Fermi/LAT and Swift/XRT observations of XSS J12270-4859/2FGL J1227.7-4853
ATel #5652; P. H.T. Tam, A. K.H. Kong, K. L. Li (National Tsing Hua University, Taiwan)
on 11 Dec 2013; 19:42 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)
Subjects: X-ray, Gamma Ray, Binary, Neutron Star
Following an earlier report of decreased brightness in optical and X-rays, and possibly gamma-rays, of XSS J12270-4859 (ATel #5647), we provide further information based on Fermi/LAT and Swift/XRT observations of this gamma-ray emitting low-mass X-ray binary.
Maximum likelihood analyses were performed for a rectangular region of 21x21 square degrees around XSS J12270-4859, using all LAT data above 200 MeV obtained since 2008. The source model includes all sources in the second Fermi catalog (Nolan et al., 2012) and the diffuse Galactic and isotropic background. Spectral parameters of point sources further away than 10 degrees were held fixed while others were allowed to vary in the iterations. XSS J12270-4859 was modeled as a power law.
We found that the gamma-ray flux (above 200 MeV) from 2013 January 01 to 2013 December 10 is (2.1+/-0.3)e-8 ph cm^-2 s^-1, which is lower than the average flux (~3.4e-8 ph cm^-2 s^-1) from 2008 to 2012. In particular, the gamma-ray flux has decreased steadily from ~4e-8 ph cm^-2 s^-1 in mid-2012 to below 2e-8 ph cm^-2 s^-1 in late 2013.
We therefore conclude that the gamma-ray flux from XSS J12270-4859/2FGL J1227.7-4853 has recently decreased from its long-term average value, consistent with the first inspection in ATel #5647.
We note that a Swift/XRT observation obtained on 2013 January 26 resulted in a 0.3-10 keV count rate of <0.02 counts/s (3 sigma). This indicates that the X-ray flux decreased to the current level (0.015 counts/s on 2013 Dec 10) at around the same time as the optical state change (see ATel #5647).
