The Interstellar Matter in the Direction of the Supernova Remnant ...

THE ASTRONOMICAL JOURNAL, 119 : 281È291, 2000 January ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE INTERSTELLAR MATTER IN THE DIRECTION OF THE SUPERNOVA REMNANT G296.5]10.0 AND THE CENTRAL X-RAY SOURCE 1E 1207.4[5209 E. B. GIACANI1 AND G. M. DUBNER1 Instituto de Astronom• a y F• sica del Espacio, C.C. 67, Sucursal 28, 1428 Buenos Aires, Argentina ; egiacani=iafe.uba.ar, gdubner=iafe.uba.ar

A. J. GREEN Department of Astrophysics, School of Physics, University of Sydney, NSW 2006, Australia ; agreen=physics.usyd.edu.au

W. M. GOSS National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801

AND B. M. GAENSLER2 Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 Received 1999 July 23 ; accepted 1999 August 31

ABSTRACT G296.5]10.0 is a high Galactic latitude supernova remnant (SNR), with a bilateral morphology in radio and X-rays. The compact X-ray source 1E 1207.4[5209, classiÐed as a radio-quiet neutron star, is located very close to the remnantÏs center. We report on a survey of the H I distribution in a region of sky, 3¡.5 ] 3¡.5, around the remnant, using the Australia Telescope Compact Array with a spatial resolution of about [email protected] and a velocity resolution of 0.82 km s~1. The H I distribution is quite smooth, with no obvious large-scale features that can explain the SNR bilateral morphology based on external factors. There are, however, three clouds that we do associate directly with the remnant. Optical Ðlaments outline two smaller features, which appear to have been overtaken by the shock front and are cooling radiatively. Also, a high-velocity cloud gives a lower limit of D35 km s~1 to the expansion velocity of the shock into the H I gas and a lower limit of D2 ] 1049 ergs for the kinetic energy injected by the supernova explosion into the surrounding interstellar medium. We estimate a distance to the SNR of d \ 2.1`1.8 kpc and a total mass of at least 1900 M of associated H I gas. A hole in the H I _ X-ray source 1E 1207.4[5209. This H I column density ~0.8 is observed at the same position of the compact hole is at the same radial velocity as G296.5]10.0. We argue that this result constitutes concrete evidence that 1E 1207.4[5209 and G296.5]10.0 are physically associated. Key words : ISM : H I È ISM : individual (G296.5]10.0\PKS 1209[51/52, 1E 1207.4[5209) È stars : neutron È supernova remnants 1.

INTRODUCTION

central compact object (Manchester 1987 ; Storey et al. 1992). The simplest extrinsic model proposes that the morphology is determined by an elongated cavity in the ISM (Bisnovatyi-Kogan, Lozinskaya, & Silich 1990). Models involving alignment and compression of well-ordered magnetic Ðelds have also been suggested (Whiteoak & Gardner 1968 ; Milne 1987 ; Roger et al. 1988). Gaensler (1998) proposes that the Galactic magnetic Ðeld produces density stratiÐcations parallel to the Galactic plane, which produces an axis of symmetry for the remnant parallel to the Galactic plane. He suggests that this mechanism is most relevant for middle-aged remnants. G296.5]10.0 (PKS 1209[51/52) is a clear example of a bilateral SNR, both at radio and X-ray wavelengths (Fig. 1). It is peculiar because it has a symmetry axis oriented almost perpendicular to the Galactic plane (the axis departs from a constant-longitude line by only 7¡), unlike most ““ barrel ÏÏ remnants. The SNR is also exceptional because of its location, well above the Galactic plane. The radio emission associated with G296.5]10.0 (Fig. 1, left) is fragmented and consists of long, thin Ðlaments, positioned tangentially with respect to the outer boundary (Dickel & Milne 1976 ; Kesteven & Caswell 1987 ; Roger et al. 1988). Roger et al. (1988) suggest that these Ðlaments may represent sheets or crushed clouds seen edge-on, partially compressed and cooled. Gaensler (1998) notes that if the morphology is determined by the ISM, then we would expect to Ðnd evidence of expansion into a cavity elongated

Galactic supernova remnants (SNRs) exhibit a wide range of morphologies resulting from the inÑuence of several factors, including the nature of the supernova explosion, the possible existence of a central compact object, the e†ects of the circumstellar material from the progenitor star, and the properties of the surrounding interstellar medium (ISM). Amid the variety of morphologies, bilateral SNRs (also called bipolar, axisymmetric, or barrel-shaped SNRs) constitute an unusual classiÐcation that has provoked considerable debate on the determinants of their appearance. These SNRs are characterized by a clear axis of symmetry, low level of emission along this axis, and two bright limbs on either side (Kesteven & Caswell 1987 ; Roger et al. 1988). The majority of members of this class have a symmetry axis parallel to the Galactic plane (Gaensler 1998). Many mechanisms for generating a bilateral appearance, relying broadly on either factors intrinsic to the original progenitor and subsequent explosion or to the external inÑuence of the surrounding ISM, have been proposed. Intrinsic models invoke either a toroidal distribution of ejecta (Kesteven & Caswell 1987) or the biconical e†ects of outÑows due to progenitor material or from the ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Member of the Carrera del Investigador Cient• Ðco, CONICET, Argentina. 2 Hubble Fellow.

