NuSTAR observation of GRO J1744−28 at low mass accretion rate





Citation of Original Publication

König, O., “NuSTAR observation of GRO J1744-28 at low mass accretion rate”, Astronomy and Astrophysics, vol. 643, 2020. doi:10.1051/0004-6361/202039064.


This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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We present the spectral analysis of the LMXB GRO J1744-28 using ∼29 ks of NuSTAR data taken in 2017 February at a low luminosity of 3.2×10³⁶ erg/s (3-50 keV). The continuum spectrum is modeled with an absorbed power-law with exponential cut-off, and an additional iron line component. We find no obvious indications for a CRSF and therefore perform a detailed cyclotron line search using statistical methods on the pulse phase-averaged as well as phase-resolved spectra. 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, which was not seen at higher flux. The 4σ upper limit for the strength of a CRSF in the 3-20 keV band is 0.07 keV, lower than the strength of the line found at higher luminosity. 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/cm2/s 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 3.2×10³⁶ erg/s, which implies a surface magnetic field ≲2.9×10¹¹ G and mass accretion rate ≲1.7×10¹⁶ g/s. 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.