4.1.1. Spectrum Fitting of Each Energy Spectrum
The energy spectrum of the Galactic ridge emission obtained in scan 2 is shown in Figure 3. The energy spectrum exhibits strong iron line emission at 6.7 keV, indicating the existence of hot gas of temperature kT D several keV.
FIG. 3b
FIG. 3.ÈFitting of the energy spectrum of scan 2. (a) Absorbed thin thermal bremsstrahlung]Gaussian line model ; (b) absorbed power-law]Gaussian line model.
824
YAMASAKI ET AL.
To characterize the spectral feature, we Ðtted the spectrum with a simple thermal emission model, an absorbed thin thermal bremsstrahlung, and a Gaussian emission-line model. The best-Ðt parameters are summarized in Table 1. Based on these spectral Ðts, the Ðve energy spectra exhibit three common features. First, they show a strong emission line whose center energy is consistent with 6.7 keV (He-like Fe ion). The line widths agree with the instrumental resolution except for scan 6. The equivalent width of the line ranges from 500 to 1000 eV. Second, the absorption column density comes out to be zero for all Ðve scans when interpreted by the single-temperature thermal bremsstrahlung model. The upper limit obtained on the absorption column density N , less than a few times 1022 cm~3, is too low for H coming from the Galactic plane. Third, the the emission electron temperature of the continuum emission ranges from 6.3 to 10.3 keV for the Ðve scans, and there still remains excess emission or hard tails above 10 keV (see Fig. 3a, for example). We note that the thermal emission model is not statistically acceptable for scans 1 and 2 at the 90% conÐdence level, because the data exceed the thermal bremsstrahlung model in the high-energy band. We Ðtted the data with another model, an absorbed power-law and a Gaussian line model, although the powerlaw emission model does not naturally explain the iron line emission. The parameters are summarized in Table 2. This model turned out to give acceptable Ðts to all of the data (see Fig. 3b, for example). This indicates that there is a hard tail or a power-lawÈlike hard X-ray component associated with the Galactic ridge emission. To estimate the intensity ratio between the thermal component and the hard tail component, we Ðtted the spectrum below 10 keV by an absorbed Raymond-Smith model. We then subtracted the best-Ðt Raymond-Smith spectrum from the data to obtain the hard tail component. In Figure 4 we plot the count rate of the best-Ðt Raymond-Smith model between 2 and 10 keV as a measure of the intensity of the thermal component and the residual count rate above 10 keV corresponding to that of the hard tail component. From this plot, we see a correlation between the two com-
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FIG. 4.ÈCount-rate ratio between the thermal component and the hard tail component ; see text for details.
ponents. It shows that the hard tail component is a common feature of the Galactic ridge emission. 4.1.2. Features of the Averaged Energy Spectrum
To study the spectral features of the Galactic ridge emission, we calculated the averaged energy spectra from scans 1, 2, 3, 5, and 6 and Ðtted the spectrum by thermal and nonthermal emission models. The results are summarized in Table 3. The Ðrst model is an absorbed thin thermal bremsstrahlung with a Gaussian emission line. The best-Ðt temperature is as high as 8.4 keV, and the model underestimates the data above 10 keV. The Ðtting result is not statistically acceptable with s2/dof \ 3.4. The equivalent width of the Gaussian line is 700 eV, corresponding to an iron abundance of 0.8 solar. The line-center energy is consistent with 6.7 keV. If the ion temperature is the same as the electron temperature, about 8 keV, one expects an emission line at 6.9 keV from H-like ionized iron. This means that the temperature of the electrons implied by the continuum emission
TABLE 1 PARAMETERS OF ENERGY SPECTRA (4.0È13.8 keV) FITTING BY ABSORBED (THIN THERMAL BREMSSTRAHLUNG]GAUSSIAN) MODELS Path
N H (1022 cm~2)
Fe Line Energy (keV)
Fe Line EW (eV)
Continuum kT (keV)
s2/dof
1...... 2...... 3...... 5...... 6......
\1.0 \0.8 \5.9 \3.4 \2.1
6.70 ^ 0.20 6.72 ^ 0.04 6.72 ^ 0.22 6.68 ^ 0.22 6.60 ^ 0.08
643 ^ 118 1060 ^ 80 544 ^ 230 623 ^ 210 906 ^ 120
8.19 ^ 1.02 7.65 ^ 0.70 6.25 ^ 1.15 10.3 ^ 1.90 7.81 ^ 0.47
18.5/12 22.8/12 14.0/12 9.7/12 10.6/12
TABLE 2 PARAMETERS OF ENERGY SPECTRA (4.0È13.8 keV) FITTING BY ABSORBED (POWER-LAW]GAUSSIAN) MODELS Path
N H (1022 cm~2)
Fe Line Energy (keV)
Fe Line EW (eV)
Continuum Photon Index
s2/dof
1...... 2...... 3...... 5...... 6......
