Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744-28 using timing analysis. (arXiv:1905.05593v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Monkkonen_J/0/1/0/all/0/1">Juhani Mönkkönen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tsygankov_S/0/1/0/all/0/1">Sergey S. Tsygankov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mushtukov_A/0/1/0/all/0/1">Alexander A. Mushtukov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Doroshenko_V/0/1/0/all/0/1">Victor Doroshenko</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Suleimanov_V/0/1/0/all/0/1">Valery F. Suleimanov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Poutanen_J/0/1/0/all/0/1">Juri Poutanen</a>
The X-ray pulsar GRO J1744-28 is a unique source which shows both pulsations
and type-II X-ray bursts, allowing studies of the interaction of the accretion
disk with the magnetosphere at huge mass accretion rates exceeding $10^{19}$ g
s$^{-1}$ during its super-Eddington outbursts. The magnetic field strength in
the source, $Bapprox 5times 10^{11}$ G, is known from the cyclotron
absorption feature discovered in the energy spectrum around 4.5 keV. Here, we
explore the flux variability of the source in context of interaction of its
magnetosphere with the radiation-pressure dominated accretion disk.
Particularly, we present the results of the analysis of noise power density
spectra (PDS) using the observations of the source in 1996-1997 by RXTE.
Accreting compact objects commonly exhibit a broken power-law shape of the PDS
with a break corresponding to the Keplerian orbital frequency of matter at the
innermost disk radius. The observed frequency of the break can thus be used to
estimate the size of the magnetosphere. We found, however, that the observed
PDS of GRO J1744-28 differs dramatically from the canonical shape. Furthermore,
the observed break frequency appears to be significantly higher than what is
expected based on the magnetic field estimated from the cyclotron line energy.
We argue that these observational facts can be attributed to the existence of
the radiation-pressure dominated region in the accretion disk at luminosities
above $sim$2$times 10^{37}$ erg s$^{-1}$. We discuss a qualitative model for
the PDS formation in such disks, and show that its predictions are consistent
with our observational findings. The presence of the radiation-pressure
dominated region can also explain the observed weak luminosity-dependence of
the inner radius, and we argue that the small inner radius can be explained by
a quadrupole component dominating the magnetic field of the neutron star.
The X-ray pulsar GRO J1744-28 is a unique source which shows both pulsations
and type-II X-ray bursts, allowing studies of the interaction of the accretion
disk with the magnetosphere at huge mass accretion rates exceeding $10^{19}$ g
s$^{-1}$ during its super-Eddington outbursts. The magnetic field strength in
the source, $Bapprox 5times 10^{11}$ G, is known from the cyclotron
absorption feature discovered in the energy spectrum around 4.5 keV. Here, we
explore the flux variability of the source in context of interaction of its
magnetosphere with the radiation-pressure dominated accretion disk.
Particularly, we present the results of the analysis of noise power density
spectra (PDS) using the observations of the source in 1996-1997 by RXTE.
Accreting compact objects commonly exhibit a broken power-law shape of the PDS
with a break corresponding to the Keplerian orbital frequency of matter at the
innermost disk radius. The observed frequency of the break can thus be used to
estimate the size of the magnetosphere. We found, however, that the observed
PDS of GRO J1744-28 differs dramatically from the canonical shape. Furthermore,
the observed break frequency appears to be significantly higher than what is
expected based on the magnetic field estimated from the cyclotron line energy.
We argue that these observational facts can be attributed to the existence of
the radiation-pressure dominated region in the accretion disk at luminosities
above $sim$2$times 10^{37}$ erg s$^{-1}$. We discuss a qualitative model for
the PDS formation in such disks, and show that its predictions are consistent
with our observational findings. The presence of the radiation-pressure
dominated region can also explain the observed weak luminosity-dependence of
the inner radius, and we argue that the small inner radius can be explained by
a quadrupole component dominating the magnetic field of the neutron star.
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