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
Credential Certification: Lara Sidoli (sidoli@mi.iasf.cnr.it)
Subjects: Optical, X-ray, Gamma Ray, Binary, Gamma-Ray Burst, Transient
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.
