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The X-Ray Afterglow of Dark GRB 970815: A Common Origin for Gamma-Ray Bursts and X-Ray Flashes?
GRB 970815 is a well-localized gamma-ray burst (GRB) detected by theAll-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) forwhich no afterglow was identified despite follow-up ASCA and ROSATpointings and optical imaging to limiting magnitude R>23. Although anX-ray source, AX/RX J1606.8+8130, was detected just outside the ASMerror box, it was never associated with the GRB because it was notclearly fading and because no optical afterglow was ever found. Werecently obtained an upper limit for this source with Chandra that is atleast a factor of 100 fainter than the ASCA detection. We also made deepoptical observations of the AX/RX J1606.8+8130 position, which is blankto limits V>25.2 and I>24.0. In view of these extreme limits, weconclude that AX/RX J1606.8+8130 is indeed the afterglow of GRB 970815,which corresponds to an optically ``dark'' GRB. AX/RX J1606.8+8130 cantherefore be ruled out as the counterpart of the persistent EGRET source3EG J1621+8203. The early light curves from BATSE and the RXTE ASM showspectral softening between multiple peaks of prompt emission. We proposethat GRB 970815 might be a case in which the properties of an X-rayflash and a ``normal'' GRB coincide in a single event.
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| Is the EGRET Source 3EG J1621+8203 the Radio Galaxy NGC 6251?
We discuss the nature of the unidentified EGRET source 3EG J1621+8203.In an effort to identify the gamma-ray source, we have examined X-rayimages of the field from ROSAT PSPC, ROSAT HRI, and ASCA GIS. Of theseveral faint X-ray point sources in the error circle of 3EG J1621+8203,most are stars or faint radio sources, unlikely to be counterparts tothe EGRET source. The most notable object in the gamma-ray error box isthe bright FR I radio galaxy NGC 6251. If 3EG J1621+8203 corresponds toNGC 6251, then it would be the second radio galaxy to be detected inhigh-energy gamma rays after Cen A, which provided the first clearevidence of the detection above 100 MeV of an active galactic nucleus(AGN) with a large-inclination jet. If the detection of more radiogalaxies by EGRET has been limited by its threshold sensitivity, thereexists the exciting possibility that new high-energy gamma-rayinstruments, with much higher sensitivity, will detect a larger numberof radio galaxies in the future.
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