SWIFT/BAT Detections of Hard X-ray Sources: IV

ATel #697; M. Ajello, J. Greiner, A. Rau (all MPE Garching), S. Barthelmy (GSFC), J. A. Kennea (PSU), A. Falcone (PSU), O. Godet (U. Leceister),D. Grupe (PSU), J. Tueller (GSFC), C. Markwardt (GSFC/UMD), R. Mushotsky (GSFC), T. Belloni (OAB), K. Mukai (GSFC), S. T. Holland (GSFC/USRA), N. Gehrels (GSFC)
on 19 Jan 2006; 14:37 UT
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

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We report on the first detection of hard X-rays (15-150 keV) from further 3 sources with the Swift Burst Alert Telescope (BAT), and subsequent follow-up observations with the Swift X-ray Telescope (XRT; 0.5-10 keV). In contrast Tueller et al. (ATEL #668, #669) and Kennea et al. (ATEL #677), BAT data from May-Nov 2005 have been used.
Sources are named by their SWIFT/BAT name/position. The XRT position is accurate to a few arcsec, but can be off the BAT position by about 3 arcmin (90%).

  
BAT Name            S/N     XRT position (2000)    D   Type     z  


  
                           RA          Dec   Err  


  
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SWIFT J0732.5-1331  5.9  07 32 37.5 -13 31 04  4  1.5  CV?    0.0000   


  
SWIFT J0823.4-0457  6.7  08 23 01.2 -04 56 04  5  3.7  AGN    0.0218   


  
SWIFT J0918.5+1618  6.7  09 18 25.9 +16 18 22  5  2.6  Sy1.5  0.0292   


  

The XRT error is given in arcsec (90%), the distance D between BAT and XRT position in arcmin. In all three cases, the XRT source is the only X-ray source in the XRT field of view, centered on the BAT position. Also, in all cases the extrapolation of the XRT spectral model to the BAT range gives consistent BAT fluxes. We therefore tentatively identify the BAT source with the XRT source.
SWIFT J0732.5-1331: In a 3400 sec XRT exposure we detect a bright source (SWIFT J073237.5-133104) with 600 cts, coinciding in position with the ROSAT all-sky survey source 1RXS J073237.6-133113. The X-ray spectrum can be fit with either a power law with photon index 1.1, or a Raymond-Smith model with a temperature of 60 keV. In both cases, the hydrogen column density is below 10^-19 cm^-2 which implies a rather small distance from Earth (bII= +2.8 deg). The X-ray position falls on top of USNO B1 star J073237.64-133109.0, which has B1=15.74, B2=14.56, R1=14.49 and R2=13.82 mag. This is coincident with 2MASS J073237.64-133109.4 with J=12.9, H=12.5 and K=12.4 mag. The 2MASS data suggest a K main-sequence star at a distance of 400 pc. The soft X-ray luminosity at that distance would be of order 10^31 erg/s. This, plus the implied B+R variability and the colors suggest a CV identification. Spectroscopic identification and search for orbital optical modulation are encouraged.
SWIFT J0823.4-0457: The XRT position (SWIFT J082301.2-045604) falls exactly onto the center of FAIRALL 0272 = PGC 023515 = MCG -01-22-006. In a 1230 sec XRT exposure we find only 24 photons. The admittedly low S/N spectrum is surprisingly flat: assuming a power law of photon index 2.0 (1.7) requires a neutral hydrogen column of 2.5(2.2)x10^23 cm^-2! At bII= +17.8 deg, this cannot be foreground galactic. According to Fairall (1979, Nat 279, 140) this (in NED unclassified) object shows strong [OIII]5007+4959 as well as narrow H-beta emission lines which leads us, in accordance with the strong X-ray absorption, to tentatively interpret FAIRALL 0272 as a Sy2 type galaxy. Spectroscopic confirmation would be valuable.
SWIFT 0918.5+1618: The XRT position (SWIFT J091825.9+161822) coincides with Mrk 704 which has been detected earlier at soft X-rays as a bright ROSAT source (1RXS J091826.2+161825 = RBS 764; Schwope et al. 2000, Astron. Nachr. 321, 1). In a 660 sec exposure we detect 62 photons. The BAT spectrum is best fit with a power law with photon index 1.1, but the XRT spectrum is not. Archival ASCA observations indicate the 0.5-10 keV spectrum to be either fit by a broken power law with 1.3 below 2.5 kev and 0.7 at higher energies, or a reflection model with all the emission being totally reflected, thus being a Compton thick object. Optical spectroscopy to check the classification as Sy1.5 seems worthwhile.