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FIG. 1.ÈL eft : New image (1997) from the Molonglo Observatory Synthesis Telescope of the continuum emission of SNR G296.5]10.0 at 843 MHz, displayed in equatorial coordinates. The gray scale varies linearly from [0.5 to 50 mJy beam~1 and the synthesized beam is 43A ] 55A. Right : ROSAT PSPC X-ray image of G296.5]10.0 in the energy range 0.1È2.4 keV represented in arbitrary units as grays and contours and shown in Galactic coordinates (P. Slane, 1999, private communication). The black spot seen near the center of the remnant corresponds to the X-ray emission of 1E 1207.4[5209.

perpendicular to the Galactic plane. The diameter of G296.5]10.0 along the symmetry axis is about 1.6 times the diameter normal to the axis, while the radius of curvature of the eastern shell is less than that of the western shell, suggesting anisotropy in the original expansion. However, Storey et al. (1992) Ðnd small-scale correlations between the structure in the two arms, which they suggest is indicative of a mechanism intrinsic to the initial explosion. Milne & Haynes (1994) investigated the polarization of G296.5]10.0 at 2.4, 4.8, and 8.4 GHz, Ðnding that the magnetic Ðeld has a remarkably well deÐned tangential alignment. None of the radio frequency observational evidence currently available has been able to distinguish between the various models for the formation of this SNR. At optical wavelengths, small Ðlamentary nebulosities associated with G296.5]10.0 were reported by Irvine & Irvine (1974), based on ultraviolet and Ha observations. Ruiz (1983) investigated the optical spectra toward selected Ðlaments. Strikingly, the optical emission is completely anticorrelated with bright radio or X-ray emission areas (Irvine & Irvine 1974 ; Roger et al. 1988). In the X-ray domain, the overall morphology of G296.5]10.0 is very similar to that in the radio wavelengths (Fig. 1, right) (Tuohy et al. 1979 ; Matsui, Long, & Tuohy 1988). The most intense X-ray emission comes from the same regions that are brighter in radio, i.e., the southeastern and southwestern sectors. This correspondence is observed even at small scales. However, to the northwest, the radio emission has no counterpart in X-rays. The existence of a compact X-ray source (1E 1207.4[5209) located D6@ from the geometric center of the remnant was Ðrst reported by Helfand & Becker (1984) (see Fig. 1, right). Its association with G296.5]10.0, as well as its nature, is still controversial. No radio or optical counterpart has been detected, nor any pulsed emission (Kellet et al. 1987 ; Matsui et al. 1988 ; Bignami, Caraveo, & Meregh-

etti 1992 ; Vasisht et al. 1997). Zavlin, Pavlov, & Trumper (1998) reinterpreted ASCA and ROSAT observations of this source, concluding that it is a radio-quiet neutron star at a distance between 1.6 and 3.3 kpc. From the X-ray data, Zavlin et al. (1998) infer a hydrogen column density of between 0.7 ] 1021 and 2.2 ] 1021 cm~2 in the direction of the compact source. The cold interstellar gas around G296.5]10.0 was formerly investigated at low resolution (half-power beamwidth 30@) over an extended area by Dubner, Colomb, & Giacani (1986). They reported the presence of a large H I shell, centered near v D [11 km s~1, which partially surrounds the western LSR half of the remnant and extends to the east, to the position of the radio source G300.1]9.4, proposed to be another SNR. A low-density tunnel in the H I, about 7¡ long and approximately parallel to the Galactic plane, encloses the SNRs. To di†erentiate between the various models for bilateral SNRs, a clear and detailed picture of the surrounding gas distribution is essential. In particular, the morphology may be dominated by small-scale e†ects. The location of G296.5]10.0 at high Galactic latitude allows sensitive observations of H I gas associated with the remnant, minimizing the e†ects of confusion due to H I along the line of sight. Furthermore, this source is a striking example of bilateral SNR morphology with a sufficiently large angular size to enable a detailed investigation of the threedimensional structure to be made. From our observations, we can investigate the H I environment of the SNR, compare these new results with images at other wavelength domains, and attempt to model the e†ect of the ISM on the evolution of this unusual SNR. In the present paper, we present new H I observations carried out with the Australia Telescope Compact Array (ATCA) in a 3¡.5 ] 3¡.5 Ðeld around G296.5]10.0, with an angular resolution of about [email protected]. To obtain a complete

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picture of the ISM and accurate estimates of masses and column densities, the low spatial frequency data have been provided from single-dish observations performed with the 30 m telescope of the Instituto Argentino de Radioastronom• a (IAR). We describe our observations and analysis in ° 2 and discuss the results in ° 3. Our conclusions are presented in ° 4. 2.

