The Astrophysical Journal, 523:L65–L68, 1999 September 20 q 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.
DISCOVERY OF 715 SECOND PULSATIONS FROM THE FAINT X-RAY SOURCE AX J17000624157 Ken’ichi Torii,1 Mutsumi Sugizaki,1 Takayoshi Kohmura,2 Takao Endo,3 and Fumiaki Nagase3 Received 1999 June 3; accepted 1999 July 16; published 1999 August 25
ABSTRACT We report here the discovery of a new X-ray pulsator, AX J17000624157, in the Galactic plane. The source was observed three times between 1994 and 1997 by the Advanced Satellite for Cosmology and Astrophysics. Significant pulsations with P = 714.5 5 0.3 s were discovered from the third observation made between MJD 50,707.8926 and MJD 50,710.2199. The X-ray spectrum is described by a flat power-law function with a photon index of .0.7. Although the spectrum could also be fitted by thermal models, the temperature obtained was unphysically high. The hard spectrum suggests that the source is a neutron star binary pulsar similar to X Persei (4U 03521309), but we cannot completely exclude the possibility that it is a white dwarf binary. Subject headings: pulsars: general — pulsars: individual (AX J17000624157) — X-rays: general operated in PH nominal mode, which had a time resolution of 1/16 and 1/2 s for high and medium bit rate data, respectively. To search for new X-ray pulsars, we employed the maximum power mapping method (Torii et al. 1998b). A barycentric correction was applied to the arrival time of events, and the data from two GIS detectors were co-added. Power spectra were calculated for events within circular regions 49 in diameter. In successive calculations, the center of the circular region was moved to form a grid of size 29. The maximum power map was thus obtained for the entire field of view in the 0.7–10 keV range. In addition to the known pulsar, OAO 16572415 (P . 38 s), we detected significant pulsations from a serendipitous faint X-ray source within the field of view. The probability of finding power larger than that of the serendipitous source is less than 10213, taking into account all the frequency and spatial regions in the map. Figure 1 shows the GIS image obtained in 1997. The source position was determined from the 1997 observation to be s (17h00m5.3, 2417579440) (J2000), or (l, b) = (3447. 0, 107. 2). Because the source was off-center within the GIS field, the position determination might be in error by .19. Because the source was first identified and named AX J17000624157 from the ASCA observation in 1996 (S99), we adopt this name hereafter. Figure 2 shows the power spectrum for AX J17000624157. A Fourier search gave a period of P = 714 5 2 s, and pulse folding refined the value to P = 714.5 5 0.3 s. Figure 3 shows the folded light curve of AX J17000624157. The pulse is singly peaked, and the pulse amplitude is .50% of the source flux, excluding the background. We did not detect a significant change in the period during the observation. By dividing the data into two time series, we find an upper limit for the de˙ ! 2 # 1025 s s21. rivative of the period of FPF The X-ray spectrum of the source was examined. The source spectra were extracted from circular regions 89 in diameter. A power-law function modified by interstellar absorption fitted them well (x2/degrees of freedom = 158/147). The spectral fit was made using the GIS2 and GIS3 data simultaneously. For clarity, Figure 4 shows the spectrum obtained by adding the GIS2 and GIS3 spectra. The spectral parameters are summarized in Table 1. Bremsstrahlung, Raymond-Smith (Raymond & Smith 1977), or MEKAL (Liedahl, Osterheld, & Goldstein 1995 and references therein) models all fitted the data well. However, the temperature ranges obtained, kT 1 67 keV (bremsstrahlung), kT 1 38 keV (Raymond-Smith), kT 1 45 keV
1. INTRODUCTION
Studies of faint X-ray sources in the Galactic plane are important for understanding the populations of the various types of X-ray sources in the Galaxy. The nature of the Galactic ridge X-ray emission has been a long-standing problem since its discovery (e.g., Warwick et al. 1985; Koyama et al. 1986). The contribution of faint pointlike X-ray sources to the ridge emission is still unclear. The recent hard X-ray survey made with the Advanced Satellite for Cosmology and Astrophysics (ASCA; Tanaka, Inoue, & Holt 1994) has shed new light on faint Xray sources in the Galactic plane (e.g., Sugizaki 1999, hereafter S99). Since many of the point sources in the plane are considered to be compact objects, studies of these faint X-ray sources give us information on the evolution and endpoints of stars, as well as setting limits to their contribution to the ridge emission. While studying faint X-ray sources in the Galactic plane, we have discovered a new X-ray pulsator among them. We report here the results of three ASCA observations of this pulsator, AX J17000624157. 2. OBSERVATIONS AND RESULTS
To search for an X-ray pulsar, we analyzed the ASCA archival data obtained in 1997 September 16 (Table 1). The screened data were retrieved from the data bank at NASA GSFC. ASCA is equipped with two kinds of X-ray detectors, two solid-state imaging spectrometers (SISs) and two gas-imaging spectrometers (GISs), at the foci of four identical X-ray telescopes. Since the new pulsator was only detected by the GIS (Ohashi et al. 1996; Makishima et al. 1996), which has a larger field of view (about 509 in diameter) than the SIS, we herein concentrate on the GIS results. The main target of the observation in 1997 was an accretion-driven binary X-ray pulsar, OAO 16572415 (e.g., Chakrabarty et al. 1993). Part of the ASCA results for OAO 16572415 has been reported by Nagase (1998), and a detailed analysis will be published elsewhere. The GIS was 1 Space Utilization Research Program, Tsukuba Space Center, National Space Develop Agency of Japan, 2-1-1, Sengen, Tsukuba, Ibaraki, 305-8505, Japan;
[email protected],
[email protected]. 2 Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1, Machikaneyama-cho, Toyonaka, Osaka, 560-0043, Japan;
[email protected]. 3 Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara-shi, Kanagawa, 229-8510, Japan;
[email protected], nagase@astro .isas.ac.jp.