\2.7 \4.2 \13 \3.8 3.4 ^ 2.7
6.70 ^ 0.09 6.71 ^ 0.05 6.74 ^ 0.20 6.71 ^ 0.25 6.62 ^ 0.09
822 ^ 140 1250 ^ 170 607 ^ 250 739 ^ 190 1140 ^ 190
2.23 ^ 0.10 2.30 ^ 0.13 2.75 ^ 0.45 2.15 ^ 0.21 2.45 ^ 0.21
8.81/11 13.3/11 12.4/11 8.5/11 8.5/11
No. 2, 1997
HARD X-RAY EMISSION FROM GALACTIC RIDGE TABLE 3
825
a Normalization K \ exp [1/kT ] (keV)/Gaunt factor (kT ). b 1 solar abundance \ 4.68 ] 10~5 iron atoms relative to hydrogen. c Normalization K \ 10~14/(4nD2 / n2 dV ). d Normalization K \ 10~12/(n2V ). e Normalization K \ photons keV~1 s~1 LAC~1 at 1 keV.
and that for the ions as shown by the line emission are di†erent from each other, as indicated by Yamauchi & Koyama (1995). This suggests that the plasma is in its heating phase, since the cooling time of such a hot plasma is much longer than the escape time, which we will show later. The second thermal emission model is a Raymond-Smith model including emission lines from several elements. We used a model included in XSPEC Version 8.4 (Shafer et al. 1994). The best-Ðt temperature is 6.7 keV with s2/dof \ 6.6, and the model underestimates the data in the high-energy band (Fig. 5a). Because this emission model assumes temperature and ionization equilibrium of the hot plasma, the Ðtting results emphasize the temperature discrepancy between the ions and the electrons. We have tried the third model, a nonequilibrium ionization model by Masai (1984). Using the energy spectrum below 10 keV, Yamauchi & Koyama (1995) obtained an acceptable Ðt for the ridge emission with this model. However, the Ðt is not statistically acceptable with s2/ dof \ 4.5, mainly because of the residual above 10 keV. In this model we obtained a positive value of the hydrogen column density. It is consistent with the existence of a lowtemperature component emitting sulfur and silicon K lines as indicated by T enma (Koyama et al. 1989a) and shown by ASCA (Yamauchi et al. 1996 ; Kaneda et al. 1997). We next turned to models including power-law emission or a nonthermal component. The Ðrst of this kind that we tried is a simple sum of an absorbed power-law model and a Gaussian emission line. The result turned out to be good, with s2/dof \ 1.4 (Fig. 5b). This infers that a hard component exists in the hard X-ray band in addition to the hot plasma component responsible for the iron emission line at 6.7 keV. In the second model with a power-law emission, an absorbed Raymond-Smith model was used instead of the Gaussian line emission. The goodness of the Ðt was s2/ dof \ 1.4, nearly the same as that of the previous simple power-law and emission-line model. The spectrum is represented by a slightly absorbed [N \ (1.3 ^ 0.6) ] 1022 H cm~2] Raymond-Smith component with a temperature of 3.1 ^ 1.4 keV mixed with a strongly absorbed (N º H 4 ] 1023 cm~2) power-law component with a photon index of 1.6 ^ 1.1. Since the ridge emission is widely extended in the Galactic disk, the large di†erence in the absorption column density between the thermal and power-law components, which would invoke distinctly separated emission
FIG. 5a
FIG. 5b
PARAMETERS OF FITTING RESULTS FOR VARIOUS MODELS Model Element
Parameter
Value
Bremsstrahlung]Gaussian Model Absorption . . . . . . . . . . . . Bremsstrahlung . . . . . . Gaussian . . . . . . . . . . . . . . s2/dof . . . . . . . . . . . . . . . . . .
N (1022 cm~2) H kT (keV) Normalizationa Center energy (keV) p (keV) Equivalent width (eV) ...
0.0 8.43 22.5 6.66 0.0 706 40.36/12 \ 3.363
Raymond-Smith Model Absorption . . . . . . . . . . . . Raymond-Smith . . . . . . s2/dof . . . . . . . . . . . . . . . . . .
N (1022 cm~2) H kT (keV) Abundanceb Normalizationc ...
0.0 6.70 0.50 222 111.6/17 \ 6.56
Masai Model Absorption . . . . . . . . . . . . Masai . . . . . . . . . . . . . . . . .
s2/dof . . . . . . . . . . . . . . . . . .
N (1022 cm~2) kTH (keV) Abundance log [nt/(s cm~3)] Normalizationd ...
1.23 7.31 0.62 10.9 19,528 72.3/16 \ 4.52
Power-Law]Gaussian Model Absorption . . . . . . . . . . . . Power law . . . . . . . . . . . . Gaussian . . . . . . . . . . . . . . s2/dof . . . . . . . . . . . . . . . . . .
