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Fermi LAT detection of increasing gamma-ray activity from the microquasar Cygnus X-3

ATEL # 2646; S. Corbel (University Paris Diderot & CEA Saclay), E. Hays (NASA/GSFC) on behalf of the Fermi Large Area Telescope Collaboration
on 27 May 2010; 21:00 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: S. CORBEL (

Subjects: Gamma Ray, >GeV, Request for Observations, Black Holes, Neutron Stars, Transients

Cygnus X-3 has recently been undergoing a transition from the hard state to the soft state. Whereas the transition already started late April (ATel #2609), the transition is still ongoing as indicated by the almost continuous decrease of hard X-ray emission of Cyg X-3 from Swift/BAT lightcurve or INTEGRAL analysis (ATel #2644, see also ATel #2635).

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 consistent with Cygnus X-3. Preliminary analysis of the Fermi-LAT data indicates that during the period from May 19 to May 26, 2010, the source was detected with an average flux (E >100 MeV) of 1.00 +/- 0.16 x 10^-6 ph cm^-2 s^-1, whereas it was not detected during the period from May 8 to May 18, 2010 (see also ATel #2609, #2611). The daily LAT analysis shows that the recent gamma-ray activity of Cygnus X-3 is greatest on May 25 and 26, during which the daily fluxes (E >100 MeV) were 2.2 +/- 0.6 x 10^-6 ph cm^-2 s^-1. This behaviour is consistent with the flaring behaviour observed by Fermi during the two recent soft states in 2008 and 2009 (Abdo et al. 2009, Science, 326, 1512). The LAT analysis uses the off-pulse phase intervals of the nearby pulsar PSR J2032+4127.

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 person is Stéphane Corbel (

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.

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R. E. Rutledge , Editor
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