Broadband X-ray characteristics of the transient pulsar GRO J2058+42. (arXiv:2007.04519v1 [astro-ph.HE])

Broadband X-ray characteristics of the transient pulsar GRO J2058+42. (arXiv:2007.04519v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kabiraj_S/0/1/0/all/0/1">Sanhita Kabiraj</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Paul_B/0/1/0/all/0/1">Biswajit Paul</a>

The Be X-ray binary GRO J2058+42 recently went through a Type-II outburst
during March-April 2019 lasting for about 50 days. This outburst was detected
with the operating all sky X-ray monitors like the Fermi-GBM, Swift-BAT and
MAXI-GSC. Two NuSTAR observations were also made, one during the rise and other
during the decay of the outburst. It gave us the unique opportunity to analyze
the broadband 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.

The Be X-ray binary GRO J2058+42 recently went through a Type-II outburst
during March-April 2019 lasting for about 50 days. This outburst was detected
with the operating all sky X-ray monitors like the Fermi-GBM, Swift-BAT and
MAXI-GSC. Two NuSTAR observations were also made, one during the rise and other
during the decay of the outburst. It gave us the unique opportunity to analyze
the broadband 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.

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