N (1022 cm~2) H Photon index Normalizatione Center energy (keV) p (keV) Equivalent width (eV) ...
1.95 ^ 2.4 2.33 ^ 0.13 165 ^ 57 6.68 ^ 0.05 \0.24 822 ^ 65 17.19/12 \ 1.432
Raymond-Smith]Power-Law Model Absorption . . . . . . . . . . . . Raymond-Smith . . . . . .
N (1022 cm~2) kTH (keV) Abundance Normalization
Absorption . . . . . . . . . . . . Power law . . . . . . . . . . . .
N (1022 cm~2) H Photon index Normalization
s2/dof . . . . . . . . . . . . . . . . . .
...
1.30`2.0 3.10~64 ^ 1.4 0.67`4.3 ~0.17 351`130 ~70 37.8`66 1.58~0.1 ^ 1.05 24.1`395 ~0.1 19.87/15 \ 1.42
FIG. 5.ÈFitting results of averaged energy spectrum. (a) Absorbed Raymond-Smith model ; (b) power-law]Gaussian line model.
826
YAMASAKI ET AL.
Vol. 481
TABLE 4 COUNT RATES OF THE DIFFUSE COMPONENT (counts s~1 LAC~1) ENERGY BAND SCAN PATH 1 2 3 4 5 6
........... ........... ........... ........... ........... ...........
2.30È16.1 keV
1.15È3.45 keV
3.45È6.33 keV
6.33È16.1 keV
23.4 ^ 0.2 29.4 ^ 0.3 27.2 ^ 0.5 1.9 ^ 0.5 34.8 ^ 0.6 23.8 ^ 0.2
10.9 ^ 0.1 12.5 ^ 0.2 10.7 ^ 0.3 1.5 ^ 0.2 11.3 ^ 0.2 7.0 ^ 0.1
9.6 ^ 0.3 12.4 ^ 0.2 10.3 ^ 0.3 0.9 ^ 0.3 14.4 ^ 0.4 10.4 ^ 0.1
6.1 ^ 0.1 8.2 ^ 0.2 6.2 ^ 0.3 0.1 ^ 0.4 10.4 ^ 0.3 6.9 ^ 0.1
and absorbing matter, may be unrealistic. This model is considered to represent the characteristic shape of the energy spectrum in such a way that the thermal emission including the 6.7 keV line is accompanied by a hard tail component. We leave a physical interpretation of this coexistence of an absorbed power-law component and a Raymond-Smith emission model for later discussion. 4.2. Spatial Distribution and T otal L uminosity of the Ridge Emission In this section we make a model of the spatial distribution of the ridge emission. Using the 6.7 keV line mapping, Yamauchi & Koyama (1993) obtained a spatial distribution model which consist of a exponential disk, arm components, and a bulge component. Here we make a simple model applicable to the hard X-ray band above 10 keV to obtain the luminosity and, for later use, to compare the c-ray Ñux with the X-ray Ñux. We assumed a thin disk of radius r which is uniform in radial direction and decreases exponentially with a scale height of b degrees perpendicular to the Galactic disk. 0 model we Ðtted the intensity proÐles of scans With this 1È6 summarized in Table 4. A reasonable Ðt was obtained for the spatial model described here for all the energy bands. The result of the Ðt is summarized in Table 5. Here the units of I are counts s~1 sr~1 LAC~1 at (l, b) \ (0, 0) ; r is 0 as r/R, r is the radius of the emission disk, and R 0 is deÐned the distance between the Earth and the Galactic center. The spatial distribution of the ridge emission above 3.5 keV is well described by the uniform disk model. Assuming that the distance R between the Earth and the Galactic center is 8 kpc (Reid 1993), the radius r is about 5 kpc and the scale height is 400 pc. The emitting volume is estimated as follows, with a scale height h and a Ðlling factor g, which 0 region in the interstellar is a volume ratio of the emitting space :
ponent obtained by 6.7 keV line mapping (Yamauchi & Koyama 1993). As our scan paths include the bulge region, the scale height is probably inÑuenced by both components. 5.
