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Gamma-ray emission from Cygnus X-3 detected by Fermi/LAT at the onset of a major radio flare

ATel #10243; Alan Loh (LESIA, Paris Observatory) & Stephane Corbel (DAP-AIM, CEA, Paris Diderot University), on behalf of the Fermi Large Area Telescope Collaboration
on 4 Apr 2017; 12:51 UT
Credential Certification: Alan Loh (alan.loh.up7@gmail.com)

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

Referred to by ATel #: 10252

The high-mass X-ray binary Cygnus X-3 (Cyg X-3) remained in an ultra-soft state since the transition from the soft state associated with gamma-ray emission detected by both Fermi and AGILE satellites (ATel#10109 and #10138). During this period, minor radio flaring occurred with typical flux densities of a few hundred milliJanskys at 4.6 and 8.2 GHz as reported in ATel#10126. Thanks to Trushkin et al. (ATel#10126), the public RATAN radio light curve indicates that Cyg X-3 is currently undergoing a major radio flare. In conjunction with this event, the Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed significant gamma-ray emission originating from the microquasar.

Preliminary analysis of LAT data has revealed an increase of gamma-ray emission originating from Cyg X-3 with a flux (E > 100 MeV) of (1.4 +/- 0.4) x 10^-6 ph cm^-2 s^-1 (errors are statistical only) on MJD 57846.5 +/- 0.5 at a significance level of nearly 5 sigma. The gamma-ray detection matches closely in time the beginning of the major radio flare while the source has not already reached the expected ~10 Jy radio flux densities, in agreement with previous findings for this peculiar source (see for e.g., Corbel et al. 2012, MNRAS 421, 2947 and Piano et al. 2012, A&A 545, 110).

We therefore strongly encourage multi-wavelength observations of Cyg X-3 during the major radio flare including VLBI radio observations.

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@obspm.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.