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TABLE 1 Summary of the Observation Log and Spectral Parametersa Observation Numbers Parameter
1
2
3
Observation date (UT) . . . . . . . . . . . . . . Start time (MJD) . . . . . . . . . . . . . . . . . . . . End time (MJD) . . . . . . . . . . . . . . . . . . . . . Exposure time (ks/GIS) . . . . . . . . . . . . . Photon index (E2G) . . . . . . . . . . . . . . . . . . Column density (cm22) . . . . . . . . . . . . . Observed flux (ergs s21 cm22) . . . . . .
1994 Mar 24 49435.4277 49436.1966 26.7 ) ) )
1996 Sep 1 50327.6526 50327.9956 9.2 20.310.6 20.3 6(!11) # 1022 6 # 10212
1997 Sep 16 50707.8926 50710.2199 69.2 10.3 0.720.5 6(!12) # 1022 4 # 10212
a
The errors for the spectral parameters are for the 90% confidence level.
(MEKAL) (90% confidence), seemed physically unreasonable. No significant iron K lines could be detected at the 3 j level. Upper limits for the equivalent width of a narrow line at 6.4, 6.7, and 6.9 keV were found to be 590, 1150, and 1160 eV, respectively. To study the nature of the source, we examined other archival ASCA data. We found that the source had been observed twice by ASCA before 1997 (Table 1). The source was first observed in 1994 March 24. The main target was the pulsar OAO 16572415. AX J170006-4157 was serendipitously positioned very close to the edge of the detector. The source was out of the field of view of GIS2, and it was seen only at the edge of the GIS3 field of view. This position made it impossible to conduct any reliable spectral analysis. Also, limited statistics (414 events from a circular region 89 in diameter) made it impossible to conduct a meaningful search for pulsation. An upper limit for the pulse amplitude reached 100%. In 1996 September 1, the source was observed by ASCA for the second time, during a Galactic plane survey (e.g., Yamauchi et al. 1998). The source was clearly detected, and it was named after its position (S99). Timing and spectral analyses were carried out (S99). No significant pulsations could be detected from
Fig. 1.—ASCA GIS image of AX J17000624157 as observed in 1997 September. The bright source at the center of the field of view is OAO 16572415. AX J17000624157 is the faint source to the southwest.
the data, probably because of the short exposure and limited statistics. The best-fit spectral parameters are reproduced from S99 in Table 1. The observed X-ray flux was 6 # 10212 ergs s21 cm22 in the 1–10 keV range. Spectral analyses of the 1996 and 1997 data suggest that the source spectrum is extremely hard. Although it is difficult to obtain the source flux accurately from the 1994 data, the source does not seem to show significant flux variations of more than a factor of .3 during the 3.5 yr interval. 3. DISCUSSION
The 715 s pulsation suggests that we are observing the spin period of a compact object, either a white dwarf or a neutron star. Most white dwarf binaries have relatively low X-ray luminosity (L X ! 10 34 ergs s21) compared with neutron star binaries. We thus try to estimate the source distance in order to understand the nature of the compact object. The H i column density for the direction of AX J17000624157 is 1.9 # 10 22 cm22 (Dickey & Lockman 1990). Taking into account the molecules and dust grains for X-ray absorption (e.g., Sugizaki et al. 1997), we estimate the effective total column density to be 6.9 # 10 22 cm22. Here we have used a CO measurement (Dame et al. 1987) and a CO-to-H2 conversion factor of 1.9 # 10 20 mol cm (K km s21) (Strong & Mattox 1996). The observed column density (Table 1) is consistent with this estimate, and the source distance is not constrained at all. If the
Fig. 2.—The power spectrum of AX J17000624157 calculated from the 1997 observation. The power is normalized so that the random fluctuation has an averaged value of 2. The data were extracted from a circular region of 29 radius, and the energy range was 0.7–10 keV.