RESULT FROM WELCOME-1
5.1. T he Energy Spectrum of the Galactic Plane and the Di†use Component The energy spectrum obtained from the Galactic plane region by Welcome-1 after subtracting the o†-source spectrum is shown in Figure 6. It is not corrected for the angular response of Welcome-1. GX 339[4 was the only known hard X-ray source in this Ðeld of view. We estimated the contribution of GX 339[4 to the measured Ñux using data obtained by other observations. From the separation angle for GX 339[4, the time-averaged detection efficiency for GX 339[4 was 0.62 ^ 0.05 at 100 keV during our observation. GX 339[4 has been continuously monitored by BATSE on board the CGRO satellite. On 1991 December 3, the intensity of GX 339[4 was below the sensitivity of BATSE, which is about 100 mcrab (Harmon et al. 1993). In Figure 6 we indicate the level for a source of 100 mcrab with a photon index 2.1, which was an upper limit obtained by BATSE on the day. The Ñux observed by Welcome-1 is consistent with the BATSE observation. OSSE also observed GX 339[4 on 1991 November 7È12, and the best-Ðt value for the Ñux at 100 keV is (3.9 ^ 0.5) ] 10~6 photons cm~2 s~1 keV~1 (Grabelsky et al. 1995). The Ñux is shown by the lower dashed line in Figure 6. However, the time variation of the intensity of GX
V \ nr2 ] 2h ] g \ (2.6 ^ 0.9) ] 1066g cm3 . (2) tot 0 The obtained scale height D400 pc is in between the disk component of 100 pc at l [ 10¡ and that of the bulge comTABLE 5 PARAMETERS OF THE EXPONENTIAL THIN DISK Energy Band (keV) 2.3È16.1 . . . . . . . 1.15È3.45 . . . . . . 3.45È6.33 . . . . . . 6.33È16.1 . . . . . .
I
0 (7.2 ^ 1.1) ] 104 (2.7 ^ 0.3) ] 104 (3.0 ^ 0.5) ] 104 (2.3 ^ 0.5) ] 104
b 0 (deg) 3.0 ^ 0.7 4.1 ^ 0.8 3.2 ^ 0.8 2.3 ^ 0.9
r
0 0.72 ^ 0.19 0.57 ^ 0.05 0.73 ^ 0.20 0.68 ^ 0.16
NOTE.ÈI is in units of counts s~1 sr~1 LAC~1 at (l, b) \ (0, 0). 0
s2/dof 1.60/3 2.70/3 0.20/3 0.48/3
FIG. 6.ÈFlux from the Galactic plane observed by Welcome-1 compared with the LAC extension and the OSSE observation. The upper limit of the Ñux of GX 339[4 obtained by BATSE and the Ñux level on November 7È12 by OSSE are also shown.
No. 2, 1997
HARD X-RAY EMISSION FROM GALACTIC RIDGE
339[4 in its low state is not well known. If the Ñux from GX 339[4 during our observation was the same as that of the OSSE observation in November, the contribution of GX 339[4 was about 10% of the Welcome-1 observed Ñux. Next, we compared the measured Ñux to previous observations of the Galactic ridge. The middle long-dashed line shows a power-law extrapolation of the Galactic ridge emission obtained by the LAC below 16 keV. The e†ective solid angle of Welcome-1 is calculated as (2.1 ^ 0.1) ] 10~2 sr by convolving the spatial distribution model of the ridge emission and the angular response of the detector. The lower dot-dashed line shows a di†use emission obtained by OSSE at l \ 25¡ (Purcell et al. 1996). The Ðeld of view of OSSE is 1.3 ] 10~2 sr and is normalized to that of Welcome-1. In the Ñux obtained by OSSE, the pointsource contribution was subtracted using the estimation from the simultaneous SIGMA observation. The ridge emission obtained by OSSE is consistent with the extension of the LAC Ñux level within a factor of 2. Assuming that the Ñux of GX 339[4 was at the same level as the November OSSE observation, the Ñux observed by Welcome-1 is consistent with both results. So the Ñux measured by Welcome1 is considered to be dominated by the Galactic ridge emission between 40 and 600 keV, which would include about 10% contamination from GX 339[4. 5.2. Combined Energy Spectrum Figure 7 shows the wide-band spectrum of the Galactic ridge emission obtained by the LAC and Welcome-1, together with the previously reported c-ray emission (Mandrou et al. 1980 ; Bertsch & Kni†en 1983 ; Strong et al. 1994 ; Paul et al. 1978). The unit is the Ñux from the central radian of the Galactic plane. The energy spectrum obtained by the LAC shows a power-law shape between 3 and 16 keV, whereas the strong iron K line indicates that the emission below 10 keV is dominated by thermal emission from a hot plasma. The surface brightness obtained by the LAC and the Ñux observed by Welcome-1 are translated assuming that the spatial distribution of the intensity in this energy band is the same as that obtained by the LAC. The comparison between the Ñux of our LAC and Welcome-1 data and those of c-ray bands indicates that this power-lawÈshape emission in the X-ray band extends
FIG. 7.ÈWide-band spectrum of the Galactic ridge emission ; the intensity is normalized to that from the central radian. The data are taken from Mandrou et al. (1980), Strong et al. (1994), Bertsch & Kni†en (1983), and Paul et al. (1978).
827
smoothly to the c-ray region (Yamasaki et al. 1994 ; Yamasaki et al. 1996). We note that the e†ects of the detectorÏs FOV, observed region, and assumed spatial distribution of the di†use emission for each c-ray observation are not corrected. The dot-dashed line in Figure 7 shows a power-law spectrum with photon index 2.1 between 1 keV and 100 MeV. It is interesting that this single power-law spectrum seems to connect the wide-band emission from the LAC to the COMPTEL band. 6.
