MAXI/GSC detection of a big flare from UX Ari

ATel #6315; A. Kawagoe, Y. Tsuboi (Chuo U.), H. Negoro (Nihon U.), N. Kawai (Tokyo Tech), T. Sakamoto (AGU), S. Nakahira (JAXA), M. Serino (RIKEN) , S. Ueno, H. Tomida, M. Kimura, Y. E. Nakagawa (JAXA), T. Mihara, M. Sugizaki, M. Morii , J. Sugimoto, T. Takagi, A. Yoshikawa, M. Matsuoka (RIKEN), R. Usui, T. Yoshii, Y. Tachibana (Tokyo Tech), A. Yoshida, Y. Nakano, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, M. Sasaki (Osaka U.), M. Nakajima, K. Fukushima, T. Onodera, K. Suzuki (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro, T. Hori (Kyoto U.), M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada, Y. Morooka (Miyazaki U.), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team
on 16 Jul 2014; 10:09 UT
Credential Certification: Yohko Tsuboi (tsuboi@phys.chuo-u.ac.jp)

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At 20:05:22 UT on July 15 2014, the MAXI/GSC nova alert system triggered on a bright source at a position consistent with that of an RS CVn star UX Ari. The flux increased further and peaked at the next scan transit at 21:38:03 UT on July 15 with a value of 283^+26.8_-35.0 mCrab (90% error) in the 4-10 keV band. At the scan following the peak (23:08:11 UT), the source started to fade. This is the largest flare from UX Ari among those detected by the MAXI/GSC. In the past, the MAXI/GSC detected a flare with a flux of 130⁺_-30 mCrab in April 2011 (Matsumura et al. 2011, ATEL#3308), and three more with comparable fluxes in 2011 and 2012, which were the highest flux records before this detection. It is also larger by an order of magnitude than the long and bright X-ray flares observed during pointed observations with Ginga in 1987 (Tsuru et al. 1989, PASJ 31, 679) and BeppoSAX in 1997 (Franciosini et al. 2001, A&A 375, 196), both of which exhibited X-ray fluxes of about 30 mCrab.

The position was determined from the data at the flare peak, as (R.A., Dec) = (51.505 deg, 28.600 deg) = (03 26 06, +28 35 09) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.14 deg and 0.09 deg respectively. The roll angle of long axis from the north direction is 0 deg. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). This position is consistent with that of an RS CVn star UX Ari. No other source is listed in the ROSAT Bright Source Catalog in this error region.

Trigger by the Swift BAT at 21:14:11 UT (59 minutes after the first MAXI/GSC trigger, Baumgarther et al. GCN Circ. 16594) from UX Ari confirms the identification. Also, the reported flux by the Swift XRT is roughly consistent with our measurement 26 minutes later.