XMM-Newton Observations of the TeV \gamma-ray Source HESS J1804-216

TL;DR

This study analyzed XMM-Newton observations of the TeV gamma-ray source HESS J1804-216, identifying likely X-ray counterparts and characterizing their properties, but found no direct evidence linking these sources to the TeV emission.

Contribution

First detailed X-ray analysis of HESS J1804-216's central region, identifying and characterizing potential counterparts and their properties.

Findings

Src 1 shows a 2.93 hr X-ray periodicity, likely an eclipsing polar.

Src 2 exhibits a strong Fe emission line and variability, probably an intermediate polar.

No clear extended X-ray emission linked to the supernova remnant G8.7-0.1 or the pulsar wind nebula PSR J1803-2137 was detected.

Abstract

We have analyzed three XMM-Newton observations of the central part of the unidentified TeV \gamma-ray source HESS J1804-216. We focus on two X-ray sources 2XMMi J180442.0-214221 (Src 1) and 2XMMi J180432.5-214009 (Src 2), which were suggested to be the possible X-ray counterparts to the TeV source. We discover a 2.93 hr X-ray periodicity from Src 1, with the pulse profile explained with a self-eclipsing pole in an eclipsing polar. Src 2 exhibits a strong Fe emission line (FWHM ~0.3 keV and equivalent width ~0.8 keV) and large X-ray variability on timescales of hours and is probably an intermediate polar. Thus Src 1 and Src 2 are probably two field sources not responsible for the TeV emission. The observations were contaminated by strong straylight from a nearby bright source, and we see no clear extended X-ray emission that can be attributed to the supernova remnant G8.7-0.1, a popular possible association with the TeV source. The other possible association, the pulsar wind nebula candidate PSR J1803-2137, shows little long-term variability, compared with a previous Chandra observation. Many point sources were serendipitously detected, but most of them are probably normal stars. Three new candidate compact object systems (other than Src 1, Src 2 and PSR J1803-2137) are also found. They are far away from the TeV source and are probably also magnetic cataclysmic variables, thus unlikely to be responsible for the TeV emission.