DISCUSSION
6.1. Unresolved Point-Source Contribution In this analysis we removed the contribution of point sources whose Ñux is above 0.3 mcrab. We estimate the possible contribution of point sources fainter than the detection limit of the LAC. A source of 0.3 mcrab located at the Galactic center has a luminosity of 8 ] 1034 ergs s~1 between 2 and 10 keV. Using the log NÈlog S relation for the Galactic and noncoronal (hard) sources obtained by the IPC (Hertz & Grindlay 1984), the sum of the point sources with luminosity less than 8 ] 1034 ergs s~1 is estimated to be about 4 ] 1036 ergs s~1. This is only 5% of the observed ridge emission. Therefore, discrete X-ray sources observed with the IPC are unlikely to be the main origin of the present Galactic ridge emission. Many authors have discussed the possibility that the ridge emission consists of faint point sources. RS CVn stars and cataclysmic variables (CVs) have been considered to be the most probable candidates (Worrall et al. 1982) because their X-ray temperatures are high and their spectra show strong iron line emission. Ottmann & Schmitt (1992) showed that the emission from RS CVn binaries contributes to the ridge emission at a level of 27% in the 2È6 keV band but only 1% of the iron line emission. Yamauchi & Koyama (1993) pointed out that the scale height of late-type stars is larger than that of the ridge emission. If all of the ridge emission consists of the point sources, the variation of the temperature of the ridge emission observed by T enma gives a lower limit to the luminosity of the sources of about 1033 ergs s~1 (Koyama et al. 1986a). From a Ñuctuation analysis of the surface brightness of the ridge emission in an ASCA observation, Yamauchi et al. (1996) concluded that the luminosity of discrete sources that contribute to the ridge emission is less than D2 ] 1033 ergs s~1. The rest of the candidates that may contribute signiÐcantly to the ridge emission are CVs, especially intermediate polars. The scale height and the spatial density are estimated to be 100È250 pc and D10~7 pc~3, respectively. They can produce hard X-ray spectra with iron emission lines with luminosities around 1 ] 1033 ergs s~1 (Ishida 1991 ; Patterson 1984). However, the typical temperature of the continuum emission is above 10 keV, which is higher than that of the ridge emission. If the emitting volume is a few times 1066 cm3 or 1011 pc3, the luminosity from the intermediate polars is of order 1037 ergs s~1. These results suggest that the contribution from the point sources in the ridge emission is less than 20% and that the Galactic ridge emission has a di†use origin. 6.2. Origin of the Hard X-Rays The existence of the Galactic ridge emission above 10 keV is revealed by the LAC and Welcome-1. The power-law spectral nature of the di†use emission in the hard X-ray
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YAMASAKI ET AL.
band suggests that nonthermal electrons are involved. The possible production mechanisms of the hard X-rays are (1) electron bremsstrahlung, (2) inverse Compton scattering, and (3) synchrotron radiation. Because magnetic Ðelds in interstellar space are thought to be as weak as a few microgauss, the third possibility can be ruled out. The Ñux and the spectrum of the inverse Compton emission can be estimated from the observed spectrum of cosmic-ray electrons. Skibo & Ramaty (1993) estimate that the intensity of inverse Compton emission in interstellar space is weaker than that of bremsstrahlung below 50 MeV. Also, the photon index of the inverse Compton spectrum should be about 1.5 when the index of the electron number spectrum is 2. This is Ñatter than the overall index of D2 inferred from the available data in the hard X-ray and c-ray band. The bremsstrahlung process is thus the most probable mechanism for the hard X-ray/low-energy c-ray emission. The spectrum shown in Figure 7 suggests that the emission follows a power-law form between 10 keV and 10 MeV, even though the spectrum is obtained using di†erent observations. If bremsstrahlung is the underlying mechanism, the energy spectrum of electrons therefore must extend down to about 10 keV. This prohibits the passage of the electrons through interstellar matter. A simple calculation shows that the amount of matter that causes signiÐcant change in the electron spectrum at 10 keV due to ionization losses is only 135 kg cm~2 (Zombeck 1990). If the ISM has a hydrogen density of 1 cm~3, this thickness corresponds to only 26 pc. The power-law spectrum of electrons, as suggested from the hard X-ray spectrum, would imply that the hard X-rayÈemitting region is very close to the site where the electrons are accelerated. If we are to explain low-energy c-ray Ñux in terms of the cosmic-ray electrons, we need a large amount of newly accelerated low-energy electrons. 