Swift follow-up of BL Lacertae after a bright TeV flare

ATel #3468; F. D'Ammando (INAF-IASF Palermo and CIFS), K. Sokolovsky (MPIfR/ASC Lebedev/SAI MSU), A. Falcone (Penn State Univ.), M. Stroh (Penn State Univ.)
on 1 Jul 2011; 22:11 UT
Credential Certification: Filippo D'Ammando (filippo.dammando@iasf-roma.inaf.it)

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Following the VHE flare of the blazar BL Lacertae detected by VERITAS on June 28, 2011 (ATel #3459), a Swift target of opportunity observation was performed on June 30, 2011.

Swift/XRT data were taken in Photon Counting mode for a total exposure of 2.5 ksec. The X-ray spectrum (0.3-10 keV) of BL Lacertae can be fit 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 2.35+/-0.16. The corresponding unabsorbed 2-10 keV flux is (6.65+/-0.73) x10^-12 erg cm^-2 s^-1. The flux observed is a factor of almost three lower with respect to that observed by XRT on 23 May 2011 (ATel #3377), soon after the GeV gamma-ray flare detected by Fermi-LAT (ATel #3368). A continuous decrease of the X-ray activity was observed since 29 May 2011 (see http://www.swift.psu.edu/monitoring/source.php?source=BLLacertae ).

The steep X-ray spectrum observed on 30 June indicates that the X-ray emission is likely to be representative of the tail of the synchrotron emission. This is a clue that the synchrotron emission peak shifted towards higher energies. Taking into account that the Swift observation is not simultaneous with the VERITAS and Fermi ones, it suggests a possible shift also of the inverse Compton emission peak during the VHE flare observed by VERITAS, in accordance with the hardening of the spectrum observed in the MeV-GeV domain by Fermi-LAT (ATel #3462).

Simultaneous Swift/UVOT observations on JD(TT) 2455742.86146 found BL Lacertae at V= 14.21 +/-0.03. The UV-optical flux level is close to the one reported in ATel #3377 after the onset of the current activity period and is average compared to variations seen in recent weeks (see http://scan.sai.msu.ru/~kirx/ATel/BL_Lac/ ). A clear ~0.15m decline is seen in all UVOT filters between the two Swift pointings on the source separated by 95 minutes (one orbital revolution) manifesting short-timescale variability.

Swift will continue monitoring BL Lacertae during the next weeks. We thank the Swift Team for their rapid scheduling of this observation.