GRO J1750–27: A neutron star far behind the Galactic Center switching into the propeller regime

Abstract

We report on analysis of properties of the X-ray binary pulsar GRO J1750−27 based on X-ray (Chandra, Swift, and Fermi/GBM), and near-infrared (VVV and UKIDSS surveys) observations. An accurate position of the source is determined for the first time and used to identify its infrared counterpart. Based on the VVV data we investigate the spectral energy distribution (SED) of the companion, taking into account a non-standard absorption law in the source direction. A comparison of this SED with those of known Be/X-ray binaries and early type stars has allowed us to estimate a lower distance limit to the source at > 12 kpc. An analysis of the observed spin-up torque during a giant outburst in 2015 provides an independent distance estimate of 14 − 22 kpc, and also allows to estimate the magnetic field on the surface of the neutron star at B ≃ (3.5 − 4.5) × 10 12 G. The latter value is in agreement with the possible transition to the propeller regime, a strong hint for which was revealed by Swift/XRT and Chandra. We conclude, that GRO J1750−27 is located far behind the Galactic Center, which makes it one of the furthest Galactic X-ray binaries known.

Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750−27

“…This difference is below the critical value indicated in Tsygankov et al (2017b, |∆C| = 10), therefore we could not statistically prefer one model over the other. 11 Lutovinov et al (2019) constrained the distance towards the source to a range of 14 to 22 kpc, very similar to what we assume in our paper. models) 12 and the radius of the emission region (for the blackbody model) using both distances.…”

“…We do detect GRO J1750-27 in three of our five Chandra monitoring observations but the spectral parameters (the spectra are relatively hard) as well as the variability seen in quiescence (also taking into account the non-detections) argue that we likely see X-ray emission due to low-level accretion (see Section 3.2). Alternatively, the observed X-ray emission could potentially be originated by the companion star, which, most probably, is an early B-type star (see Lutovinov et al 2019). Such stars are known to be (variable) X-ray emitters as well (e.g., Nazé 2009;Nazé et al 2011).…”

Observational constraints on the magnetic field of the bright transient Be/X-ray pulsar SXP 4.78

“…Assuming a blackbody spectrum with the temperature of kT = 0.5 keV, that is expected and observed for sources in the propeller regime (see, e.g., Tsygankov et al 2016;Wijnands & Degenaar 2016), and the absorption column of 2 × 10 21 cm −2 (Kalberla et al 2005), we estimated an unabsorbed source flux of 3.7 × 10 −15 erg s −1 cm −2 and the corresponding intrinsic luminosity of L q 1.6 × 10 33 erg s −1 for the distance of 60.3 kpc. Such a luminosity agrees well with usual values for HMXBs in the propeller state (Tsygankov et al 2016;Wij-nands & Degenaar 2016;Tsygankov et al 2017a;Lutovinov et al 2017Lutovinov et al , 2019.…”

First characterization of Swift J1845.7–0037 with NuSTAR

“…In fact, the broadband continuum spectrum of the source can be well described by different phenomenological models without the necessity to include any absorption line in any of the models. The source's spin period remained constant, which, together with the unknown orbit and distance, precludes any estimate of the magnetic field by modeling spin evolution with accretion torque models (see, e.g., Tsygankov et al 2016Tsygankov et al , 2017aDoroshenko et al 2018;Lutovinov et al 2019, for the current results).…”