NuSTAR observation of GRO J1744–28 at low mass accretion rateстатья
Статья опубликована в высокорейтинговом журнале
Информация о цитировании статьи получена из
Web of Science,
Scopus
Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 30 декабря 2020 г.
Аннотация:Context. Neutron stars in low-mass X-ray binaries are important systems for studying the physics of accretion onto compact objects. The system GRO J1744-28 is particularly interesting as it usually shows clear pulsations as well as X-ray bursts. Additionally, there are claims for a magnetic field of 5 × 10^11 G through the detection of a cyclotron resonant scattering feature (CRSF).Aims: We present the spectral analysis of GRO J1744-28 using ∼29 ks of NuSTAR data taken in 2017 February at a low luminosity of 3.2 × 1036 erg s^-1 (3-50 keV). Our goal is to study the variability of the source spectrum with pulse phase and to search for the claimed CRSF.Methods: The continuum spectrum was modeled with an absorbed power law with exponential cutoff, and an additional iron line component. We found no obvious indications for a CRSF, and therefore performed a detailed cyclotron line search using statistical methods. We performed this search on pulse phase-averaged spectra and on phase-resolved spectra.Results: GRO J1744-28 was observed in a low-luminosity state. The previously detected Type II X-ray bursts are absent. Clear pulsations at a period of 2.141124(9) Hz are detected. The pulse profile shows an indication of a secondary peak that was not seen at higher flux. The upper limit for the strength of a CRSF in the 3-20 keV band is 0.07 keV (90% CL), lower than the strength of the line found at higher luminosity.Conclusions: The detection of pulsations shows that the source did not enter the "propeller" regime, even though the source flux of 4.15 × 10^-10 erg cm^-2 s^-1 was almost one order of magnitude below the threshold for the propeller regime claimed in previous studies on this source. The transition into the propeller regime in GRO J1744-28 must therefore be below a luminosity of 3.2 × 10^36 erg s-1 (3-50 keV), which implies a surface magnetic field ≲2.9 × 10^11 G and mass accretion rate ≲1.7 × 10^16 g s^-1. A change of the CRSF depth as function of luminosity is not unexpected and has been observed in other sources. This result possibly implies a change in emission geometry as function of mass accretion rate to reduce the depth of the line below our detection limit.