MAXI/GSC detection of weak X-ray activity from the Be/X-ray binary pulsar XTE J1858+034

ATel #13217; M. Nakajima, H. Negoro (Nihon U.), K. Kurogi (Miyazaki U.), T. Mihara (RIKEN), W. Maruyama, M. Aoki, K. Kobayashi (Nihon U.), T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Oeda, K. Shiraishi (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, N. Isobe, R. Shimomukai, M. Tominaga (JAXA), Y. Ueda, A. Tanimoto, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, S. Iwahori, Y. Kurihara, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
on 23 Oct 2019; 12:31 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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The MAXI/GSC nova-alert system (Negoro et al. 2016) triggered on the weak X-ray activity on 2019-10-22 (MJD 58778). Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (284.554 deg, 3.501 deg) = (18 58 12, +03 30 03) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.35 deg and 0.31 deg, respectively. The roll angle of long axis from the north direction is 0.0 deg. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). This position is consistent with that of the Be/X-ray binary pulsar XTE J1858+034. This is the fifth detection of the outburst from XTE J1858+034 since its discovery in 1998 February (IAUC 6826 :2). Since then, outbursts were recorded in 2004 (ATel#274), 2010 (ATel#2846), and 2011 (ATel#3766). As there was an interruption in the MAXI observation from 2019-10-18 to 2019-10-20, the onset of the present brightening was some time in this period. After the MAXI/GSC observation resumed, the flux exhibited a small variation, 23 +- 10 mCrab on 2019-10-21 (MJD 58777) and 18 +- 8 mCrab on 2019-10-22 (MJD 58778). Since the peak fluxes in the past outbursts were distributed in 30-50 mCrab, if the two days observations indicate the flux decreasing, XTE J1858+034 seems to have just passed the flux peak. The 15-50 keV flux was also detected by Swift/BAT, and reached 18 +- 8 mCrab on 2019-10-22 (MJD 58778).

The latest lightcurve can be checked at the following pages;
MAXI(http://maxi.riken.jp/pubdata/v6l/J1858+034/index.html),
Swift/BAT(https://swift.gsfc.nasa.gov/results/transients/weak/XTEJ1858p034/),
Fermi/GBM(https://gammaray.msfc.nasa.gov/gbm/science/pulsars/lightcurves/xtej1858.html),and
the BeXRB monitor page(http://integral.esac.esa.int/bexrbmonitor/webpage_oneplot.php).