OBSERVATIONS

The interferometric H I observations were carried out with ATCA (Frater, Brooks, & Whiteoak 1992), located near Narrabri, Australia, during three sessions of 13 hr each, on 1998 October 9È11. To achieve optimal u-v coverage in the radial direction, the array was used in the nonstandard 0.210 km conÐguration. We used only the Ðve movable 22 m diameter antennas, which are positioned on an east-west track, in this compact conÐguration. The source was surveyed in a mosaic of 109 di†erent pointings, following a hexagonal grid to cover over 12 deg2 in the sky. The separation between grid points of about 16@ satisÐes the Nyquist sampling criterion. A third of the pointings were observed during each session and later combined after appropriate calibration. The observations were made using 1024 channels over a total bandwidth of 4 MHz, centered at 1420 MHz. This corresponds to a singlechannel width of about 4 kHz, which is 0.82 km s~1 at 21 cm. The primary Ñux density calibration was related to the source PKS B1934[638 ; PKS B1215[457 was used to calibrate the phases. Initial processing was carried out using the MIRIAD software package (Sault, Teuben, & Wright 1995). The continuum emission was subtracted from the line data in the image plane using the MIRIAD task CONTSUB. The data were then jointly deconvolved using the task MOSMEM (Sault, Staveley-Smith, & Brouw 1996) into a single cube of 700 spectral planes covering the velocities between [225 and ]350 km s~1. All velocities given in this paper are with respect to the local standard of rest. The conversion between Ñux density in mJy beam~1 and brightness temperature in kelvins for the interferometric data is 0.014 K mJy~1 beam~1, and the synthesized beam is [email protected] ] [email protected], P.A. \ [0¡.1. To recover structures at all spatial frequencies, the same area was observed on 1999 May 25, with the single-dish, 30 m radiotelescope of the IAR, located in Villa Elisa, Argentina. A 1008-channel correlator was used, with a total e†ective bandwidth of 4 MHz, centered at 1420 MHz. The velocity resolution of the single-dish data is 1 km s~1, and the rms noise per channel is 0.13 K in brightness temperature. Interferometric and single-dish data were normalized to a common temperature scale and uniform velocity intervals of 0.82 km s~1. Both databases were then combined in the Astronomical Image Processing System using the task IMERG, which Fourier-transforms the cubes and merges high- and low-resolution data in the u-v plane. The rms noise was determined from the Ñux in line-free channels, resulting in a 1 p level of 0.25 K channel~1. 3.

RESULTS

3.1. Distribution of Neutral Hydrogen Emission After inspection of the entire velocity range, we Ðnd that in the direction of G296.5]10.0 neutral hydrogen is present

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between about [60 and ]25 km s~1. Figure 2 shows the brightness temperature distribution within this velocity interval, where the images were convolved to a 5@ circular beam for presentation purposes. The Ðgure displays nine H I emission images, obtained after averaging over 9.6 km s~1 intervals, between v D [58 km s~1 and v D ]27 km s~1. The same gray scale, ranging from 5 to 40 K, is used for all the images in order to provide a comparative view of the H I emission distribution. In Figure 1, we have shown equatorial and Galactic coordinate frames in the radio continuum and X-ray images, respectively. Since the inclination between the two coordinate frames is relatively small in this direction, for the rest of this paper we will refer to east and west directions (left and right, respectively) in our images displayed in Galactic coordinates. From v D [53 to v D [43 km s~1 (Fig. 2, panels 1 and 2), faint emission is observed in projection against the northern part of G296.5]10.0, near (l, b) \ (296¡.50, 10¡.50). This feature appears as an extension toward high latitudes of emission parallel to the Galactic plane. This feature moves toward the south and the east across the remnant in the image centered at v D [33.8 km s~1 (panel 3). At v D [24 km s~1 (panel 4), the most conspicuous feature is an almost complete, thick shell centered near (l, b) D (295¡.50, ]10¡.50). At v D [14.6 km s~1 (panel 5), a depression in the H I emission is observed toward the southern half of G296.5]10.0, near (l, b) D (295¡.50, ]9¡.50), with the deepest minimum coinciding with the maxima in the radio and X-ray emission. The void is part of the lowdensity H I cavity reported by Dubner et al. (1986) that surrounds G296.5]10.0. The hole moves signiÐcantly westward at v \ [5 km s~1 (panel 6). At v \ ]4.6 km s~1, it has disappeared. No H I features that might be associated with G296.5]10.0 are apparent at positive velocities. Near v D ]50 km s~1, two small structures are detected approximately 1¡ to the southeast and approximately 1¡ to the southwest of the southern limits of the remnant. Also, between v D ]252 km s~1 and v D ]257 km s~1, an unresolved feature with an average brightness temperature of 20 K is detected at (l, b) D (297¡.16, ]8¡.80), or (297¡9@, 8¡[email protected]). These H I features are too far away from G296.5]10.0 to be related. A prominent Ðlament was detected in the ATCA cube with a velocity v D [26 km s~1, near the position (l, b) D (296¡.50, ]10¡.50). However, after combination with the single-dish data, this feature is no longer conspicuous and does not seem to be associated with the SNR. By comparing the morphology of the H I distribution with that observed from the SNR in di†erent spectral domains, we aim to establish which neutral gas features are physically connected with the remnant. We now investigate the correlations observed between the H I and emission from the optical, radio continuum, and X-ray regimes. 3.2. Comparison with Optical Emission Based on UV and Ha plates, Irvine & Irvine (1974) reported the existence of three di†erent optical features associated with G296.5]10.0. After careful inspection of the H I cube, we Ðnd a close morphological agreement between the H I in the velocity range v D [15.5 to [17 km s~1 and the bright Ðlamentary nebulosity located near (l, b) D (296¡.20, ]10¡.50) (R.A. 12h08m28s, decl. [51¡48@44A ; J2000.0). The optical feature, seen in both the UV image

