Fermi-LAT and Swift detection of a large GeV and optical flare from J123939+044409

ATel #1888; A. Tramacere(CIFS Torino/SLAC Stanford), N. Rea(University of Amsterdam) on behalf of the Fermi Large Area Telescope Collaboration
on 4 Jan 2009; 01:22 UT
Credential Certification: andrea tramacere (tramacer@slac.stanford.edu)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope (formerly GLAST, launched on June 11, 2008), has observed a large flare from the GeV source J123939+044409 at: RA = 189.9 and Dec = 4.7 (error circle at 95% = 0.1 deg).

This position is coincident with the EGRET source 3EG J1236+0457, and a has probability of 98.3% to be associated with the CRATES source J1239+0443. 3EG J1236+0457 was previously associated with the object SDSS J123932.75+044305 (z~1.76; Mattox, Hartman & Reimer 2001, ApJS 135), which lies at 0.15 arcseconds from CRATES J1239+0443.

Preliminary analysis of the Fermi-LAT data indicates that in the period between December 27-31, 2008, the source flux rose up to a flux level about 40 times higher than its quiescent state observed by Fermi-LAT in the past months, when the source showed a flux (E>100 Mev) of 0.04+/-0.01 x 10^-6 ph cm^-2 s^-1. On December 29th, J123939+044409 reached a flux of 1.5+/-0.5 x10^-6 ph cm^-2 s^-1 with a spectral index of -2.3+/-0.3.

Swift observed the Fermi-LAT field on January 2, 2009, for 3 ks. Using these data, we detect only 1 X-ray source within the Fermi-LAT error circle, positionally coincident with the ROSAT counterpart to SDSS J123932.75+044305 (see http://www.slac.stanford.edu/~tramacer/ds9.jpg for an image of the Swift-XRT field with the error circles overplotted). The Swift-XRT flux in the 2-10 keV band was about 6.2 x 10^-13 erg cm^-2 s^-1, which is consistent with the ROSAT flux (after re-scaling for the different energy bands).

A preliminary analysis of the Swift-UVOT data showed a single bright source within the Swift-XRT error circle, possibly the optical counterpart to SDSS J123932.75+044305. We find a U magnitude of 15.98+/-0.04, while in the archival SDSS the source u magnitude was 20.60+/-0.02.

No X-ray enhancement has been observed after 3 days from the GeV peak of the flare, while a large optical outburst is on-going. Further analyses of the Fermi-LAT and Swift data are in progress to confirm these preliminary findings.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the violent outburst we strongly encourage multiwavelength observations to look for possible lags connected both to the radiative/acceleration processes and to the geometry of the object.

For this source the Fermi-LAT contact persons are Andrea Tramacere (tramacer@slac.stanford.edu) and Nanda Rea (N.Rea@uva.nl)

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.

We would like to thank the Swift Team for their support of these observations.