A1 Refereed original research article in a scientific journal
RX J0440.9+4431: another supercritical X-ray pulsar
Authors: Salganik Alexander, Tsygankov Sergey S, Doroshenko Victor, Molkov Sergey V, Lutovinov Alexander A, Mushtukov Alexander A, Poutanen Juri
Publisher: OXFORD UNIV PRESS
Publication year: 2023
Journal: Monthly Notices of the Royal Astronomical Society
Journal name in source: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Journal acronym: MON NOT R ASTRON SOC
Volume: 524
Issue: 4
First page : 5213
Last page: 5224
Number of pages: 12
ISSN: 0035-8711
DOI: https://doi.org/10.1093/mnras/stad2124
Web address : https://doi.org/10.1093/mnras/stad2124
Preprint address: https://arxiv.org/abs/2304.14881
Abstract
In the beginning of 2023, the Be transient X-ray pulsar RX J0440.9+4431 underwent a first-ever giant outburst observed from the source peaking in the beginning of February and reaching peak luminosity of & AP;4.3 x 10(37) erg s(-1). Here, we present the results of a detailed spectral and temporal study of the source based on NuSTAR, Swift, INTEGRAL, and NICER observations performed during this period and covering wide range of energies and luminosities. We find that both the pulse profile shape and spectral hardness change abruptly around & AP;2.8 x 10(37) erg s(-1), which we associate with a transition to supercritical accretion regime and erection of the accretion column. The observed pulsed fraction decreases gradually with energy up to 20 keV (with a local minimum around fluorescence iron line), which is unusual for an X-ray pulsar, and then rises rapidly at higher energies with the pulsations significantly detected up to & AP;120 keV. The broad-band energy spectra of RX J0440.9+4431 at different luminosity states can be approximated with a two-hump model with peaks at energies of about 10-20 and 50-70 keV previously suggested for other pulsars without additional features. In particular, an absorption feature around 30 keV previously reported and interpreted as a cyclotron line in the literature appears to be absent when using this model, so the question regarding the magnetic field strength of the neutron star remains open. Instead, we attempted to estimate field using several indirect methods and conclude that all of them point to a relatively strong field of around B & SIM; 10(13) G.
In the beginning of 2023, the Be transient X-ray pulsar RX J0440.9+4431 underwent a first-ever giant outburst observed from the source peaking in the beginning of February and reaching peak luminosity of & AP;4.3 x 10(37) erg s(-1). Here, we present the results of a detailed spectral and temporal study of the source based on NuSTAR, Swift, INTEGRAL, and NICER observations performed during this period and covering wide range of energies and luminosities. We find that both the pulse profile shape and spectral hardness change abruptly around & AP;2.8 x 10(37) erg s(-1), which we associate with a transition to supercritical accretion regime and erection of the accretion column. The observed pulsed fraction decreases gradually with energy up to 20 keV (with a local minimum around fluorescence iron line), which is unusual for an X-ray pulsar, and then rises rapidly at higher energies with the pulsations significantly detected up to & AP;120 keV. The broad-band energy spectra of RX J0440.9+4431 at different luminosity states can be approximated with a two-hump model with peaks at energies of about 10-20 and 50-70 keV previously suggested for other pulsars without additional features. In particular, an absorption feature around 30 keV previously reported and interpreted as a cyclotron line in the literature appears to be absent when using this model, so the question regarding the magnetic field strength of the neutron star remains open. Instead, we attempted to estimate field using several indirect methods and conclude that all of them point to a relatively strong field of around B & SIM; 10(13) G.
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