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X-Ray Emission of Galaxies; Rosat Latest Results

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X-Ray Emission of Galaxies; Rosat Latest Results 67 X-RAY EMISSION OF GALAXIES; ROSAT LATEST RESULTS Wolfgang Pietsch Max Planck Institut fiir extraterrestrische Physik D-8046 Garching, Federal Rep. of Germany ABSTRACT First results of pointed and All Sky Survey observations of galaxies with the X-ray observatory satellite ROSAT are reported. Survey correlations with IRAS galaxies led to the detection of spiral galaxies with X-ray luminosities of up to 1043 erg s-1 . During observations of the Magellanic Clouds and the Andromeda galaxy new super-soft X-ray sources have been detected. This new class of luminous X-ray sources may help to solve the millisecond pulsar progenitor problem. Due to the improved sensitivity and longer observation times of ROSAT new X-ray point sources have been resolved in several nearby galaxies. The diffuse emission of the LMC that was already reported by EINS TEIN has been been mapped in detail. It shows a lot of fine structure and K. ROSAT temperatures around 5 x 106 The improved low energy response of led to the discovery of 106 K gas from the spiral galaxy MlOl and the halo of the starburst galaxy NGC 253. No diffuseemission was detected from the halo of the edge-on spiral galaxy NGC 5907. 68 W. PIETSCH 1. INTRODUCTION 10 EINSTEIN More than years aftPr t. hP observations with the satellite a com­ prehensive analysis of normal galaxies is collected in An X-Ray Catalog and Atlaa of Galaziea1) and The X-Ray Spectra of Galazies2),3). The catalog consists of 493 galaxies, 238 of which are detected, for 212 upper limits are given, some are at the edge of the fieldof view or confused with other X-ray sources. Spectral parameters are presented for43 galaxies and X-ray colors for 127 galaxies. Detailed analysis of individual galaxies have added important information to our knowledge of the X-ray emission of normal galaxies. Some galaxies were detected with enough resolution and sensitivity to study their X-ray morphology, spectra and individual sources within the galaxy. Comparison with infrared,optical, and radio data gave further information. For a recent review see 4). Detailed descriptions of the EINSTEIN view of the local group galaxies LMC, SMC and M31 are given in 5),5),7) and references therein. The EINSTEIN observations have shown that normal galaxies of all morphological types are spatially extended sources of X-ray emission with luminosities in the range of � 1038 erg s-1 to 1042 erg s-1 with spirals only reaching a few10° erg s-1 . On average for spiral galaxies the X-ray spectra are harder than for ellipticals. This is explained by the commonly accepted view that the X-ray emission of spirals is dominated by accreting binaries and supernova remnants (SNR) while a hot interstellar medium is present in ellipticals. However also some spirals show a very soft component in their spectra. This may manifest that an extended gaseous component is also present in some spirals as has been resolved in the starburst galaxies NGC 253 and M82. In the following I will report on X-ray observations with the Rontgen Observatory SATellite (ROSAT )8) during it's calibration and verificationphase, during the ROSAT All Sky Survey (RASS) and on first pointed observations of galaxies. They were devoted to the local group galaxies LMC, SMC, and M31. Further observations were planed to search for the hot interstellar medium or hot halo gas with temperatures of � 106 K in the face-on respectively edge-on spirals MlOl and NGC 5907 and in the starburst galaxy NGC 253. The ROSAT X-ray telescope consists of 4 nested Wolter type I mirrors9). The ROSAT design reduces mirror scattering to a minimum. This leads to high-contrast images that are essentially free of non-X-ray background and allow to directly map the diffuse sky background. For all observations reported a position sensitive proportional counter PSPC was the focal plane instrument in use. The PSPC covers the energy ( ) lO) range 0.1 -2.4 keV and hasa fieldof view of 57 arcmin radius. The on axis resolution is limited by the PSPC to 25 arcsec and degrades for off axis angles larger than 20 arcmin due to the telescope blur, the energy resolution is 45% FWHM at 1 keV. With respect to EINSTEIN the main improvements are collecting area, spatial and energy resolution, lower intrinsic background and especially the highly improved low energy response in the 0.1 - 0.4 keV band. ROSAT LA TEST RESULTS 69 Before ROSAT all sky X-ray surveys have been performed by collimated photon counters aboard the UHURU11), Ariel-512),l3), and HEA 0-114) satellites detecting some 970 sources. During the EINSTEIN and EXOSAT missions that carried pointed X-ray telescopes about 5,000 sources