NICER observations of GRO J1008-57
ATel #13749; Mark Reynolds (University of Michigan); Micheal Wolff (NRL); Jon Miller (University of Michigan); Zaven Arzoumanian (NASA/GSFC); Keith Gendreau (NASA/GSFC); Deepto Chakrabarty (MIT); Peter Jenke (NASA/MSFC)
on 21 May 2020; 18:20 UT
Credential Certification: Mark Reynolds (markrey@umich.edu)
Subjects: Radio, Infra-Red, Optical, X-ray, Gamma Ray, Request for Observations, Binary, Neutron Star, Transient
Referred to by ATel #: 13750
GRO J1008-57 is a well known Be/X-ray binary system (P_orb ~ 249.5 days; e=0.68; P_spin ~ 93.7s). Regular outbursts are observed from the system during periastron passage (phi ~ 0). Monitoring observations with Swift/BAT (15 - 50 keV) reveal that after a recent period of low-level activity (atel #13544), an outburst is currently underway that began outside periastron (phi ~ 0.3, assuming the Kuhnel et al. 2013 ephemeris) suggesting the onset of a large type-II outburst, similar to those observed from this system in 2012, 2015 and 2016 (atel #4561, #6917, #10626).
NICER has observed the source regularly during this outburst. The current time-averaged spectrum is well described by a model consisting of an accretion disk with associated comptonized emission (gamma = 1.3, kT_e = 7.6 keV, kT_dbb = 0.65 keV), plus a blackbody (kT_bb = 2.1 keV). Moderate local absorption is present in the form of a partially covering absorber with N_H = 3.7e22 cm^-2 and c_frac = 0.6. Prominent emission lines are clearly detected in the iron band, including Fe K (E = 6.41 keV, EW = 33 eV), Fe XXV (E = 6.68 keV, EW = 10 eV) and Fe XXVI (E = 7.03 keV, EW = 9 eV).
Symmetric soft X-ray pulsations are clearly detected in the 0.3 - 12.0 keV band, with P = 93.205s measured on 2020 May 14 00:05:00 UT (MJD 58983.0035). NICER measurements of the neutron star spin are consistent with measurements at higher energies from the Fermi/GBM (https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/groj1008.html). Further NICER observations are planned and analysis is ongoing.
The source flux is currently measured to be ~300 mCrab (~30% L_Edd for a distance of 5.8 kpc) and rising. Type-II outbursts can reach luminosities in excess of the Eddington limit and present an excellent opportunity to study high m_dot accretion in the presence of both a large magnetic field and strong gravity.
Further multi-wavelength observations are encouraged.
NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
