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Fermi-LAT Observation of Increased Gamma-ray Emission from the Microquasar Cygnus X-3

ATel #8591; Alan Loh, Stephane Corbel (CEA, University Paris Diderot ), Guillaume Dubus (IPAG), on behalf of the Fermi Large Area Telescope Collaboration, Robin Corbet (UMBC/NASA GSFC)
on 26 Jan 2016; 14:20 UT
Credential Certification: S. CORBEL (stephane.corbel@cea.fr)

Subjects: Radio, X-ray, Gamma Ray, >GeV, Request for Observations, Black Hole, Transient

Referred to by ATel #: 8597, 10109

Recently, the hard X-ray emission from the high-mass X-ray binary Cygnus X-3 has drastically dropped since 2016 Jan 11 (MJD 57398, as observed by Swift/BAT http://swift.gsfc.nasa.gov/results/transients/CygX-3/, Krimm et al. 2013, ApJS 209, 14) indicating a possible transition to the soft state. Such transitions have previously been associated with high-energy gamma-ray emissions (see e.g. Fermi/LAT Collaboration et al. 2009, Science 326, 1512; ATel#2646 and Corbel et al. 2012, MNRAS 421, 2947). An epoch of recurrent increased gamma-ray activity was previously reported by AGILE, INTEGRAL and Fermi in the period 2010-2011 (ATel#2609, ATel#2644, ATel#2645, ATel#3141, ATel#3233, ATel#3386).

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed an increasing gamma-ray flux from a source positionally consistent with Cyg X-3. Preliminary analysis of the Fermi-LAT data indicates that, from MJD 57398 to 57412, the source was significantly detected with an average flux (E > 100 MeV) of (6.5 +/- 0.9) x 10^-7 photons cm^-2 s^-1 (errors are statistical only). The most significant gamma-ray detection occurred on MJD 57408 +/- 2 days with a flux (E > 100 MeV) of (9.6 +/- 1.9) x 10^-7 photons cm^-2 s^-1 and a fitted power-law photon index of 2.7 +/- 0.2, consistent with the values reported in Corbel et al. (2012). Gamma-ray flux modulation at a period of 4.80 +/- 0.02 hours is detected in agreement with the orbital period of the system. We strongly encourage multi-wavelength observations of Cygnus X-3 as it enters an active state.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. For this source the Fermi LAT contact persons are Alan Loh (alan.loh@cea.fr) and Stephane Corbel (stephane.corbel@cea.fr).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.