# GROJ1750-27: a neutron star far behind the Galactic Center switching into the propeller regime

## TL;DR

This study characterizes the X-ray binary pulsar GROJ1750-27, determining its distance, magnetic field, and accretion state, revealing it as one of the most distant known Galactic X-ray binaries and indicating a transition into the propeller regime.

## Contribution

First precise localization and infrared identification of GROJ1750-27, with combined multi-wavelength analysis estimating its distance, magnetic field, and accretion regime transition.

## Key findings

- Distance estimated at 14-22 kpc, placing it behind the Galactic Center.
- Magnetic field strength on neutron star surface estimated at (3.5-4.5)×10^{12} G.
- Evidence of transition into the propeller regime during outburst.

## Abstract

We report on analysis of properties of the X-ray binary pulsar GROJ1750-27 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 $B\simeq(3.5-4.5)\times10^{12}$ 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 GROJ1750-27 is located far behind the Galactic Center, which makes it one of the furthest Galactic X-ray binaries known.