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FIG. 2.ÈGray scale and contours of brightness temperature distribution obtained after averaging over an interval of 9.6 km s~1 (11 spectral channels). Central velocities for images in panels labeled from 1 to 9 are [53.0, [43.4, [33.8, [24.2, [14.6, [5.0, ]4.6, ]14.2, and ]23.8 km s~1, respectively. Images are smoothed to a 5@ Gaussian beam (original resolution of [email protected] ] 2@7 is used for all other H I Ðgures). The gray scale ranges from 5 to 40 K. The H I contours are 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 36, 40, 44, 48, 52, and 56 K. The 3 p level rms noise is 0.75 K. A few contours of the radio continuum emission of G296.5]10.0 are included for comparison.

from Irvine & Irvine (1974) and the Digitized Sky Survey data, resembles an inverted V and has many thin Ðlaments extending to the north. Figure 3 (top) shows the optical image extracted from the Digitized Sky Survey (IIIa-J plate plus GG 395 Ðlter), with a few H I contours overlapped. For reference, the H I is displayed (Fig. 3, bottom) on a gray scale with the same contours. To emphasize the small-scale structure, the H I image was produced by integrating the interferometric data alone (without the single-dish data) between v D [15.5 km s~1 and [17 km s~1. Excellent correspondence is found between the brightest optical Ðlaments and the H I features. The thin optical Ðlaments, which extend to the north, outline the two H I clouds [near (l, b) D (296¡.08, ]10¡.50) and (l, b) D (296¡.25, ]10¡.53)]. A similar correlation between optical ““ upstream ÏÏ Ðlaments and H I clouds was recognized previously by Dubner et al. (1998), toward a

Ðlament located in the southwest of the Vela SNR. We propose that the supernova (SN) shock has overtaken the small interstellar H I clouds ; the optical emission then arises from shocked interstellar material that is cooling radiatively. The weaker H I emission area to the northeast (upper left) coincides with the presence of di†use optical emission, suggesting reduced optical extinction. To the southeast (lower left), di†use red emission is observed near an H I feature. The presence of the bright star HD 105610 (R.A. 12h09m36s. 66, decl. [52¡08@38A. 5 ; J2000.0) precludes a detailed comparison of the H I with the optical emission in this direction. Ruiz (1983) investigated the optical spectra toward three selected positions along this Ðlament (indicated in Fig. 3, bottom) : toward the east and west extremes (named F1 and F3, respectively) of this inverted V-shaped optical feature

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FIG. 3.ÈT op : Gray-scale image of the optical emission as extracted from the Digitized Sky Survey (IIIa-J plate plus GG 395 Ðlter) with contours of the interferometric H I data integrated between v \ [15.5 and [17 km s~1. Plotted contours are 0.1, 0.2, 0.3, 0.4, and 0.5 Jy beam~1. Bottom : Same as top, but showing the interferometric H I data on a gray scale with the same isocontours as in the optical image, overlapped for reference. The gray scale ranges from [10 to 500 mJy beam~1. F1, F2, and F3 show the positions of the spectra taken by Ruiz (1983) at (296¡.29, ]10¡.35), (296¡.07, ]10¡.43), and (296¡.01, ]10¡.36), respectively.

and close to the center, near the brightest part of the V (named F2). Ruiz (1983) found that in F2 the [O III] 5007 Ó line is the strongest line in the spectrum, while F1 exhibits a ““ normal ÏÏ spectrum (as observed in most SNRs), and F3 is intermediate in its characteristics. From the [S II] j6731/ j6717 line ratios, Ruiz (1983) suggests a density for the ISM on the order of 5 cm~3. From Figure 3, we note that F1 appears inside the larger central H I structure, that F2 is in

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contact with the border of the western H I cloud, and that F3 is associated with a fainter extension of the structure containing the two bright H I features. From the present data, we have estimated an H I mass of about 8 M for each of the clouds. By assuming spherical _ the small clouds, an approximate diameter of geometry for 6@, and a distance of 2.1 kpc (see ° 3.6 for a discussion of the distance determination), a volume density of n ^ 13 cm~3 is derived. For the mass estimates, the complete data set (interferometric plus single-dish contribution) was used. It is noteworthy that this is the only opticalÈH I correlation observed within the velocity range [225 to ]350 km s~1. No obvious H I counterparts were found for the other two faint features described by Irvine & Irvine (1974), which are located near the southern and southwestern boundaries of the SNR. 3.3. Comparison with X-Ray and Radio Continuum Emission The X-ray and radio continuum images are quite similar (Fig. 1), and the H I cube has been investigated for morphological correlation with both spectral ranges. From the interferometric H I images, many small clouds are seen randomly distributed in the observed Ðeld. The best morphological correspondences between the SNR and the interstellar H I clouds are, however, observed in the velocity range between about [14 and [17 km s~1 ; likewise, it was found in the opticalÈH I comparison. In Figure 4a, we display on a gray scale the H I distribution (based on the interferometric H I data) in the four velocity channels where the particular features are present. The contours correspond to the ROSAT Position Sensitive Proportional Counter (PSPC) X-ray image. Figure 4b shows the same H I data, displayed as contours overlapping the Molonglo Observatory Synthesis Telescope (MOST) 843 MHz radio continuum image, shown on a gray scale. The brightest Ðlaments in synchrotron radiation are expected to form in higher compression regions behind the shock front, where the expanding SNR shell encounters and overtakes denser material. Hence, the bright continuum Ðlaments might be expected to occur adjacent to H I enhancements. From the present data, the existence of such an association between the H I and radio continuum is not obvious in general. One reason for the failure to observe this pattern is the fact that if the bright radio continuum Ðlaments are sheets of emission viewed edge-on (as suggested by Roger et al. 1988), then the velocity crowding at this Galactic longitude (i.e., small increments in radial velocity corresponding to large changes in distance) prevents the recognition of distinct small-scale H I structures along the line of sight. The expected change in radial velocity with respect to distance along the line of sight in this direction is estimated to be about 5 km s~1 kpc~1. Nevertheless, some good correlations are identiÐed. Three main H I features can be associated with G296.5]10.0 : 1. The long, broad H I structure of size D1¡ ] 25@ to the northeast that terminates at the northeastern extreme of the X-ray/radio emission, where both radio continuum and X-rays split into two branches, near (l, b) D (296¡.67, ]10¡.50) or (296¡40@, ]10¡30@). In this case, a good match is found between the southwestern border of the H I cloud and the X-ray/radio Ðlament that bends to the west. The correspondence is difficult to show in a printed reproduction

