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Swift follow-up of the gamma-ray flaring blazar PKS 0521-36

ATEL # 2689; F. D'Ammando (INAF - IASF Palermo), K. V. Sokolovsky (MPIfR/ASC Lebedev), G. Iafrate (INAF - OATs, Trieste) on behalf of the Fermi Large Telescope Collaboration and M. Stark (PSU)
on 22 Jun 2010; 3:33 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Filippo D'Ammando (

Subjects: Optical, Ultra-Violet, X-ray, Gamma Ray, >GeV, Request for Observations, AGN, Quasars

Following the gamma-ray flare of PKS 0521-36 detected by Fermi LAT on June 17, 2010 (ATel #2683, Iafrate et al. 2010), we performed a Swift target of opportunity observation on June 19.

Swift/XRT data were taken in Photon Counting mode for a total exposure of 2.8 ksec. The X-ray spectrum (0.3-10 keV) can be fit by an absorbed power law model with a HI column density consistent with the Galactic value in the direction of the source (n_H = 3.58 x10^20 cm^-2, Kalberla et al. 2005) and a photon index of 1.66+/-0.06. The corresponding unabsorbed 0.3-10 keV flux is (2.62+/-0.14) x10^-11 erg cm^-2 s^-1. The flux observed is more than a factor of 2 higher than that previously observed by XMM-Newton on October 2002 (1.2 x10^-11 erg cm^-2 s^-1 with Gamma = 1.847+/-0.001, Foschini et al. 2006, A&A, 453, 829) with also a hardening of the X-ray spectrum.

Simultaneous Swift/UVOT observations found PKS 0521-36 about 0.5 mag. brighter in V-band (V = 14.58+/-0.02) and 1.2 mag. brighter in W2-band (W2 = 14.56+/-0.01) compared to the UVOT observations performed on 8 and 13 February 2008. The host galaxy is clearly visible in the V-band image.

An intermediate level of activity was observed by Swift on 13 March 2010 (unabsorbed 0.3-10 keV flux of 1.95+/-0.15 x10^-11 erg cm^-2 s^-1, V = 14.90+/-0.03, W2 = 15.54+/-0.02) giving an indication that the current flare could be the culmination of an active period of PKS 0521-36 over the last months, as indicated also by the gamma-ray outburst detected by Fermi LAT on January 29, 2010.

In consideration of the ongoing activity of this source, confirmed also by the Swift observations, we strongly encourage further multiwavelength observations. For this source the Fermi LAT contact person is F. D'Ammando (

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

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