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ATEL # 1675; J. A. Kennea (PSU) and V. Mangano (INAF-IASF PA)
on 22 Aug 2008; 20:32 UT
Password Certification: Jamie A. Kennea (firstname.lastname@example.org)
Subjects: X-ray, Gamma Ray, Gamma-Ray Bursts, Transients
We have analyzed an archival ROSAT observation of the newly discovered soft Gamma-ray repeater SGR 0501+4516 (GCN #8112, #8113, #8114, #8115). The ROSAT PSPC observed the a field containing SGR 0501+4516 between September 21st and 24th, 1992 for a total exposure time of 4.2 ks. An off-axis point source, 2RXP J050107.7+451637, was clearly detected in the observation which is coincident within errors the XRT position of the SGR.
We have extracted the 0.2-2.4 keV spectrum of 2RXP J050107.7+451637, and compared it with the Swift/XRT spectrum from the first orbit of observations after the initial BAT trigger on SGR 0501+4516. Performing a simultaneous fit to both spectra, utilizing an absorbed power-law model and allowing the normalization to be independent for the two spectra, we find that both spectra are well fit (reduced chi2 = 0.9 for 47 degrees of freedom) by the following model parameters:
N_H = (0.65 +/- 0.18) x 1022 cm-2
Photon Index = 1.7 +/- 0.3
The flux, uncorrected for absorption in the ROSAT band (0.2-2.4 keV) is 1.6 x 10-12 erg/s/cm2 for the ROSAT source, and 2.8 x 10-11 erg/s/cm2 for the XRT source. Assuming that these two sources are indeed the same object, the XRT detection is approximately a factor of 6 brighter than in the ROSAT detection. The ROSAT light curve does not show any obvious signs of variability.
We therefore suggest on the basis of the positional coincidence and spectral similarity that 2RXP J050107.7+451637 is emission from SGR 0501+4516 in quiescence. We note also that a ROSAT All-Sky Survey detected source, 1RXS J050107.9+451631, is positionally coincident and has consistent flux with 2RXP J050107.7+451637, so is likely the same source.