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FIG. 4a FIG. 4.È(a) H IÈX-ray comparison of the region around G296.5 ] 10.0. The gray-scale images correspond to the interferometric H I data in the velocity range from v \ [16.9 to [14.4 km s~1. The gray scale varies from 0.2 to 1 Jy beam~1. Contours correspond to the ROSAT PSPC image, smoothed to a resolution of [email protected]. (b) H IÈradio continuum comparison. Interferometric H I data in the velocity range v \ [19.6 to [14.4 km s~1 displayed in contours. Plotted contours correspond to 0.3, 0.4, and 0.6 Jy beam~1. The 843 MHz MOST radio continnum image is displayed on a gray scale (5 to 50 mJy beam~1).

because of the faintness of the radio and X-ray emission in this direction. We conclude that the presence of this H I cloud may have contributed in forming the northeastern side of the SNR, where two main boundary Ðlaments bifurcate. From the interferometric plus single-dish data, we have estimated for this H I feature a column density N D 4 ] 1020 cm~2, an H I mass of approximately 1300 M H, and _ to a volume density n D 8 cm~3. We assumed a distance G296.5]10.0 of 2.1 kpc (see ° 3.6) and an ellipsoidal geometry for the cloud, with a dimension along the line of sight equal to the minor axis. 2. The H I cloud that lies along the southwestern limb of the SNR, near (l, b) D (296¡.00, ]9¡.50). This cloud is adjacent to the outermost limit of the western lobe of

G296.5]10.0 (particularly from [16.9 to [15.2 km s~1). The observed agreement suggests that the SN shock is in fact interacting with an interstellar cloud. For this feature, we have estimated a column density N D 2 ] 1020 cm~2, a H a volume density total H I mass of about 110 M , and _ n D 13 cm~3, where again we have assumed an ellipsoidal shape for the cloud. 3. The H I cloud that crosses the eastern limb of G296.5]10.0, close to the location of the brightest Ðlaments centered near (l, b) D (296¡.67, ]9¡.67) or (296¡40@, 9¡40@). Since the cloud is observed in projection, covering part of the SNR shell and a portion of the interior, we cannot conclude whether they are mutually interacting and whether the H I cloud inÑuences the formation of the bright X-ray and radio Ðlaments. Based on the interferometric

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and single-dish data, we have estimated for this structure a column density N D 2 ] 1020 cm~2, a total H I mass of about 95 M , and Ha volume density n D 13 cm~3. _ A small cloud is seen in projection against the western limb of the SNR, near (l, b) D (296¡.0, ]10¡.0). Since this feature has a size comparable to the beam and is detected in only one channel, its association with G296.5]10.0 is doubtful and will not be further discussed. Matsui et al. (1988) proposed that the density of the ISM swept up at the eastern side of the remnant should be up to 6 times higher than that on the western side, to explain the anisotropic expansion. In fact, the present observations indicate a trend in the opposite sense.

From the correspondence between the H I features and the optical/X-ray/radio continuum emissions, we propose v ^ [16 km s~1 as the systemic velocity of G296.5]10.0. 3.4. High-V elocity H I Gas and the Energetics of G296.5]10.0 In addition to the distribution of the neutral gas in the vicinity of the SNR, the only higher velocity feature that can be clearly associated with G296.5]10.0 is a faint H I cloud detected between [53 and [45.5 km s~1, projected against the northern part of the SNR (Fig. 5). For this cloud, we have estimated a mass of about 390 M and a volume _ density of 1.5 cm~3.

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FIG. 5.ÈOverlay of the 843 MHz radio continuum image of G296.5]10.0 with the H I distribution integrated between [50.4 and [45.5 km s~1. Contours correspond to 42, 46, and 50 K.

