MAXI and NICER observations of a long flare from UX Ari

ATel #12248; W. Iwakiri, R. Sasaki (Chuo U.), K. Gendreau, Z. Arzoumanian, M. F. Corcoran, K. Hamaguchi (NASA/GSFC), Y. Tsuboi, H. Kawai, T. Sato (Chuo U.), T. Mihara, S. Nakahira (RIKEN), M. Serino (AGU), H. Negoro (Nihon U.), M. Shidatsu (Ehime U.), T. Enoto (Kyoto U.), Y. Yonekura (Ibraki U.), K. Fujisawa, K. Niinuma, T. Aoki, Y. Furuya, K. Shiotani, Y. Kojima, S. Sukehiro, Y. Imaizumi, T. Ogura, A. Hirano, K. Fujimoto, M. Nakamura, K. Mori (Yamaguchi U.), T. Umemoto (NRO),, K. Takefuji (NICT)
on 29 Nov 2018; 18:19 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)

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MAXI detected a large X-ray flare from the RS CVn-type star UX Ari at 2018-11-24 15:54 UT. The flare peak occurred near 2018-11-25, and the peak spectrum is well reproduced by a distribution of thermal emission with a maximum temperature kT > 3.5 keV. The estimated flux in the 2-10 keV band is 1.1 +/- 0.3 x 10^-9 erg/s/cm². Assuming a distance of 52 pc, the peak luminosity corresponds to 3.6 x 10³² erg/s. NICER follow-up observations began on 2018-11-26 21:59 UT with monitoring approximately every 6 hours. The NICER 0.4-8.0 keV lightcurve is well reproduced by a constant + an exponential decay with a time constant of 1.34 days. Based on this decay rate, we expect that the flare will continue through 2018-12-01 UT, making this the longest X-ray flare ever seen from UX Ari. The time-averaged X-ray spectrum during the interval 11-26 21:59 to 11-28 23:40 shows relatively strong Ne and Ar lines and is well described by four thermal components (the highest temperature kT is about 3.9 keV). The flare was also detected in a 6.7 GHz VLBI observation using the Hitachi 32-m and Yamaguchi 34-m radio telescopes in Japan. Follow-up observations are encouraged.