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Swift XRT and UVOT flares accompany brightest ever gamma-ray flare of CTA 102

ATel #9924; Roopesh Ojha (NASA/GSFC/UMBC), Bryce Carpenter (CUA/NASA/GSFC), and Filippo D'Ammando (DIFA - University of Bologna, INAF-IRA) on behalf of the Fermi-LAT Collaboration
on 1 Jan 2017; 18:19 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)

Subjects: Optical, Ultra-Violet, X-ray, Gamma Ray, >GeV, AGN, Black Hole, Blazar, Quasar

Referred to by ATel #: 10007

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, observed the highest-ever level of gamma-ray activity from a source positionally consistent with the flat spectrum radio quasar CTA 102 (also known as 4C +11.69, PKS 2230+11, 3FGL J2232.5+1143). This source has coordinates, (J2000.0), R.A.: 22h32m36.4089s, Dec.: +11d43m50.904s (Johnston et al. 1995, AJ, 110, 880) and redshift z=1.037 (Schmidt 1965, ApJ, 141, 1295).

Preliminary analysis indicates that on 2016 December 28, CTA 102 had a daily averaged gamma-ray flux (E>100 MeV) of (21.7+/-0.7) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), about 135 times greater than its four-year average flux reported in the third Fermi-LAT source catalog (3FGL, Acero et al. 2015, ApJS, 218, 23) making this source the brightest in the whole gamma-ray sky. The corresponding daily averaged photon spectral index (E>100 MeV) of 1.9+/-0.1 (statistical uncertainty only) is harder than the 3FGL catalog value of 2.34+/-0.03. This is almost twice the daily averaged flux reported during the recent peak on 2016 December 14 (ATel#9869). Other past Fermi LAT Collaboration announcements of gamma-ray activity in CTA 102 are ATel#3320, ATel#4409, ATel#6631, ATel#8478, ATel#8722. Enhanced gamma-ray activity has also been reported by AGILE (ATel#9743, ATel#9788, ATel#9840, ATel#9911) and DAMPE (ATel#9901). Recent enhanced activity in radio (ATel#9884), optical/NIR (ATel#9821, ATel#9868), and X-ray (ATel#9841) have also been reported.

Following this gamma-ray flare, a Swift TOO observation was performed on 2016 December 29, and the results compared to a Swift observation on 2016 December 26 before the current unprecedented gamma-ray flare. The total exposure was about 1.8 and 1.7 ks respectively. The X-ray spectrum (0.5-10 keV) can be fit by an absorbed power law model with an HI column density set to the Galactic value of 5.35 x 10^20 cm^-2 (Kalberla et al. 2005, A&A, 440, 775) using the abundances of Wilms et al. (2000, ApJ, 542, 914) and the cross sections of Verner et al. (1996, ApJ, 465, 487). The observed fluxes are (4.2+/-0.3) x 10^-11 and (3.4+/-0.3) x 10^-11 erg cm^-2 s^-1; while the photon indexes are 1.4+/-0.1 and 1.3+/-0.1, respectively. This represents an increase in the flux after the latest gamma-ray flare from an already high state before it.

Simultaneous Swift/UVOT observations are shown below. The source is clearly much brighter after the flare:

Filter 2016-12-26 (mag) 2016-12-29 (mag)
V 12.71+/-0.02+/-0.01 < 11.46
B 13.20+/-0.04+/-0.02 < 12.68
U 12.49+/-0.02+/-0.02 < 11.91
W1 12.71+/-0.02+/-0.03 11.33+/-0.05+/-0.03
M2 12.98+/-0.02+/-0.03 11.51+/-0.02+/-0.03
W2 13.16+/-0.02+/-0.03 11.71+/-0.02+/-0.03

Fermi is currently in a Target of Opportunity mode with enhanced exposure to CTA 102. The TOO will continue through 2017 January 03. Preliminary, uncalibrated estimation of the daily gamma-ray flux observed by Fermi LAT is publicly available at http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/CTA_102. For this source the Fermi LAT contact person is S. Cutini (sara.cutini at 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.