have been looked at. The RASS is the first all sky survey in X-rays by means of a true imaging telescope. It is a factor of hundred more sensitive than HEA0-1, allows much higher angular resolution, and source positions are determined to ,..., 30 arcsec or better depending on source strength. During the RASS ROSAT scanned the sky with constant speed on great circles perpendicular to the sun direction covering all the sky within half a year starting July 30, 1990. The PSPC was the focal plane instrument in use. The angular resolution of the ROS AT telescope/PSPC system in survey mode is about 1 arcmin, if all the photons accumulated fora source at different PSPC off axis angles are used. The typical exposure in the ecliptic plane is 400 sec adding up to 50,000 sec at the north ecliptic pole. Due to the position close to the south ecliptic pole the LMC region got a rather deep exposure during the RASS. For the analysis of ROSAT data a Standard Analysis Software System (SASS)15l hasbeen developed at the Max Planck Institut fiir extraterestrische Physik (MPE). 2. RASS OBSERVATIONS OF GALAXIES During the first processing of the RASS with SASS X-ray data have been accu­ mulated in 2 degree wide strips that pass through the ecliptic poles. On strip typ­ ically was ready for processing every two days during the survey. Fields with 17 degree radius around the ecliptic poles, where the strips strongly overlap have been processed separately. The data have been analysed using source detection algorithms. Sources have been checked for variability and extend. Information on the energy spec­ tra was achieved by fitting Raymond-Smith thermal plasma and power low spectra to the brighter sources; for the fainter ones only hardness ratios have been determined. About 60,000 sources have been detected during this first processing. RASS sources have been correlated with with a special version of the SIMBAD catalog at MPE and with so-called 'user-supplied' catalogs16). The nearby galaxy group at MPE provided a galaxy catalog to SASS containing 9 2 l 2 ,..., 30, 000 entries from the UGC17), RC218l, RSA 1 l, ES0 0 , and Tully 1> catalogs. Cross-correlation of this catalog with the RASS detected sources yields of about 600 identifications, ,..., 150 of which are AGN. Detailed analysis of this sample of RASS galaxies is in progress. Another catalog correlated contained ,..., 15, 000 extragalactic infrared sources se­ lected from the IRAS Point Source Catalog via statistical classification22l. 244 of the IRAS galaxies are positionally coincident with RASS sources; 222 have optical coun­ terparts identified by comparison with the NASA/IPAC Extragalactic Database. The sample of X-ray emitting IRAS galaxies contains infrared luminous nearby galaxies, spirals and ellipticals, Seyfert galaxies, and QSOs. More than half of the sample are 70 W. PIETSCH galaxies with active nuclei which only make up 6% in the unselected sample of IRAS galaxies. The number of X-ray selected ellipticals ("' 3%) is enhanced by a factor of the sources versus the X-ray luminosity more than 3. Figure 1 shows a lii:sLug,rau1 of for thesubgroup s. The interesting result is the discovery of spiral galaxies with X-ray luminosities of up to 1044erg s-1 , more than a factor of 10 above the highest lumi­ 2 x nosities quoted for spirals from EINSTEIN observations. Follow up optical observations showed that a few of these spiral galaxies with very high X-ray luminosity (up to a few times 1043erg s-1 ) are misclassified Seyfert galaxies or AGN; most of them however are ROSAT detected X-ray luminous narrow emission line galaxies. These may hide an AGN nucleus that is not detected in the optical regime23). Seyfert Spirals flux high IRAS QSOs Fig. 1. X-ray luminosity forRASS detected IRASgalaxies (Fig. 14 of 22)) The first RASS processing did only cover 97% of the sky. The way the source detection was done has inhibited the detection of sources in a few percent of the sky. In a second processing run of the RASS SASS will bin photons in slightly overlapping images similar to optical plate surveys, allowing a 100% detection capability. The images will take the full RASS exposure of that region of the sky (including the remaining 3% of the sky, that have been filled by follow up survey observations) and therefore the source flux limit will decrease. This will increase the number of galaxy detections since most of the galaxies are found at the detection limit of the survey. 71 ROSAT LATEST RES ULTS The investigations of the RASS reported above are using the final output of the SASS source detection programs that have been correlated with 'user defined' catalogs. Individual galaxies however can also be analysed by starting from the information col­ lected for each individual photon of the survey.
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