Broad-band X-ray characteristics of the transient pulsar GRO J2058+42

Abstract

ABSTRACT The Be X-ray binary GRO J2058+42 recently went through a Type-II outburst during 2019 March–April lasting for about 50 d. This outburst was detected with the operating all sky X-ray monitors like the Fermi-GBM, Swift-BAT, and MAXI-GSC. Two Nuclear Spectroscopic Telescope Array(NuSTAR) observations were also made, one during the rise and other during the decay of the outburst. It gave us the unique opportunity to analyse the broad-band characteristics of the pulsar for the first time and accretion torque characteristics of the pulsar over a range of X-ray luminosity. The pulse profiles are strongly energy-dependent, with at least four different pulse components at low energy (< 20 keV), which evolves to a single-peaked profile at high energy (> 30 keV). In each of the narrow energy bands, the pulse profiles are nearly identical in the two NuSTAR observations. The spectra from both the observations are fitted well to a power-law with a Fermi–Dirac-type high-energy cutoff. We ruled out presence of a cyclotron line in the pulse phase averaged X-ray spectrum in the NuSTAR band with an optical depth greater than 0.15. An iron emission line is detected in both the NuSTAR spectra with an equivalent width of about 125 eV. We looked at the dependence of the spin-up rate on the luminosity and estimated the magnetic field strength from that, which came out to be much higher compared to other known BeXRB pulsars. Lastly, we discussed the inadequacy of the torque–luminosity relation for determination of magnetic field strength of neutron stars.