6.3. Supernovae as an Energy Source of Di†use Emission The previous discussion suggests that the hard X-ray photons are emitted when the electrons are still freshly accelerated in the Galactic plane. The large-scale association of the hard X-ray emission with the thermal X-rays implies that these two components are tied by underlying physical process. The spectral shape shows that the hard power-law component has a luminosity comparable to that of the thermal soft component. This leads to the idea that the thermal electrons in the hot plasma are seed particles that get accelerated to make the nonthermal electrons responsible for the hard X-rays. Young SNRs are possible candidates for the acceleration site. There shock waves may heat up the hot plasma and produce cosmic-ray electrons. We Ðrst examine whether SNRs can be the sites for the thermal soft X-ray emission in the Galactic ridge. The mean electron density in the emission region is estimated as (n2)1@2 D 2.5 ] 10~3g~1@2 from the emissivity of the thermal e emission. Assuming the temperature of the hot plasma is 8 keV, a value obtained from a single-temperature thermal model, the kinetic energy density of the plasma is (9.6 ^ 1.2) ] 10~11g~1@2 ergs cm~3 and the entire energy in the hot plasma in the Galactic plane is E \ (2.5 ^ 0.9) tot ] 1056g1@2 ergs. The hot plasma with kT \ 8 keV cannot be conÐned in the Galactic disk by the gravitational force only. Taking the escape velocity to be v \ v d \ 8.8 ] 107 cm s~1, the esc 105 sound escape time is (4 ^ 1) ] d~1 yr, which is shorter than
Vol. 481
the radiative cooling time by several orders of magnitude for d [ 10~6. To maintain the hot plasma, the energy supply is E/v \ (6 ^ 2) ] 1050g1@2 d1 ergs yr~1. esc This huge energy of hot plasma is thought to be supplied by supernova explosions (for example, Koyama et al. 1989b). However, there are two serious problems in this picture : (1) it requires too high a supernovae rate, and (2) the process in SNRs that produce hot plasma of several keV is not clearly understood. We will estimate the Ðlling factor based on the generally assumed supernova rate and the size of SNRs. If each supernova supplies E \ v ] 1051 ergs to the SN Galactic ridge plasma, we need a supernova rate (6 ^ 2) ] 10~1g1@2 dv~1 SNe yr~1 to maintain the hot plasma. If the Ðlling factor is as small as g \ 1 ] 10~3 and v D 1, one supernova should occur every 50 yr in the Galaxy. The total number and lifetime of supernovae contributing to the Galactic ridge emission become 8 ] 103v~1 and 4 ] 105 d~1 yr, respectively. If the shape of the acceleration region of each supernova is a shell with a radius of 30 pc and thickness 1 pc, the volume is about 3 ] 1059 cm3. This gives a Ðlling factor of the order of 10~3, which is consistent with the previous assumption. It means that the supernova scenario can explain the energy supply of thermal X-ray emission in the Galactic ridge if we assume the small value of the Ðlling factor. The remaining problem is the mechanism that heats up the plasma to several keV and produces the hard tail component. The simple mechanism is shock heating by the blast waves in SNRs. The observed ridge temperature of several keV is higher than that of young SNRs such as Cas A (Tsunemi et al. 1986). 6.4. Electron Acceleration in SNRs If a fraction of the thermal electrons is continuously accelerated in the emission region, the basic feature of our observations can be explained. The thermal and nonthermal emission from the Galactic ridge are produced by the bremsstrahlung in the accelerated hot plasma in the Galactic plane. Particle acceleration in SNRs has been discussed in the past, mostly in relation to high-energy cosmic rays and Galactic c-ray emission. In these studies the Ñux and spectrum of injected particles are a priori assumed. Not much attention has been paid to their origin (see reviews in Draine & McKee 1993 and Achterberg 1990). Achterberg (1990) pointed out that the injected particles should come from the thermal pool at the shock front and that electrons have to be accelerated to some threshold momentum before they can be picked up by the shock acceleration process. We propose a model in which the injected electrons are in the form of the hot plasma responsible for the X-ray ridge emission. The question is whether thermal electrons can be efficiently accelerated up to energies of 10 keVÈ100 MeV. If such an acceleration is possible, these freshly accelerated electrons can sustain the hard tail component of the Galactic ridge emission. This picture would give a basis for the uniÐed understanding of the energy spectrum of the Galactic ridge from the keV band to the 100 MeV band. The efficiency of the acceleration process is determined by the balance between the energy gain from the shock and the loss due to the cooling of electrons. The energy loss of low-energy electrons in the optically thin and fully ionized gas is dominated by the excitation of plasma oscillations.
