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Fermi LAT Observations of Cyg X-3

ATEL # 2611; S. Corbel (Univ. Paris Diderot & CEA Saclay) on behalf of the Fermi Large Area Telescope Collaboration
on 11 May 2010; 21:25 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: S. CORBEL (

Subjects: Gamma Ray, >GeV, Transients
Referred to by ATEL #: 2644, 2646

Following the possible detection of Cyg X-3 by AGILE (ATel #2609) that occurred from May 7 to 9, we report on the analysis of the >100MeV emission from this region with the Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope.

Preliminary LAT analysis indicates that the gamma-ray emission (E >100 MeV) observed during the same time interval as reported by AGILE (2010 May 7-9; ATel #2609) at the location of Cyg X-3 is (0.89 +/- 0.33) x 10^-6 ph cm^-2 s^-1. While this is consistent with the flux reported from the region by AGILE ( > 1.0 x 10^-6 ph cm^-2 s^-1), it is also consistent with the baseline flux (E >100 MeV) of (0.53 x 10^-6 ph cm^-2 s^-1, Abdo et al. 2009, Science, 326, 1512) that has been measured previously at this location when Cygnus X-3 is not active in gamma rays. Excess emission from Cyg X-3, which would be indicative of a new flare, is not significantly detected by the LAT with a flux (E >100 MeV) contribution of the order of (0.36 +/- 0.33) x 10^-6 ph cm^-2 s^-1 . In addition, preliminary LAT analysis of daily (>100 MeV) fluxes from 2010 May 1st to 10th indicates no sign of a flare onset thus far.

Note that Fermi-LAT and/or AGILE observed flaring gamma-ray emission from Cyg X-3 during the previous three soft X-ray states. The RXTE/ASM and Swift/BAT recent data may indicate that a transition to a new soft X-ray state may be currently occurring.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing X-ray activity of this source we will keep searching in the next days for any gamma-ray activity. 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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