Abstract
We report the second-generation Energetic X-Ray Imaging Telescope Experiment (EXITE2) hard X-ray imaging of the sky around 3C 273. A hr observation on 1997 May 8 shows a ~260 mcrab source detected at ~4 σ in each of two bands (50-70 and 70-93 keV) and located ~30' from 3C 273 and consistent in position with the Granat SIGMA source GRS 1227+025. The EXITE2 spectrum is consistent with a power law with photon index 3 and large low-energy absorption, as indicated by the SIGMA results. No source was detected in more sensitive follow-up EXITE2 observations in 2000 and 2001 with 3 σ upper limits of 190 and 65 mcrab, respectively. Comparison with the flux detected by SIGMA shows the source to be highly variable, suggesting it may be nonthermal and beamed and thus the first example of a type 2 (absorbed) blazar. Alternatively, it might be an unprecedented very highly absorbed binary system undergoing accretion-disk instability outbursts, possibly either a magnetic cataclysmic variable or a black hole X-ray nova.
Highlights
GRS1227+025 was discovered serendipitously with SIGMA (Bassani et al 1991; Jourdain et al 1992) in the 40-80 keV energy band during an observation of the QSO 3C273
The source has a significantly steeper spectrum (photon index 3+−10..39, Bassani et al (1991)) compared to 3C273, which was detected in subsequent observations, with its usual flatter photon index of ∼1.5
No obvious soft X-ray counterpart was found in the SIGMA error circle (5 arcmin, 1σ) with ROSAT (e.g. Leach & McHardy (1996)), an Einstein source (1E 1227+0224) in the SIGMA error circle (1σ) was identified with a QSO at z=0.57 that would have to be of “type 2” if it were the counterpart (Grindlay 1993)
Summary
GRS1227+025 was discovered serendipitously with SIGMA (Bassani et al 1991; Jourdain et al 1992) in the 40-80 keV energy band during an observation of the QSO 3C273. Upper limits (15-30 keV) with ART-P (simultaneous with SIGMA) showed that the spectrum must be heavily absorbed: NH ∼1.5×1025 cm−2 for a power law spectrum with photon index 3 (Bassani et al 1993). Followup observations done with SIGMA and OSSE (which could only measure the combined flux of 3C273 and GRS1227) showed that GRS1227 is probably variable by a factor 2-5 on ∼1 day time scales (Bassani et al 1993; Johnson et al 1995). Leach & McHardy (1996)), an Einstein source (1E 1227+0224) in the SIGMA error circle (1σ) was identified with a QSO at z=0.57 that would have to be of “type 2” (heavily self-absorbed) if it were the counterpart (Grindlay 1993) No obvious soft X-ray counterpart was found in the SIGMA error circle (5 arcmin, 1σ) with ROSAT (e.g. Leach & McHardy (1996)), an Einstein source (1E 1227+0224) in the SIGMA error circle (1σ) was identified with a QSO at z=0.57 that would have to be of “type 2” (heavily self-absorbed) if it were the counterpart (Grindlay 1993)
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