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Fig. 3.—The folded light curve of AX J17000624157 is shown for two cycles. The data were extracted from a circular region of 39 radius, and the energy range was 0.7–10 keV. The background level is shown by a dashed line.
source distance is relatively large (d . 10 kpc), then its X-ray luminosity is L X . 5 # 10 34d10 ergs s21 assuming a powerlaw model. Since the source should be located within the Galaxy, an upper limit for the luminosity is L X ! 2 # 10 35d 20 ergs s21 for the farthest reasonable distance of d ! 20 kpc. However, it may be fainter than 1032 ergs s21 for a smaller distance (d ≤ 1 kpc). The former is comparable to the luminosity of binary pulsars and the latter to white dwarf binaries. White dwarf binaries show line emission from an optically thin thermal plasma (e.g., Ezuka & Ishida 1999). On the other hand, neutron star binaries show hard power-law–type spectra (e.g., Nagase 1989). The X-ray spectrum for AX J17000624157 suggests that it is more likely a neutron star binary pulsar. The relation between the spin and orbital periods for neutron star binaries (e.g., Corbet 1986) can give us a clue to the nature of the binary companion. If AX J17000624157 is a neutron star binary pulsar, we expect that the companion might be an underfilled Roche lobe supergiant or Be transient. Considering the faintness of the source, the former possibility may be excluded. Since AX J17000624157 was detected at a comparable flux level during all three ASCA observations, it is unclear whether the source shows large flux variability during its orbital revolution. With the information currently available, we cannot conclude what kind of companion is the most likely. The long spin period of AX J17000624157 resembles that of the prototype system of persistent Be/X-ray binaries, X Persei (4U 03521309). The pulse period of P . 840 s from X Per was first discovered by White et al. (1976) using the Copernicus satellite. It has been studied extensively by subsequent generations of X-ray missions (Haberl et al. 1998; Salvo et al. 1998 and references therein). Unlike many other Be transient binaries, X Per does not show a strong orbital modulation. The hard spectrum, the low X-ray luminosity, and the lack of significant long-term flux variability of AX J17000624157 resemble the characteristics of X Per. Another example of this type of object may be 1WGA J1958.213232
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Fig. 4.—The X-ray spectrum of AX J17000624157 as observed in 1997 September. The spectrum is fitted with a power-law function modified by interstellar absorption. The residuals are shown in the bottom panel.
(Israel et al. 1998). Due to the long spin period of this class of objects, the minimum luminosity at which accretion is possible against centrifugal inhibition is small (e.g., Stella, White, & Rosner 1986). This effect may be the reason why AX J17000624157, as well as X Per, is underluminous. This discovery, together with the recent discoveries of Xray pulsars in the Galactic plane (e.g., Angelini et al. 1998; Kinugasa et al. 1998; Sugizaki et al. 1997; Torii, Kinugasa, & Kitamoto 1997; Torii et al. 1998a, 1998b), suggests the presence of a large number of faint and dormant X-ray pulsars in the Galaxy (e.g., Waters & van Kerkwijk 1989). The presence of a large number of X-ray pulsars was clarified observationally by Koyama et al. (1990), based on their discoveries of a number of X-ray pulsars and transients in the Galactic plane, particularly in the Scutum arm region. If most of the faint hard Xray sources are neutron star binaries as inferred in S99, deeper observations will make it possible to detect pulsations from some of these faint sources. The population of neutron star binaries at this flux level (&1 mcrab) is still unclear. This problem will be addressed by the next generation of observatories to be launched within a year. Since faint X-ray pulsars are considered to be slowly rotating, with a small centrifugal force against accreting matter, the moderate time resolution of X-ray CCDs will be useful for conducting a pulsation search. The imaging capability will be useful for studying this kind of object since they are often located in crowded regions in the Galactic plane. For AX J17000624157, follow-up observations by imaging X-ray instruments will be useful so as to avoid source confusion with OAO 16572415. Optical and infrared observations are also needed to identify its binary companion. The authors are grateful to all the members of the ASCA team. We thank B. Paul for useful comments. This research has made use of data obtained through the High-Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA Goddard Space Flight Center. T. K. and T. E. are supported by JSPS Research Fellowships for Young Scientists. The authors would like to acknowledge the anonymous referee for invaluable suggestions.
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