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Swift uncovers that SAX J0840.7+2248 is not a X-ray Binary, but BeppoSAX X-ray Rich GRB980429

ATEL # 1089; L. Sidoli (INAF-IASF Milano), P. Romano (Univ Bicocca&INAF-OABrera), A. Cucchiara (PSU), S. Mereghetti (INAF-IASF Milano), N. Masetti (INAF-IASF Bologna), C. Guidorzi (Univ Bicocca&INAF-OABrera), E. Montanari (Univ Ferrara& IIS "Calvi"), F. Frontera (Univ Ferrara & INAF-IASF Bologna), L. Amati (INAF-IASF Bologna), F. Rossi (Univ Ferrara)
on 31 May 2007; 13:03 UT
Password Certification: Lara Sidoli (sidoli@mi.iasf.cnr.it)

Subjects: Optical, X-ray, Gamma Ray, Binaries, Gamma-Ray Bursts, Transients

The transient source SAX J0840.7+2248 has recently been observed by Swift/XRT and UVOT for a total of 10.6ks on May 29 00:35:49 to 23:21:57 UT. The Swift/XRT data show no X-ray counterpart within the BeppoSAX/WFC position error circle of 3 arcmin radius centered on RA,Decl(J2000)=08h 40m 40s, +22d 48m 18s (Heise et al., 1998, IAUC 6892). A 3sigma upper limit on a source within the SAX error box can be placed at 4.5E-14 erg cm-2 s-1 (for a spectrum of photon index 2, no intrinsic absorption, and a Galactic Hydrogen column of NH=(3.45)10^+20 cm-2). Assuming a distance of 8 kpc (1 kpc), we obtain a 3sigma upper limit on the luminosity of 3x10^32 (5x10^30) erg s-1. These values are quite low for a Galactic X-ray Binary.

Prompted by this finding, we performed a reanalysis of the BeppoSAX/GRBM data on this transient. We discovered that the X-ray Fast Transient (IAUC 6892) is an X-ray rich gamma-ray burst, ~100s long, with an onset time of 1998-04-29 08:46 UT. The GRBM spectrum was fit with a simple power law. We find a total 40-700 keV fluence of 1.7 (-0.4, +0.5) x10^-6 erg cm-2 and a photon index of 3.4 (-0.6,+0.7). The peak flux evaluated over 16s (40-700 keV) is (2.7 +/- 0.9)x 10^-8 erg cm-2 s-1 with a photon index of 2.5+/-0.8. All quoted errors are at the 90% level.

We would like to thank the Swift Team for performing these observations.


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