Fermi and Swift observations of a rapid flare in the BL Lac object SDSS J084411.67+531250.7 (NVSS J084411+531250)

ATel #7358; Stefano Ciprini (ASDC Rome & INFN Perugia, Italy), on behalf of the Fermi Large Area Telescope Collaboration.
on 9 Apr 2015; 10:28 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed increasing gamma-ray flux from a source positionally consistent with the optically-selected and polarized (Smith et al. 2007, ApJ, 663, 118) BL Lac object SDSS J084411.67+531250.7 (also known as NVSS J084411+531250, BZB J0844+5312, and 3FGL J0843.9+5311, Acero et al. 2015, arXiv:1501.02003), with optical counterpart coordinates (J2000.0) R.A.: 131.04872 deg, Dec.: 53.21407 deg (Adelman-McCarthy et. al 2008, ApJS, 175, 297).

Preliminary analysis indicates that SDSS J084411.67+531250.7 on April 6, 2015 was in a high state with a daily averaged gamma-ray flux (E>100MeV) of (0.3+/-0.1)X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only) corresponding to about 50 times the averaged flux reported in the third Fermi LAT source catalog (3FGL). In a 6 hour interval of the same day this BL Lac object reached a gamma-ray flux (E>100MeV) of (1.3+/-0.6)X 10^-6 photons cm^-2 s^-1. The gamma-ray photon spectral index (2.2+/-0.3) is in agreement with the value reported in the 3FGL catalog.

A single high-urgency follow-up target of opportunity Swift observation was made between April 7 and 8, 2015 for a total exposure of 4.0ks. XRT data were taken in Photon Counting mode. The X-ray spectrum (0.3-10 keV) can be fit by an absorbed power law model with an HI column density set to the Galactic value of 3.0X10^20 cm^-2 (Kalberla et al. 2005, A&A, 440, 775). The unabsorbed flux was (1.1+/-0.3)X10^-12 erg cm^-2 s^-1 and the photon index 1.4+/-0.4.

Simultaneous Swift/UVOT observations using all the filters are shown below.

The X-ray spectral energy distribution of SDSS J084411.67+531250.7 has brightened of a factor of about 5 with respect to archival observations of Swift XRT performed on March 11, 2010 and September 5, 2009, while is roughly comparable to the XMM-Newton observation performed on November 3, 2007 (Saxton et al. 2008, A&A, 480, 611). The optical-UV spectral energy distribution of the source has brightend by a factor of about 6 with respect to archival observations of Swift UVOT, with a possible hint of synchrotron peak frequency shift towards the UV. This correlated and simultaneous gamma-ray/X-ray/UV/optical brightening confirms the identification of 3FGL J0843.9+5311 (2FGL J0843.9+5312) with SDSS J084411.67+531250.7.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source, we encourage multiwavelength observations. For this source the Fermi LAT contact person is Stefano Ciprini (stefano.ciprini@asdc.asi.it).

We thank the Swift Team for making these observations possible, in particular D. Malesani as the Swift Observatory Duty Scientist.

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.