If we assume that this cloud was accelerated to this velocity by the SN shock, a lower limit of about 35 km s~1 can be estimated for the expansion velocity of the shock into the clouds. Based on the mass estimates of all the associated H I features, which amount to about 1900 M , and an expansion velocity of 35 km s~1, a lower limit of _ 2 ] 1049 ergs can be estimated for the kinetic energy injected by G296.5]10.0 into the surrounding ISM. 3.5. Absorbing Material and 1E 1207.4[5209 A knowledge of the distribution of the H I column density is essential to accurately model the X-ray spectrum. In Figure 6 (top), we show the neutral hydrogen column density distribution obtained from the integration between 0 and [16 km s~1, assuming optically thin gas. This is the velocity range for all the gas between the observer and the systemic velocity of the remnant. Because of the distance ambiguity inherent in the Galactic rotation curve at this longitude, some of the H I gas in this velocity range will be located beyond the SNR. Hence, the estimates presented here represent an upper limit for the column density, although it is reasonable to assume there will be only minimal neutral gas in this direction at b \ 10¡ (which corresponds to z ^ 370 pc at d \ 2.1 kpc ; see ° 3.6 below). No large spatial variations in the column density are observed across G296.5]10.0. The N varies between D1.1 ] 1021 H D1.6 ] 1021 cm ~2 along cm ~2 on the western side and the eastern side. Strikingly, a hole in the H I column density, approximately 4@ ] 7@ in size, with an average brightness temperature of about 38 K (0.53 Jy beam~1) is found exactly at the position of the compact X-ray source 1E 1207.4[5209, classiÐed as an isolated neutron star (Helfand & Becker 1984 ; Kellet et al. 1987 ; Matsui et al. 1988 ; Zavlin et al. 1998). Figure 6 (bottom) shows an enlargement of the central area with the position of the X-ray source marked by a plus

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sign. It is noteworthy that no similar dips are found in the data cube over a velocity range from about [30 to ]20 km s~1. After analyzing the H I distribution channel by channel, we conclude that the small hole is present from v \ [15.8 km s~1 and attains the minimum emission at v \ [16.1 km s~1. Beyond v \ [17.4 km s~1, the hole disappears. This velocity range coincides with the range in which the best morphological correlations were found, between the H I and the optical, radio, and X-ray remnants. The fact that we detect this H I depression, which agrees in position with 1E 1207.4[5209 and in velocity with G296.5]10.0, is strongly suggestive that (1) the X-ray point source is located at the same distance as the SNR and (2) the H I is inÑuenced in some way by the presence of the neutron star. The X-ray Ñux of 1E 1207.4[5209 has been measured several times since 1979 with di†erent instruments and is D4 ] 10~12 ergs cm~2 s~1 between 0.1 and 4 keV (Mereghetti, Bignami, & Caraveo 1996). The ROSAT PSPC data were analyzed looking for pulsations down to periods as short as 26 ms (Mereghetti et al. 1996), and ASCA data were searched for periodic variations on timescales ranging from a few minutes to a few hours (Vasisht et al. 1997). Both searches for pulsations gave negative results. Also, deep searches for an optical counterpart were conducted by Bignami et al. (1992) and Mereghetti et al. (1996). No optical source was detected to a limit of V D 25 mag. The two closest objects discovered are normal K stars. In the radio range, Mereghetti et al. (1996) conducted ATCA observations at 4.8 GHz (5A FWHM). No point source was detected above the 3 p limit of 0.1 mJy. Roger et al. (1988) set an earlier upper limit of 3 mJy at 843 MHz. The radio nebula that Vasisht et al. (1997) reported from their interpretation of the Roger at al. (1988) image would appear to be spurious and a result of an artifact introduced in the process of combining the MOST and Parkes Telescope data. The existence of any nebula is not evident in the more recent MOST image shown in Figure 1, which has an rms noise level of D1.5 mJy beam~1 at 843 MHz. This unusual class of radio-quiet neutron stars, which may be slowly, thermally cooling, has been described by Caraveo, Bignami, & Trumper (1996) and Brazier & Johnston (1999). Zavlin et al. (1998) have reanalyzed the X-ray data collected with ROSAT and ASCA and Ðnd that a model for the neutron star incorporating a hydrogen atmosphere can be used to derive a distance of 1.6È3.3 kpc and an intervening column density of D(0.7È2.2) ] 1021 cm~2, in very good agreement with the present results. Zavlin et al. (1998) consider these estimates to be more reasonable than those derived from simple blackbody models, from which a distance of 11È13 kpc is inferred. We suggest that the X-ray Ñux from 1E 1207.4[5209 heats local gas in an area of a few arcminutes (inferred from the fact that its e†ects are detectable with the present resolution). This hot gas around the neutron star would provide the hot background against which the emission of the cold foreground H I is self-absorbed. High-resolution and high-sensitivity radio observations and polarization studies toward 1E 1207.4[5209 are required to constrain the physical parameters of the local gas and elucidate the origin. The fact that the closest stars are K type ( Mereghetti et al. 1996) allows us to reject stellar winds as the origin for the small hole. Stellar winds for late-type stars are known to be weak.

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11 30

00

GALACTIC LAT.

10 30

00

09 30

00

08 30 297 30

00


295 30

00

10 15 10 05

GALACTIC LAT.

00 09 55 50 45 40 35

FIG. 6.ÈT op : H I column density integrated between 0 and [16 km s~1. The gray scale varies between 1.1 ] 1021 and 1.5 ] 1021 cm~2. The plotted contours, in units of 1021 cm ~2, are 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, and 1.7. For reference, a few gray-tone contours of the ROSAT PSPC image of G296.5]10.0 have been included. Bottom : Enlargement of the central region, with the position of the X-ray source 1E 1207.4[5209 indicated by a plus sign.

