SGR 0501+4516: Comparison with ROSAT archival data
ATel #1675; J. A. Kennea (PSU) and V. Mangano (INAF-IASF PA)
on 22 Aug 2008; 20:32 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Gamma Ray, Gamma-Ray Burst, Transient
Referred to by ATel #: 1683
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.
