XMM-Newton discovery of very high obscuration in the candidate Supergiant Fast X-ray Transient AX J1714.1-3912

TL;DR

This paper reports the discovery of very high obscuration and unique spectral features in the candidate SFXT AX J1714.1-3912, suggesting it may belong to a rare class of highly obscured high mass X-ray binaries with evolved supergiant companions.

Contribution

It presents the first detection of intense iron emission and extreme absorption in AX J1714.1-3912, indicating it could be a new member of highly obscured HMXBs, expanding understanding of such systems.

Findings

Detected large absorption (NH ~ 1e24 cm^-2) in the source.

Identified an intense iron emission line at 6.4 keV.

Observed variability and spectral properties similar to highly obscured HMXBs.

Abstract

We have analysed an archival XMM-Newton EPIC observation that serendipitously covered the sky position of a variable X-ray source AX J1714.1-3912, previously suggested to be a Supergiant Fast X-ray Transient (SFXT). During the XMM-Newton observation the source is variable on a timescale of hundred seconds and shows two luminosity states, with a flaring activity followed by unflared emission, with a variability amplitude of a factor of about 50. We have discovered an intense iron emission line with a centroid energy of 6.4 keV in the power law-like spectrum, modified by a large absorption (NH around 1e24 cm-2), never observed before from this source. This X-ray spectrum is unusual for an SFXT, but resembles the so-called "highly obscured sources", high mass X-ray binaries (HMXBs) hosting an evolved B[e] supergiant companion (sgB[e]). This might suggest that AX J1714.1-3912 is a new member of this rare type of HMXBs, which includes IGR J16318-4848 and CI Camelopardalis. Increasing this small population of sources would be remarkable, as they represent an interesting short transition evolutionary stage in the evolution of massive binaries. Nevertheless, AX J1714.1-3912 appears to share X-ray properties of both kinds of HMXBs (SFXT vs sgB[e] HMXB). Therefore, further investigations of the companion star are needed to disentangle the two hypothesis.