If the observed depression is a consequence of the presence of the neutron star, the present results would provide strong observational support to the contention that 1E 1207.4[5209 is the compact remnant left after the explosion of the G296.5]10.0 supernova. Similar radio-quiet, thermally cooling neutron stars were found associated with several SNRs (Brazier & Johnston 1999), including Puppis A (Petre, Becker, & Winkler 1996) and RCW 103 (Gotthelf, Petre, & Hwang 1997 ; Gotthelf, Petre, & Vasisht 1999). 3.6. Distance The convincing opticalÈH I correlation described in ° 3.2 suggests that the systemic velocity for the associated H I gas must lie in the range between [15.5 and [17.5 km s~1. This velocity range was conÐrmed from the morphological

comparison with the radio continuum and X-ray emission and from the coincidence of the N hole with the location of H 1E 1207.4[5209. Based on this evidence, we can set constraints for the radial velocities of the interstellar material associated with G296.5]10.0 of v \ [[15.8, [17.4] km s~1. The most probable central velocity is v D [16 km s~1. At l D 296¡.50, this velocity roughly corresponds to 2.1 or 5.6 kpc, as obtained by applying the Galactic circular rotation model of Fich, Blitz, & Stark (1989 ; with # \ 220 km s~1 and R \ 8.5 km s~1). The high z-height0 of several 0 hundred parsecs for this remnant does introduce some uncertainty. However, the major uncertainty arises from peculiar motions, which are on the order of 7 km s~1. It seems reasonable to assume that this SNR is located at the near distance, based on the values estimated for its linear

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GIACANI ET AL.

size and height z above the plane. At the far distance of 5.6 kpc, the remnant becomes unrealistically large (D150 pc), with a z of D970 pc above the plane. We therefore propose for G296.5]10.0 a near distance of d \ 2.1`1.8 kpc, where ~0.8 the statistical 7 km s~1 for peculiar motions are included in the quoted errors. 4.

CONCLUSIONS

We have surveyed more than 12 deg2 of sky in the direction of the bilateral SNR G296.5]10.0 and its environment, with an angular resolution of about [email protected]. The results represent a combination of ATCA interferometric data with IAR single-dish observations, with all spatial frequency contributions recovered. From our H I observations and the comparison with the emission at other wavelengths, the main Ðndings are the following : 1. The environs of G296.5]10.0 are quite homogeneous, as expected at this high Galactic latitude. The observed distribution is compatible with the suggestion of Bisnovatyi-Kogan et al. (1990) and Roger et al. (1988) that G296.5]10.0 may be expanding into the boundaries of the stellar windblown bubble of the precursor star. The hypothesis of a wind-blown bubble is favored by the argument summarized in point 7 below, that the compact central source 1E 1207.4[5209 is indeed the neutron star left after the explosion of G296.5]10.0. Massive stars blow powerful winds during their lifetimes and end their life cycles exploding as Type Ib or II SNe, which are known to leave compact cores. The present results do not conÐrm the Matsui et al. (1988) suggestion that the interstellar material along the eastern side is about 6 times denser than that on the western side, to explain the asymmetric expansion and the di†erences between the radii of curvature of the arms. We therefore conclude that the bilateral symmetry of G296.5]10.0 does not appear to have been determined from a special tangential alignment of the interstellar gas. However, our results are still compatible with models involving magnetic Ðeld compression (Whiteoak & Gardner 1968 ; Roger et al. 1988 ; Gaensler 1998) and do not exclude some intrinsic models, such as that of Storey et al. (1992). 2. From the comparison of the H I distribution with the optical emission in G296.5]10.0, we were able to detect two small clouds, near (l, b) D (296¡.17, ]10¡.50), with v D [16 km s~1, that appear to have been overtaken by the shock front and are currently cooling radiatively. 3. From the comparison of the H I distribution with the radio continuum and the X-ray emission, we identiÐed three di†erent H I structures between v D [14 and [17 km s~1, which may have contributed to shape the observed remnant. The main features are (a) a northeastern structure with volume density n D 8 cm~3, (b) the cloud close to the southwestern limb with n D 13 cm~3, and (c) the cloud placed in the south-central region, close to the bright southeastern Ðlaments with n D 13 cm~3. For the third feature, the connection with the radio Ðlaments is less evident because of the velocity crowding that occurs in this direction of the Galaxy. 4. At higher velocities, the only H I feature that appears associated with G296.5]10.0 is an isolated cloud detected between [53 and [45 km s~1. Assuming that the cloud has been accelerated to this velocity by the SN shock, a lower limit of D35 km s~1 is derived for the expansion

