The X-ray modulation of PSR J2032+4127/MT91 213 during the Periastron Passage in 2017

TL;DR

This study analyzes multi-wavelength observations of PSR J2032+4127/MT91 213 during its 2017 periastron, revealing X-ray dips, flares, and potential gamma-ray modulation linked to binary interactions.

Contribution

It provides the first detailed multi-wavelength analysis of the 2017 periastron passage, proposing explanations for X-ray dips and flares, and discusses the challenges in detecting gamma-ray modulation.

Findings

X-ray flux increased before periastron, then dipped sharply during periastron

A post-periastron X-ray flare lasted about 20 days

No significant gamma-ray variability detected in Fermi-LAT data

Abstract

We present the Neil Gehrels Swift Observatory (Swift), Fermi Large Area Telescope (Fermi-LAT), and Karl G. Jansky Very Large Array (VLA) observations of the gamma-ray binary PSR J2032+4127/MT91 213, of which the periastron passage has just occurred in November 2017. In the Swift X-ray light curve, the flux was steadily increasing before mid-October 2017, however, a sharp X-ray dip on a weekly time-scale is seen during the periastron passage, followed by a post-periastron X-ray flare lasting for ~20 days. We suggest that the X-ray dip is caused by (i) an increase of the magnetization parameter at the shock, and (ii) the suppression due to the Doppler boosting effect. The 20-day post-periastron flare could be a consequence of the Be stellar disk passage by the pulsar. An orbital GeV modulation is also expected in our model, however, no significant variability is seen in the Fermi-LAT light curve. We suspect that the GeV emission resulted from the interaction between the binary's members is hidden behind the bright magnetospheric emission of the pulsar. Pulsar gating technique would be useful to remove the magnetospheric emission and recover the predicted GeV modulation, if an accurate radio timing solution over the periastron passage is provided in the future.