Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744-28 using timing analysis

Abstract

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, $B\approx 5\times 10^{11}$ 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.

10 pages, 9 figures, accepted by A&A

References in corpus (13)

• Modelling the behaviour of accretion flows in X-ray binaries
• Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
• Optically thick envelopes around ULXs powered by accreating neutron stars
• Super-Eddington accretion discs with advection and outflows around magnetized neutron stars
• The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii
• High-Resolution X-Ray Spectroscopy of the Bursting Pulsar GRO J1744-28
• Can disk-magnetosphere interaction models and beat frequency models for quasi-periodic oscillation in accreting X-ray pulsars be reconciled?
• Magnetic fields of neutron stars in X-ray binaries
• Photon Bubbles and the Vertical Structure of Accretion Disks
• Propagating mass accretion rate fluctuations in X-ray binaries under the influence of viscous diffusion
• GRO J1744-28: an intermediate B-field pulsar in a low mass X-ray binary
• Broadband aperiodic variability in X-ray pulsars: accretion rate fluctuations propagating under the influence of viscous diffusion
• Simultaneous NuSTAR/Chandra observations of the Bursting Pulsar GRO J1744-28 during its third reactivation

Cited by in corpus (15)

• Ultraluminous X-Ray Sources
• M51 ULX-7: super-orbital periodicity and constraints on the neutron star magnetic field
• Low-Mass X-ray Binaries
• Hot disk of the Swift J0243.6+6124 revealed by Insight-HXMT
• A shared accretion instability for black holes and neutron stars
• A Bayesian approach for torque modelling of BeXRB pulsars with application to super-Eddington accretors
• Insight-HXMT observations of Swift J0243.6+6124: the evolution of RMS pulse fractions at super-Eddington luminosity
• Timing analysis of 2S 1417-624 observed with NICER and Insight-HXMT
• Observations of GRO J1744-28 in quiescence with XMM-Newton
• X-Ray Views of Galactic Accreting Pulsars in High-Mass X-Ray Binaries
• Inertial oscillation modes of an inclined dipolar magnetosphere as a source of band-limited noise in X-ray pulsars
• The Black Hole Candidate Swift J1728.9$-$3613 and the Supernova Remnant G351.9$-$0.9
• Discovery of a bimodal luminosity distribution in persistent Be/X-ray pulsar 2RXP J130159.6-635806
• Exploring the magnetic field of the ultraluminous X-ray pulsar NGC 4631 X-8
• Constraints on the magnetic field structure in accreting compact objects from aperiodic variability