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velocity of the shock into the H I clouds. The mass of all the features that we have associated with the remnant totals at least 1900 M . If this gas were set in motion at _ velocities up to 35 km s~1 by the SN shock, then a lower limit of 2 ] 1049 ergs can be estimated for the kinetic energy injected by G296.5]10.0 into the surrounding medium. 5. Based on the good morphological agreement found between the H I and the optical/X-ray/radio remnants of G296.5]10.0 in the velocity range [[17.5, [15.5] km s~1, we were able to reÐne the distance determination for this SNR. A distance of d \ 2.1`1.8 kpc is obtained. ~0.8 6. The distribution of an upper limit of the neutral hydrogen column density between the observer and the SNR was obtained. No large spatial variations are observed across the source ; the column density varies only between 1.1 ] 1021 and 1.6 ] 1021 cm~2. 7. A remarkable coincidence in the three dimensions (l, b, v) is found between the compact X-ray source 1E 1207.4[5209, proposed to be an isolated neutron star, and a hole in the N . We speculate that the H I emission void, about 4@ ] 7@ inH size, is produced by self-absorption of the cold foreground H I gas against an extended hot hydrogen nebula surrounding the neutron star. This nebula, at least several arcminutes in size, would then be heated by the X-ray Ñux of the neutron star. If the association between the H I void and the X-ray source is conÐrmed, then the present results constitute concrete observational evidence that 1E 1207.4[5209 and G296.5]10.0 are associated. Higher resolution and higher sensitivity H I observations around the compact source 1E 1207.4[5209 are needed to constrain the physical parameters. They will also provide conÐrmation that this object is the compact remnant of G296.5]10.0 and allow a determination of the nature of the system producing the observed H I hole. G. M. D. and E. B. G. are grateful to C. Cappa for her invaluable assistance during the IAR observations. They also thank M. Mayochi and M. Ortega for their help during the initial processing of the data. E. B. G. acknowledges the Australia Telescope National Facility and the Special Research Center for Theoretical Astrophysics at the University of Sydney for their support during her visit to Australia. G. M. D., E. B. G., and A. J. G. acknowledge the hospitality at NRAO, where part of the processing was carried out. B. M. G. acknowledges the support of NASA through Hubble Fellowship grant HF 01107.01-98A, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. We would like to thank P. Slane for providing us with the processed X-ray image. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility, managed by CSIRO. The MOST is owned and operated by the University of Sydney with support from the Australian Research Council and the Science Foundation for Physics at the University of Sydney. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. This project has made use of the High Energy Astrophysics Science Archive Research Center Online Service. The Digitized Sky Survey was produced at the Space Telescope Science Insti-

No. 1, 2000

SNR G296.5]10.0

tute under US government grant NAGW-2166, based on photographic data obtained using the UK Schmidt Telescope. The UK Schmidt Telescope was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council, until 1988 June,

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and thereafter by the Anglo-Australian Observatory. This research was partially funded through a Cooperative Science Program between CONICET (Argentina) and the NSF, through CONICET grant 4203/96 and through ANPCYT (Argentina) grant 03100000-02305.

REFERENCES Bignami, G. F., Caraveo, P. A., & Mereghetti, S. 1992, ApJ, 389, L67 Matsui, Y., Long, K. S., & Tuohy, I. R. 1988, ApJ, 329, 838 Bisnovatyi-Kogan, G. S., Lozinskaya, T. A., & Silich, S. A. 1990, Ap&SS, Mereghetti, S., Bignami, G. F., & Caraveo, P. A. 1996, ApJ, 464, 842 166, 277 Milne, D. K. 1987, Australian J. Phys., 40, 771 Brazier, K. T. S., & Johnston, S. 1999, MNRAS, 305, 671 Milne, D. K., & Haynes, R. F. 1994, MNRAS, 270, 106 Caraveo, P. A., Bignami, G. F., & Trumper, J. E. 1996, A&A Rev., 7, 209 Petre, R., Becker, C. M., & Winkler, P. F. 1996, ApJ, 465, L43 Dickel, J. R., & Milne, D. K. 1976, Australian J. Phys., 29, 435 Roger, R. S., Milne, D. K., Kesteven, M. J., Wellington, K. J., & Haynes, Dubner, G. M., Colomb, F. R., & Giacani, E. B. 1986, AJ, 91, 343 R. F. 1988, ApJ, 332, 940 Dubner, G. M., Green, A. J., Goss, W. M., Bock, D. C.-J., & Giacani, E. B. Ruiz, M. T. 1983, AJ, 88, 1210 1998, AJ, 116, 1842 Sault, R. J., Staveley-Smith, L., & Brouw, W. N. 1996, A&AS, 120, 375 Fich, M., Blitz, L., & Stark, A. A. 1989, ApJ, 342, 272 Sault, R. J., Teuben, P. J., & Wright, M. C. H. 1995, in ASP Conf. Ser. 77, Frater, R. H., Brooks, J. W., & Whiteoak, J. B. 1992, J. Electrical Electron. Astronomical Data Analysis Software and Systems IV, ed. R. A. Shaw, Eng. Australia, 12, 103 H. E. Payne, & J. J. E. Hayes (San Francisco : ASP), 433 Gaensler, B. M. 1998, ApJ, 493, 781 Storey, M. C., Staveley-Smith, L., Manchester, R. N., & Kesteven, M. J. Gotthelf, E. V., Petre, R., & Hwang, U. 1997, ApJ, 487, L175 1992, A&A, 265, 752 Gotthelf, E. V., Petre, R., & Vasisht, G. 1999, ApJ, 514, L107 Tuohy, I. R., Mason, K. O., Clark, D. H., Cordova, F. A., Charles, P. A., Helfand, D. J., & Becker, R. H. 1984, Nature, 307, 215 Walter, F. M., & Garmire, G. P. 1979, ApJ, 230, L27 Irvine, N. J., & Irvine, C. E. 1974, ApJ, 192, L111 Whiteoak, J. B., & Gardner, F. F. 1968, ApJ, 154, 807 Kellet, B. J., Branduardi-Raymont, G., Culhane, J. L., Mason, I. M., Vasisht, G., Kulkarni, S. R., Anderson, S. B., Hamilton, T. T., & Kawai, N. Mason, K. O., & Whitehouse, D. R. 1987, MNRAS, 225, 199 1997, ApJ, 476, L43 Kesteven, M. J., & Caswell, J. L. 1987, A&A, 183, 118 Zavlin, V. E., Pavlov, G. G., & Trumper, J. E. 1998, A&A, 331, 821 Manchester, R. N. 1987, A&A, 171, 205