MAXI observation of a bright X-ray flare with multiple peaks from Mrk 421

ATel #4978; H. Negoro, K. Suzuki (Nihon U.), N. Kawai (Tokyo Tech), M. Serino (RIKEN), S. Ueno, H. Tomida, S. Nakahira, M. Kimura, M. Ishikawa (JAXA), T. Mihara, M. Sugizaki, M. Morii, T. Yamamoto, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Nakano (AGU), H. Tsunemi, M. Sasaki (Osaka U.), M. Nakajima, K. Fukushima, T. Onodera, (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.), M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada (Miyazaki U.), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team
on 15 Apr 2013; 02:32 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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As reported from observations by NuSTAR (Balokovic et al. ATel #4974), MAGIC and VERITAS (Cortina et al. ATel #4976), and Fermi-LAT and Swift-XRT (Paneque et al. ATel #4977), Mrk 421 recently exhibited an extremely bright flaring in X-rays to the very high energy gamma-rays. MAXI-GSC has also detected this bright flare (MAXI transient email alert ID = 6394252311), and the 1-day averaged 2-10 keV flux observed mostly doubled from ~37 mCrab on 2013 April 11 (UT) to ~85 mCrab on April 12 and ~75 mCrab on April 13. (Note that MAXI-GSC observation temporally stopped from 01:47 to 10:57 on April 13 due to the system reboot.) This flare clearly consisted of multiple peaks on timescales of 1-2 hours as usually observed in this source (also see ATel #4977), and the 2-10 keV flux often varied by more than a factor of 2 between the scan transits. The maximum peaks in the 2-10 keV band appeared during the scan transits, for instance, at 06:31 (136+-11 mCrab) and 12:42 (139+-11 mCrab) on April 12. These fluxes are consistent with the NuSTAR observation, >2.0e-9 erg/s/cm2 in the 2-10 keV at 2013 April 12 UT 09:35, equivalent to 100 mCrab (ATel #4974). The X-ray flux of the current flare is almost comparable to (but not as high as) that at the historically brightest X-ray flare observed by MAXI in February 2010, in which the maximum 6-hour averaged 2-10 keV flux was 164+-17 mCrab (Isobe et al. PASJ 62, L55, 2010), and the peak flux averaged over a single scan transit (~60 sec) reached 187+-18 mCrab in the 2-10 keV band. We also note that the hardness ratios (counts in the 4-10 keV band to those in the 2-4 keV band) were 0.73-0.90 at the flare in February 2010, while it is 0.46 on 2013 April 12-13, indicating that the current flare has a softer X-ray spectrum, despite of the detection of high-energy gamma-rays. Current fluxes of this source can be seen at http://maxi.riken.jp/top/index.php?cid=000000000001&jname=J1104+382 .

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