# Chandra, MDM, Swift, and NuSTAR observations confirming the SFXT nature of AX J1949.8+2534

## TL;DR

This study confirms AX J1949.8+2534 as a supergiant fast X-ray transient through multi-instrument observations, optical spectroscopy, and identification of its supergiant companion star.

## Contribution

First multi-instrument observational confirmation of AX J1949.8+2534 as an SFXT, including optical counterpart identification and spectral classification.

## Key findings

- Rapid X-ray variability observed on ks timescales.
- Optical spectrum shows Hα emission and He I absorption.
- Star identified as an early B-type Ia supergiant.

## Abstract

AX J1949.8+2534 is a candidate supergiant fast X-ray transient (SFXT) observed in outburst by INTEGRAL (IGR J19498+2534). We report on the results of six Neil Gehrels Swift-XRT, one Chandra, and one NuSTAR observation of the source. We find evidence of rapid X-ray variability on a few ks timescales. Fortunately, Chandra observed the source in a relatively bright state, allowing us to confidently identify the optical/NIR counterpart of the source. We also obtained an optical spectrum of this counterpart, which shows an H$\alpha$ emission line and He I absorption features. The photometry and spectrum of the source allow us to constrain its distance, $\sim7-8$ kpc, and reddening, $A_V=8.5-9.5$. We find that the star is likely an early B-type Ia supergiant, confirming that AX J1949.8+2534 is indeed an SFXT.