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Swift/XRT follow-up observation confirms the flaring activity of Mrk 421

ATEL # 2295; F. D'Ammando (INAF-IASF Roma, Univ. Tor Vergata), M. Perri (ASI Science Data Center), A. Tramacere (ISDC), N. Gehrels (NASA/GSFC), E. Hoversten (PSU), H. A. Krimm (CRESST/GSFC/USRA)
on 13 Nov 2009; 20:25 UT
Password Certification: Hans A. Krimm (

Subjects: X-ray, AGN

Motivated by recent flaring activity detected on November 10, 2009 by the Burst Alert Telescope (BAT) hard X-ray transient monitoring program (Krimm et al. 2009, ATel #2292), we have performed a Swift target of opportunity observation of Mrk 421.

We report on the analysis of the Swift/XRT data obtained on November 12. The spectrum of the Swift/XRT observation can be fitted (0.3-10 keV) with a log-parabolic spectral law absorbed by a column density consistent with the Galactic one in the direction of the source (n_H = 1.61e20 cm-2, Lockman & Savage 1995), a photon index of 1.59+/-0.01 and a spectral curvature of 0.28+/-0.01. The corresponding unabsorbed 2-10 keV (0.3-10 keV) flux is 2.0e-09 (3.5e-09) erg cm-2 s-1. This flux is only slightly lower than the maximum flux ever observed from Mrk 421 in soft X-ray by Swift/XRT on June 2008 (Donnarumma et al. 2009, ApJ, 691, L13) and higher than previous observations in flaring state (Fossati et al. 2008, ApJ, 677, 906; Lichti et al. 2008, A&A, 486, 721). The very high level of flux observed by XRT confirms the flaring activity previously observed in hard X-rays by BAT.

Swift will continue monitoring Mrk 421 over the next two weeks with five more observations. Further observations at all wavelengths are encouraged. We thank the Swift Team for making this observation possible.

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