Swift UV and X-ray follow-up of the gamma-ray flare of PKS 2155-304

ATel #6165; A. Stamerra (INAF OATO, Torino & SNS, Pisa), L. Pacciani (INAF IAPS, Rome) and S. Cutini (ASDC & INAF OAR, Rome), S. Ciprini (ASDC & INAF OAR, Rome), on behalf of Fermi LAT Collaboration.
on 23 May 2014; 17:16 UT
Credential Certification: Antonio Stamerra (antonio.stamerra@to.infn.it)

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Following the flare in the GeV band of the HBL blazar PKS 2155-304 detected by Fermi-LAT on 17 May 2014 (ATel#6148), we have requested a follow-up with Swift XRT and UVOT.

Preliminary analysis of Swift-XRT data collected on 21 May 2014 (OBSID:00030795126 in Window Timing (WT) mode, exposure time: 1480 s) shows the source in a high state in the X-ray band, with an average rate of 10 counts per second (cps), to be compared with the historical maximum of ~15 cps. The count rate of previous Swift-XRT observations in WT mode on April, 25th 2014 counted in average ~2 cps, http://www.swift.psu.edu/monitoring/source.php?source=PKS2155-304.

The measured flux in the 2-10 keV range, corrected for Galactic absorption nH=1.5e20 cm^2 (Kalberla, P. M. W. et al. 2005, A&A 440, 775), is (88.5+/-2.1)e-12 erg/cm2/s. The spectrum in the 0.3-10 keV range is described by a log-parabola with spectral slope alpha=2.29+/-0.03, curvature parameter beta=0.42+/-0.07, reduced chi2/NDF=1.06/199 whereas a simple power law function cannot describe the data (reduced chi2/NDF=1.74/200). Limiting the fit in the >1keV range, the spectrum can be described satisfactorily with a simple power law with spectral index 2.55+/-0.05 (reduced chi2/NDF=1.02/130).

This spectrum is comparable to the spectrum obtained by RXTE during the second major flare in July 2006 (fig. 2 in Abramoswki et al., 2012, A&A 539, 149).

The source clearly shows X-ray variability on a hour-timescale. There is an indication of hardening of the X-ray spectrum during the flare respect to X-ray spectra in low state (Foschini et al. 2008, A&A 484, L35), though the overall UV to X-ray behavior shows global increase in amplitude at all wavelengths without any major shift of the synchrotron peak, located ~10^(16-15) Hz.

A preliminary analysis of Swift-UVOT data gives following magnitudes, not de-reddened: V:13.25 B:13.55 U:12.59 UW1:12.43 UM2:12.34 UW2:12.45. The dereddened fluxes are about a factor 2 higher.

Previous observations showed a delayed high state at longer wavelengths after the occurrence of a TeV gamma-ray flare (Abramoswki et al., 2012, A&A 539, 149); therefore multiwavelength observations are strongly encouraged.

The Swift ToO is ongoing and further observations are planned.

We acknowledge the support of the Swift team and Swift Observatory Duty Scientist.