Fermi/LAT observations of Cygnus X-1
ATel #7327; P. H. Thomas Tam (Sun Yat-Sen University), Y.-J. Yang (National Tsing Hua University)
on 31 Mar 2015; 14:40 UT
Distributed as an Instant Email Notice Transients
Credential Certification: P.H.Thomas Tam (grbtom@gmail.com)
Subjects: Gamma Ray, >GeV, Binary, Black Hole
We report the Fermi/LAT observations of the high mass X-ray binary Cygnus X-1, following a recent, possible transition from a soft state to a hard state (e.g., ATels #7316 and #7322).
We marginally detected 0.1-300 MeV gamma-ray emission from Cygnus X-1, using data from 2015-03-06 UT 00:00:00 to 2015-03-30 UT 19:04:40, roughly corresponding to the recent high radio flux state as reported in ATel #7316. A maximum likelihood analysis gives a TS value of 12 (corresponding to detection significance of 3.5 sigma), a spectral index of ~0.9, and a photon flux of 1.1e-9 cm^-2 s^-1.
We also report the 0.1-300 MeV observations over the previous X-ray outburst period. An analysis using data from 2014-12-09 to 2015-01-28 returns a TS value of 16 (corresponding to detection significance of 4 sigma), a spectral index of ~3.4, and a photon flux of 1.5e-7 cm^-2 s^-1.
Significant gamma-ray emission up to 2 MeV from Cygnus X-1 was published and 400 keV to 2 MeV emission was shown to be polarized (Laurent, P., et al., 2011, Science, 332, 438). Evidence of >100 MeV gamma-ray emission from Cygnus X-1 during the hard state have been reported before (Del Monte, E., et al., 2010, A&A, 520, 67; Bodaghee, A., et al., 2013, ApJ, 775, 98; Malyshev, D., et al., 2013, MNRAS, 434, 2380).