Swift XRT/UVOT follow-up of Fermi J1717-5156 after a gamma-ray flare

ATel #4045; C. C. Cheung (NRC/NRL), D. Donato (NASA GSFC), F. K. Schinzel (UNM), on behalf of the Fermi Large Area Telescope Collaboration
on 16 Apr 2012; 02:09 UT
Credential Certification: Teddy Cheung (ccheung@milkyway.gsfc.nasa.gov)

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Following the Fermi-LAT detection of a flare on April 7, 2012 from Fermi J1717-5156, a new gamma-ray source near the Galactic Plane (ATel #4023), we obtained Swift observations on April 9, 11, and 13. These observations confirm that the radio source found within the LAT error circle, PMN J1717-5155, is also in a high X-ray and optical state.

A new X-ray source consistent with the radio position of PMN J1717-5155 is well detected in each XRT observation with 10, 4.6, 2.7 ks exposures, respectively. The data at the three epochs were best represented by a power law model with consistent slopes (photon index, Gamma = 1.8 +/- 0.3, 2.0 +/- 0.5, and 1.5 +/- 0.6) with some additional absorption (NH ~ 4e21 cm-2) over the Galactic value (NH = 1.51e21 cm-2; Kalberla et al. 2005, A&A, 440, 775). In the first observation, the unabsorbed source flux (0.3-10 keV), with a value of 4.9e-12 cm-2 s-1, was an order of magnitude larger than implied by its previous RASS non-detection (Voges et al. 1999, A&A, 349, 389). Its X-ray flux increased to 8.8e-12 erg cm-2 s-1 by the second observation and then decreased by the third (4.1e-12 erg cm-2 s-1).

The radio/X-ray source is well detected in the UVOT with the following magnitudes observed during the first epoch: V = 17.84 +/- 0.09, B = 18.50 +/- 0.07, U = 17.97 +/- 0.06, and UVW1 = 18.65 +/- 0.07. The source brightened by 0.4-0.6 mag at the second epoch and then faded by 0.2-0.3 mag in the third epoch, mirroring the X-ray behavior. The overall optical brightening with respect to historical measurements confirms the findings based on GROND measurements (ATel #4032). Taken together with the X-ray brightening and WISE infrared color (ATel #4029), the radio source PMN J1717-5155 is likely a new gamma-ray blazar.

We thank the Swift team for their rapid scheduling of these observations.