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MAXI/GSC observations of X-ray outbursts from Be/X-ray binary pulsars, IGR J19294+1816, GRO J1008-57 and 4U 0115+63

ATel #13480; M. Nakajima (Nihon U.), T. Mihara (RIKEN), H. Negoro, W. Maruyama, M. Aoki, K. Kobayashi, R. Takagi (Nihon U.), C. Guo, Y. Zhou, 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, 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. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
on 14 Feb 2020; 10:29 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 13544

We report on X-ray outbursts from three Be/X-ray binary pulsars triggered by the MAXI/GSC nova-alert system (Negoro et al. 2016).

IGR J19294+1816:
The onset of X-ray outburst from IGR J19294+1816 was triggered on 2020-02-08 (MJD 58887). The day-averaged 4-10 keV flux reached 0.039 +- 0.011 photons/s/cm2 (34 +- 10 mCrab) on 2020-02-07 (MJD 58886), and it has been in same flux level for ∼5 days. The time interval between the onset of this event and the previous one (Domcek et al. 2019, Atel#13215) is consistent with the claimed binary orbital period of ∼117.2 days (Corbet et al. 2009, Atel#2009). Neil Gehrels Swift observatory and Fermi/GBM also detected this brightening. We checked the 4-10keV/2-4keV and 15-50keV/4-10 keV hardness ratios, and found no significant difference compared to the previous events. Thus this brightening might be a normal outburst again.

The latest X-ray light-curve can be followed at the following pages;
MAXI (http://maxi.riken.jp/star_data/J1929+182/J1929+182.html),
Swift/BAT (https://swift.gsfc.nasa.gov/results/transients/weak/IGRJ19294p1816/),
Fermi/GBM (https://gammaray.msfc.nasa.gov/gbm/science/pulsars/lightcurves/igrj19294.html), and
the BeXRB monitor page (http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_IGRJ19294+1816.html).

GRO J1008-57:
A weak X-ray emission from GRO J1008-57 was triggered on the same day 2020-02-08 (MJD 58887). The day-averaged flux in the 4-10 keV energy band obtained by the MAXI/GSC is 0.011 +- 0.004 photons/s/cm2 (10 +- 3 mCrab) on 2020-02-09 (MJD 58888), and it reached 0.035 +- 0.004 photons/s/cm2 (30 +- 3 mCrab) on 2020-02-13 (MJD 58892). The orbital phase of the onset of this faint brightening is about 0.89 which is derived from following orbital parameters, Porb=249.48 d and T0=54424.71(MJD) (Kühnel et al. 2013). This onset phase is slightly earlier than that of the regular outburst, which usually starts from the orbital phase of ∼0.91. The 4-10keV/2-4keV hardness ratio is 2.0 +- 0.8 which is comparable to that of the normal outburst observed from this pulsar so far.

The latest X-ray light-curve can be followed at the following pages;
MAXI (http://maxi.riken.jp/star_data/J1009-582/J1009-582.html),
Swift/BAT (http://swift.gsfc.nasa.gov/results/transients/GROJ1008-57/),
Fermi/GBM (https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/groj1008.html), and
the BeXRB monitor page (http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_GROJ1008-57.html).

4U 0115+63:
The brightening of 4U 0115+63 was confirmed on 2020-02-09 (MJD 58888). This source turned back to an active state since one orbital cycle before (2020-01-15, MJD 58863). On-going outburst started from 2020-02-06 (MJD 58885) which corresponds to the orbital phase of 0.0 (Porb=24.3174 d and T0=53243.038(MJD), Raichur et al. 2010). The latest flux in 4-10 keV band is 0.068 +- 0.008 photons/s/cm2 (59 +- 7 mCrab) on 2020-02-13 (MJD 58892). The current flux already exceeded the previous peak of 0.056 +- 0.009 photons/s/cm2 (49 +- 14 mCrab) on 2020-01-19 (MJD 58867). In addition, the 4-10keV/2-4keV hardness ratio of 1.9 +- 0.4 obtained from latest 3-days data is slightly larger than that of the previous outburst peak, 1.4 +- 0.2.

The source flux can be checked at the following pages;
MAXI(http://maxi.riken.jp/star_data/J0118+637/J0118+637.html), Swift/BAT(https://swift.gsfc.nasa.gov/results/transients/weak/4U0115p634/), Fermi/GBM(https://gammaray.msfc.nasa.gov/gbm/science/pulsars/lightcurves/4u0115.html), and
the BeXRB monitor page (http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_4U0115+63.html).