Swift observations of BL Lacertae

ATel #3377; K. V. Sokolovsky (MPIfR/ASC Lebedev/SAI MSU), F. D'Ammando (INAF-IASF Palermo and CIFS), S. Cutini (ASDC) on behalf of the Fermi-LAT Collaboration, and J. M. Gelbord (PSU)
on 24 May 2011; 20:44 UT
Credential Certification: Filippo D'Ammando (filippo.dammando@iasf-roma.inaf.it)

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Following the increase in gamma-ray, optical and near-infrared activity of BL Lacertae reported in ATel #3368, #3371, and #3375 we have initiated Swift monitoring observations of the source with 1ks-long pointings planned once every 3 days until 2011 June 04. The first observation of the series was conducted on May 23 16:55 UT and is described here.

The 0.3-10 keV X-ray spectrum of BL Lacertae observed by Swift/XRT can be described by an absorbed (N_HI = 3.6x10^21 cm^-2, Ravasio et al. 2003, A&A, 408, 479) power law model with the photon index of 1.59 +/-0.16 and a corresponding 2-10 keV (unabsorbed) flux of (1.74+/-0.27)x10^-11 erg cm^-2 s^-1. This spectrum is harder than that found in any previous Swift/XRT observation (Falcone & Stroh, the Swift Monitoring Program; see http://www.swift.psu.edu/monitoring/ ). The X-ray flux level is a factor of 2-3 higher with respect to the Swift/XRT and RXTE/PCA observations during the low activity state of the source in August - October 2008 (Abdo et al. 2011, ApJ, 730, 101). The current 2-10 keV flux is 1.5 times lower than the historical maximum observed for BL Lacertae in July 1997 by RXTE (Madejski et al. 1999, ApJ, 521, 145).

The source was observed at V=14.15 +/-0.01 during simultaneous Swift/UVOT observations on JD(TT)2455705.2084. The magnitude calibration was performed using comparison stars from Gonzalez-Perez et al. 2001, AJ, 122, 2055. Recent observations reported in vsnet-alert 13337 and the American Association of Variable Star Observers (AAVSO) database (see http://www.aavso.org/lcg ) show a gradual brightness increase during the last two months and the UVOT data point appears to be a continuation of that general trend. However, the current V-band brightness is about 1 mag. lower than the peak values observed in 1997-2004. This may be the ending of the fast flare reported in ATel #3371 or an indication of fast irregular variability.

We thank the Swift team for their rapid scheduling of this observation. Results of the upcoming Swift observations of the source will be posted on the following webpage:

Swift monitoring of BL Lacertae in May 2011