THE GREAT MARKARIAN 421 FLARE OF FEBRUARY 2010: MULTIWAVELENGTH VARIABILITY AND CORRELATION STUDIES

VERITAS COLLABORATIONS; MAGIC COLLABORATION ET AL.; A.C. ROVERO

ASTROPHYSICAL JOURNAL

IOP PUBLISHING LTD

Lugar: Londres; Año: 2020 vol. 890 p. 1 - 47

0004-637X

We report on variability and correlation studies using multiwavelength observationsof the blazar Mrk421 during the month of February, 2010 when an extraordinaryflare reaching a level of ∼27 Crab Units above 1 TeV was measured in very-high-energy (VHE) γ-rays with the VERITAS observatory. This is the highest flux statefor Mrk421 ever observed in VHE γ-rays. Data are analyzed from a coordinatedcampaign across multiple instruments including VHE γ-ray (VERITAS, MAGIC),high-energy (HE) γ-ray (Fermi-LAT), X-ray (Swift, RXTE, MAXI), optical (includ-ing the GASP-WEBT collaboration and polarization data) and radio (Metsa ̈hovi,OVRO, UMRAO). Light curves are produced spanning multiple days before and af-ter the peak of the VHE flare, including over several flare ?decline? epochs. The mainflare statistics allow 2-minute time bins to be constructed in both the VHE and opti-cal bands enabling a cross-correlation analysis that shows evidence for an optical lagof ∼25?55 minutes, the first time-lagged correlation between these bands reported onsuch short timescales. Limits on the Doppler factor (δ 􏰃 33) and the size of the emis-sion region (δ−1RB <~3.8 × 1013 cm) are obtained from the fast variability observedby VERITAS during the main flare. Analysis of 10-minute-binned VHE and X-raydata over the decline epochs shows an extraordinary range of behavior in the flux-fluxrelationship: from linear to quadratic to lack of correlation to anti-correlation. Takentogether, these detailed observations of an unprecedented flare seen in Mrk 421 aredifficult to explain by the classic single-zone synchrotron self-Compton model.