We report the results of optical-infrared follow-up observations of the gravitational wave (GW) event GW151226 detected by the Advanced LIGO in the framework of J-GEM (Japanese collaboration for Gravitational wave ElectroMagnetic follow-up). We performed wide-field optical imaging surveys with Kiso Wide Field Camera (KWFC), Hyper Suprime-Cam (HSC), and MOA-cam3. The KWFC survey started at 2.26 days after the GW event and covered 778 deg 2 centered at the high Galactic region of the skymap of GW151226. We started the HSC follow-up observations from ∼12 days after the event and covered an area of 63.5 deg 2 of the highest probability region of the northern sky with the limiting magnitudes of 24.6 and 23.8 for i band and z band, respectively. MOA-cam3 covered 145 deg 2 of the skymap with MOA-red filter ∼2.5 months after the GW alert. Total area covered by the wide-field surveys was 986.5 deg 2 . The integrated detection probability of all the observed area was ∼29%. We also performed galaxy-targeted observations with six optical and near-infrared telescopes from 1.61 days after the event. Total of 238 nearby (≤100 Mpc) galaxies were observed with the typical I band limiting magnitude of ∼19.5. We detected 13 supernova candidates with the KWFC survey, and 60 extragalactic transients with the HSC survey. Two third of the HSC transients were likely supernovae and the remaining one third were possible active galactic nuclei. With our observational campaign, we found no transients that are likely to be associated with GW151226.

High-dispersion spectroscopic monitoring of HESS J0632+057 has been carried out over four orbital cycles in order to search for orbital modulation, covering the entire orbital phase. We have measured the radial velocity of the Hα emission line with the method introduced by Shafter, Szkody, and Thorstensen (1986, ApJ, 308, 765), which has been successfully applied to some Be stars. The velocity is seen to increase much earlier than expected for the orbital period of 315 d, and much more steeply than expected at around “apastron.” The period of the Hα modulation is found to be $308^{+26}_{-23}$ d. We have also analyzed Swift/XRT data from 2009 to 2015 to study the orbital modulation, selecting the data with good statistics (≥30 counts). With additional two-year data to the previous works, the orbital period has been updated to $313^{+11}_{-8}$ d, which is consistent with the previous X-ray periods and the spectroscopic one. Previous XMM-Newton and Chandra observations prefer a period of 313 d. With the new period, assuming that Hα velocities accurately trace the motion of the Be star, we have derived a new set of orbital parameters. In the new orbit, which is less eccentric (e ≃ 0.6), two outbursts occur: after apastron and just after periastron. Also, the column density in bright phase ($4.7^{+0.9}_{-08}\times 10^{21} \, \mathrm{cm}^{-2}$) is higher than in faint phase (2.2 ± 0.5 × 1021 cm−2). These facts suggest that outbursts occur when the compact object passes nearby/through the Be disk. The mass function implies that the mass of the compact object is less than 2.5 M⊙, assuming that the mass of the Be star is 13.2–18.2 M⊙ (Aragona et al. 2010, ApJ, 724, 306), unless the inclination is extremely small. The photon index indicates that the spectra become softer when the system is bright. These suggest that the compact object is a pulsar.

We have performed the near-infrared photometric monitoring observations of two TeV gamma-ray binaries with O-stars (LS 5039 and 1FGL J1018.6−5856), using IRSF/SIRIUS at SAAO, in order to study the stellar parameters and their perturbations caused by the binary interactions. The whole orbital phase was observed multiple times and no significant variabilities including orbital modulations were detected for both targets. Assuming that the two systems are colliding wind binaries, we estimate the amplitude of flux variation caused by the difference in the optical depth of O-star wind at inferior conjunction, where the star is seen through the cavity created by pulsar wind, and other orbital phases without pulsar-wind intervention. The derived amplitude is <0.001 mag, which is about two orders of magnitude smaller than the observed upper limit. Also using the upper limits of the near-infrared variability, we for the first time obtain the upper limit of the dust formation rate resulting from wind–wind collision in O-star gamma-ray binaries.