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Fermi-LAT and Swift-XRT observe exceptionally high activity from the nearby TeV blazar Mrk421

ATel #4977; D. Paneque (MPI for Physics, Munich), F. D'Ammando (INAF-IRA Bologna), M. Orienti (INAF-IRA Bologna) on behalf of the Fermi LAT Collaboration, and A. Falcone (PSU) on behalf of the Swift team
on 13 Apr 2013; 20:30 UT
Credential Certification: David Paneque (dpaneque@slac.stanford.edu)

Subjects: X-ray, Gamma Ray, >GeV, TeV, VHE, Blazar, Transient

Referred to by ATel #: 4978, 4982, 4983, 5021, 5107

The high-synchrotron-peaked BL Lac Mrk421 (also known as 2FGL J1104.4+3812, Nolan et al. 2012, ApJS, 199, 31; R.A.= 11h04m27.3139s, Dec.= +38d12m31.799s, J2000.0, Fey et al. 2004, AJ, 127, 3587), at redshift z=0.03, is the subject of an extensive multi-year and multi-instrument program that aims at characterizing with exquisite detail the temporal evolution of the blazar emission across the electromagnetic spectrum. The multiwavelength campaign for the 2012/2013 season started in December 2012 and will continue until June 2013, involving the participation of VLBA, Swift , NuSTAR , Fermi, MAGIC, VERITAS, F-GAMMA, GASP-WEBT, and other collaborations and instruments, which are providing the most detailed temporal and energy coverage on this source to date. Further details on this program can be found at


During the last few weeks, the Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed increasing gamma-ray activity from Mrk421. Preliminary analysis for the period 2012 April 9-12 shows a daily flux (E > 100 MeV) between (0.5+/-0.1)x10^-6 ph cm^-2 s^-1 and (0.8+/-0.2)x10^-6 ph cm^-2 s^-1 (errors are statistical only), which is a factor of 3-4 greater than the average flux reported in the second Fermi-LAT catalog (2FGL, Nolan et al. 2012, ApJS, 199, 31).

During the last few days there has been a large increase in the X-ray and TeV gamma-ray activity, as reported in ATels #4974 and #4976. Moreover, the Swift-XRT instrument measured an X-ray count rate of 220 ct/s at ~22:00 UTC on MJD 56394 (April 12th), which decreased down to 130 ct/s at ~23:00 UTC. This is the highest X-ray flux measured from Mrk421 to date, and shows (additionally) extreme variability.

Due to this exceptionally high X-ray and TeV activity, the multi-instrument observations on Mrk421 have been intensified and, in response to a Target of Opportunity request, Fermi has interrupted all-sky scanning mode starting at 23:00 UT on April 12 and transitioned to a modified survey mode where Fermi stays rocked 50 deg north. The modified Fermi survey mode will continue until at least April 15th at 18:00 UTC, providing an increase in the exposure on Mrk421 by about a factor of 2.

Further multiwavelength observations are encouraged to better characterize this unprecedented broadband activity of Mrk421. The Fermi-LAT contact person for this campaign is David Paneque (dpaneque@mppmu.mpg.de).

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.