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Fermi LAT detection of increasing gamma-ray activity from the microquasar Cygnus X-3 associated with a giant radio flare.

ATel #3233; S. Corbel (University Paris Diderot & CEA Saclay, F), G. Pivato (University di Padova, It), R. Corbet (UMBC/NASA GSFC, USA), J. L. Richards, W. Max-Moerbeck, A. C.S. Readhead (OVRO/Caltech, USA) on behalf of the Fermi Large Area Telescope Collaboration, S. A. Trushkin, N. Nizhelskij (SAO RAS, Russia), G. Pooley (Cavendish Lab., Cambridge, UK), J. Miller-Jones (Curtin University, Aus).
on 24 Mar 2011; 07:46 UT
Credential Certification: S. CORBEL (corbel@discovery.saclay.cea.fr)

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

Referred to by ATel #: 3239, 3325, 3389

Cygnus X-3 has been in the soft X-ray state since early 2011 (e.g. ATel # 3139). However, analysis of the Swift/BAT light-curve indicates a recent increase in the level of hard X-ray emission from Cygnus X-3, possibly indicating a transition back to the hard state.

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 a daily average flux (E >100 MeV) of (3.00 +/- 0.60) x 10^-6 ph cm^-2 s^-1 for the date of 2011 March 22 (MJD 55642). The LAT analysis uses the off-pulse phase intervals of the nearby pulsar PSR J2032+4127.

In addition, we also report the results of radio observations conducted with the OVRO 40-m Telescope. Cygnus X-3 was observed at a very high 15 GHz flux density of (11.9 +/- 0.4) Jy at 11:51 UTC on 2011 March 23 (MJD 55643.49377) according to a preliminary analysis. This represents a brightening at 15 GHz by a factor of 90 compared to a recent measurement of (0.13 +/- 0.01) Jy on 2011 March 17 (MJD 55638.51). A similar increase is also observed with the RATAN-600 radio telescope, that measured on MJD 55643.24 flux densities of : (5.92 +/- 0.12) Jy at 4.8 GHz, (8.21 +/- 0.25) Jy at 7.7 GHz, (8.98 +/- 0.18) Jy at 11.2 GHz, (6.82 +/- 0.27) Jy at 21.7 GHz, (5.20 +/- 0.26) Jy at 30 GHz. RATAN measured flux densities of (60 +/- 5) mJy on MJD 55640.25 in the 4.8-11.2 GHz range. Furthermore, the AMI Large Array in Cambridge (UK) observations indicated also a faint level of (27 +/- 3) mJy at 15.7 GHz on MJD 55641.583.

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 Stephane Corbel (stephane.corbel@cea.fr). Further radio monitoring is ongoing, and multiwavelength observations are encouraged.

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.