A major 15 GHz radio flare in the blazar Mrk 421

ATel #4451; Talvikki Hovatta (Caltech), Joseph L. Richards (Purdue), Margo F. Aller (U. Michigan), Hugh D. Aller (U. Michigan), Walter Max-Moerbeck (Caltech), Timothy J. Pearson (Caltech), Anthony C. S. Readhead (Caltech)
on 3 Oct 2012; 20:02 UT
Credential Certification: Talvikki Hovatta (thovatta@caltech.edu)

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We have observed a major 15 GHz radio flare in Mrk 421, a high-synchrotron-peaked (HSP) BL Lac object. A major gamma-ray flare in this source was detected on 16 July 2012. This was reported by the Fermi-LAT collaboration (ATel #4261), with the source reaching its highest flux (E>100 MeV) observed during the Fermi mission. A simultaneous TeV detection was later reported by ARGO-YBJ (ATel #4272).

Since 2008, Mrk 421 has been observed approximately twice per week at 15 GHz with the Owens Valley Radio Observatory (OVRO) 40m Telescope as part of our gamma-ray blazar monitoring program (Richards et al. 2011, ApJS, 194, 29).

On 21 September 2012, Mrk 421 reached a flux density of (1.11 +/- 0.03) Jy, approximately 2.5 times its previous median flux density and 1.5 times its previous maximum observed flux density at OVRO. The flare profile during the period of rapid brightening is well fit by an exponential curve with a doubling time of 9 days. The OVRO light curve is available on the program website (log in with username 'guest' and no password).

Comparison to University of Michigan Radio Astronomy Observatory (UMRAO) 14.5 GHz long-term monitoring since 1980 shows that this is the highest flux density ever observed in this source at 15 GHz during the over 30 year period. Rapid, high-intensity radio flares have not previously been observed in this source.

The flux density on 27 September 2012 was 0.99 +/- 0.03 Jy indicating that the flare may have reached its peak. OVRO will continue to observe the source within the regular monitoring program. We strongly encourage multiwavelength observations of this source during this unusual event.