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HARD X-RAY EMISSION FROM GALACTIC RIDGE
The loss rate is as follows (Hayakawa 1969) :
A B
CA B
dE dt
D
b2c n \ 7.64 ] 10~9 e ln ] 74.8 eV s~1 . (3) n b e osc Here n is the electron number density in cm~3, c is the e Lorentz factor for the electrons, and b \ v/c. The acceleration efficiency depends on the frequency at which electrons cross the shock front and the energy gained per crossing. This, in turn, depends on many parameters, including the particle density, geometry, magnetic Ðeld, and others, in the shock region and cannot be modeled easily for numerical estimation. Here we use the simple di†usive shock acceleration theory of Bell (1978a, 1978b) and examine in a semiquantitative way whether the electrons in the hot plasma can be accelerated efficiently. We assume that the strong shock condition that gives the most efficient acceleration is met in SNRs and apply the formulation by Bell (1978b) to estimate the acceleration rate. The fractional increase of the electron kinetic energy E per shock passage is given by *E/E D 2u /v, where u is the s s shock velocity and v is the electron velocity. The acceleration time t for the electrons to obtain a kinetic energy E is given by tacc \ t (E/*E), where t is a cycling time and acc ascyc(4/v)(5i/u ) withcyc is approximated a di†usion constant i s the di†usion constant i for (Bell 1978a). The estimation of electrons is a very difficult problem because the intensity and the geometry of the magnetic Ðeld in the shock region are not known. We simply approximate i as lv/3, where l is the mean free path of the electrons. The value of l must be smaller than the thickness of the shell in SNRs in order for the acceleration process to take place within the shell. Based on these simpliÐcations, the acceleration rate is roughly estimated as follows : E t
\
829
3Eu2 3Ecb2 s\ s (for v ? u ) . s 10lv 10lb
(4)
acc Here b and b denote the velocities of a particle and a shock, s respectively, divided by the speed of light c. This rate is compared with the energy loss rate by the excitation of plasma oscillations. Apart from the 1/b dependence, the ionization loss rate is roughly in proportion to the electron density n , and the acceleration rate is in proe portion to b2/l (pc~1). We hereafter call b2/l the s s accel““ acceleration parameter.ÏÏ The possible value of this eration parameter in a shock region in SNRs is about 10~4 pc~1 for typical values of b \ 0.01 and l \ 1 pc. To make a numerical comparison, we splot the energy loss rate and the energy gain rate in Figure 8 for n \ 1 ] 10~3 to 1 ] 10~1 e acceleration parameter cm~3 (g \ 1 to 1 ] 10~4) and the 10~5 to 10~3. Figure 8 shows that the acceleration rate is higher than the loss rate at high electron energies. The crossing point between the acceleration line and the energy loss line gives the critical kinetic energy E for electrons to be accelerated. It ranges from a few keV to cr a few tens of keV. Electrons with energies lower than E lose more energy by exciting plasma oscillations than theycrgain by shock acceleration. In other words, these low-energy electrons convert the shock energy to the thermal energy of the plasma. The electrons with kinetic energies higher than E are likely to gain energy cr shock front. during successive crossings of the If E is around 10 keV, only a portion of the electrons in cr the thermal plasma have a chance of being accelerated to
FIG. 8.ÈEnergy loss and shock acceleration rate as a function of the electron kinetic energy. The three solid lines show the ionization loss rate for di†erent electron densities, and the three dashed lines show the acceleration rate for di†erent shock velocities. The crossing point between the solid and the dashed line gives a critical electron energy E above which an cr e†ective acceleration can occur.
high energies. This condition must be met for our picture to succeed. If E were much higher than the electron thermal energy in thecr plasma, then we would have to consider a di†erent population of electrons which are accelerated to cosmic-ray electrons. On the other hand, if E were the cr same or less than the thermal energy of the plasma in the SNRs, most electrons would have been accelerated away from the system. Our numerical calculation shows that the thermal hot plasma in SNRs can in fact work as the source of the injected electrons for the cosmic-ray acceleration if the critical energy E is of order 10 keV. Here we study bycr simple numerical simulation how the energy spectrum of electrons evolves with time during the acceleration. We assume that the electrons initially have a Maxwellian distribution with temperature 3 keV and density n \ 8 ] 10~2 cm~3, and that the shock velocity b e the electron mean free path l is 0.1 pc. Here wes is 0.01 and neglect the electron-electron collision because its timescale is much longer than that of the excitation of plasma oscillations. The critical energy E is then about 10 keV. In cr Figure 9 the kinetic energy distributions of the electrons are given at every t \ 6 ] 109 s. The escape probability of the cyc shock region in each time step is assumed electrons from the to be u /v. The resultant electron spectrum is shown in Figure 9.s It shows that a hard tail component is produced from the thermal pool of the electrons by the di†usive shock acceleration. The number index above 30 keV is about 1.5, which is consistent with the strong shock condition at the nonrelativistic approximation (Bell 1978a). The spectrum makes a transition between 10 and 30 keV from the thermal distribution to the power-law shape, and the number index is close to 2. The emission spectrum from these electrons via bremsstrahlung will be similar to the observed ridge emission, which consists of thermal bremsstrahlung with a temperature of several keV and accompanied by a hard tail component above 10 keV. Our model suggests the interesting possibility that a fraction of thermal electrons can be accelerated to much higher energies by di†usive shock acceleration. There remain several unresolved problems in this scenario. We assumed continuous nonthermal emission
