Thermal and non-thermal X-ray emission from the rotation-powered radio/$\gamma$-ray pulsar PSR J1740+1000

TL;DR

This study presents detailed X-ray observations of pulsar PSR J1740+1000, revealing distinct thermal and non-thermal emission components with phase differences, challenging simple magnetic field models and suggesting complex thermal surface maps.

Contribution

First detailed phase-resolved X-ray analysis of PSR J1740+1000, showing complex thermal surface emission and non-thermal pulsed emission, indicating more intricate magnetic and thermal structures.

Findings

Non-thermal emission is pulsed with ~30% fraction above 2 keV.

Thermal emission fits two blackbodies with distinct temperatures and radii.

Thermal and non-thermal peaks are in anti-phase, not aligned with radio pulses.

Abstract

We report the results of new XMM-Newton observations of the middle-aged ($\sim$10$^5$ yr) radio pulsar PSR J1740+1000 carried out in 2017-2018. These long pointings ($\sim$530 ks) show that the non-thermal emission, well described by a power-law spectrum with photon index $\Gamma=1.80\pm0.17$, is pulsed with a $\sim$30% pulsed fraction above 2 keV. The thermal emission can be well fit with the sum of two blackbodies of temperatures $kT_1=70\pm4$ eV, $kT_2=137\pm7$ eV, $R_1=5.4_{-0.9}^{+1.3}$ km and $R_2=0.70_{-0.13}^{+0.15}$ km (for a distance of 1.2 kpc). We found no evidence for absorption lines as those observed in the shorter XMM-Newton observations ($\sim$67 ks) of this pulsar carried out in 2006. The X-ray thermal and non-thermal components peak in anti-phase and none of them is seen to coincide in phase with the radio pulse. This, coupled with the small difference in the emission radii of the two thermal components, disfavors an interpretation in which the dipolar polar cap is heated by magnetospheric backward-accelerated particles. Comparison with the other thermally-emitting isolated neutron stars with spectra well described by the sum of two components shows that the ratios $T_2$/$T_1$ and $R_2$/$R_1$ are similar for objects of different classes. The observed values cannot be reproduced with simple temperature distributions, such as those caused by a dipolar field, indicating the presence of more complicated thermal maps.