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Vol. 481
-1
10
Sum of step 0-100
-2
Electron density (Arbitary unit)
10
Step 0 Maxwell distribution (3keV) Step 10 -3
10
Step 20 α=1.5 -4
10
-5
10
10
2
10
3
10
4
10
5
Energy(eV) FIG. 9.ÈEnergy spectra of the accelerated electrons from the Maxwell distribution of 3 keV (n \ 8 ] 10~2 cm~3, b \ 0.01, l \ 0.1 pc, and E \ 10 keV) e s cr
between 1 keV and 100 MeV. The results from the Welcome1 observation give, in a strict sense, only an upper limit of the Ñux between 50 and 600 keV. Future observations in this energy band without any contamination of point sources are necessary to conÐrm the continuity of the power-law component. As clear evidence of the shock acceleration in SNRs, recent ASCA observation of SN 1006, a young shell-like SNR, revealed that the X-ray emission from the edge of the shell is dominated by the nonthermal emission. Thermal emission containing many emission lines also was detected from the center of the SNR (Koyama et al. 1996). It indicates that the electrons are accelerated at the shock front. Observations of the middle-aged SNR, the Cygnus Loop (D20,000 yr), show bright knots produced by the blast wave or the reverse shock (Graham et al. 1995 ; Miyata et al. 1994), supporting a view that shock acceleration can occur in evolved SNRs. However, temperatures of typical SNRs are lower than that of the Galactic ridge emission. Also, we do not know how long shock acceleration lasts in a SNR and how much energy is used to accelerate the particle. As described above, the required supernova rate is about (1/50) ] dv~1 (SNe yr~1). If v, the portion of the energy used to accelerate the particles, is about 0.01, d \ v /v is of order 0.01 in order to keep the supernova esc ratesound low. It means that a conÐnement of the hot plasma in the Galactic plane is necessary. Whether the hot plasma is escaping or is conÐned in the Galactic plane is a very important question, and we hope that future obser-
vations in X-rays and c-rays will bring us new clues on these problems. 7.
CONCLUSIONS
We summarize our results below : 1. A high-quality spectrum of the Galactic ridge X-ray emission has been obtained with the LAC in the energy band between 3 and 16 keV. The energy spectra obtained in Ðve di†erent scan regions are similar and consist of thermal emission, as implied by strong iron line emission at 6.7 keV and a hard tail component above 10 keV. The presence of a hard X-ray tail in the Galactic ridge emission has been shown unambiguously for the Ðrst time. 2. The observation of the Galactic plane at l \ 345¡ with the balloon-borne experiment Welcome-1 measured the spectrum of the Galactic ridge emission between 50 and 600 keV. The measured spectrum Ðlls in the gap between the hard X-ray data obtained by the LAC and the c-ray data obtained by COMPTEL if we assume GX 339[4 was at the Ñux level measured by OSSE 1 month before. 3. The estimated contribution from the faint point sources below GingaÏs detection limit is only 5% using the Einstein log NÈlog S relation for the Galactic hard sources. 4. We have reexamined the possibility that the supernova origin can supply the energy and heavy elements for the di†use Galactic ridge emission by taking account of the inÑuence of the Ðlling factor. We have found that the Ðlling
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HARD X-RAY EMISSION FROM GALACTIC RIDGE
factor of 10~2 to 10~3 is suitable to explain the energy supply and the iron mass with a reasonable supernova rate of one per several tens of years. 5. The di†use Galactic ridge emission in the hard X-ray/ c-ray band is considered from the spectral shape to be produced by bremsstrahlung of low-energy cosmic electrons. The data suggest that the spectra of the cosmic electrons keep the same power-law form down to several times 10 keV with a number index D2, without su†ering from Ñattening in low energies due to ionization loss in the interstellar space. This requires that the hard X-ray ridge emission is produced in the region where the electrons are still freshly accelerated. 6. A simple calculation has been performed based on the di†usive shock acceleration model to obtain a consistent model for the Galactic ridge emission. In a thin, hot plasma with temperature of a few keV and an electron density n D e
831
10~2 to 10~3 cm~3, the thermal electrons can be accelerated to nonthermal velocities for a realistic set of parameters. This model can produce the thermal emission spectrum accompanied by a ““ hard tail ÏÏ component, similar to the observed spectrum from the Galactic ridge. Shock acceleration in SNRs is shown to be a promising model for explaining the smooth extension of the Galactic ridge spectrum from the keV band to the º100 MeV band. The authors thank all of the members of the Ginga team for satellite operation and data acquisition. The Welcome-1 experiment was supported by a grant-in-aid from the Ministry of Education, Culture, and Science (Monbusho) of Japan (03041022, 04554006, 03218102) and was achieved by the collaboration of the University of Tokyo, ISAS, KEK, Rikkyo University, and INPE.
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