Probing the MSP prenatal stage: the optical identification of the X-ray burster EXO 1745-248 in Terzan 5

TL;DR

This paper reports the optical identification of the neutron star burster EXO 1745-248 in Terzan 5, using HST images to observe a brightening consistent with an X-ray outburst, shedding light on its evolutionary stage.

Contribution

It provides the first optical identification of EXO 1745-248 during an X-ray outburst, linking optical brightening to the pre-millisecond pulsar evolutionary phase.

Findings

Detected a star brightening by ~3 magnitudes during outburst.

Confirmed the star's position matches the X-ray source.

Supported the scenario of a companion filling its Roche Lobe.

Abstract

We report on the optical identification of the neutron star burster EXO 1745-248 in Terzan 5. The identification was performed by exploiting HST/ACS images acquired in Director's Discretionary Time shortly after (approximately 1 month) the Swift detection of the X-ray burst. The comparison between these images and previous archival data revealed the presence of a star that currently brightened by ~3 magnitudes, consistent with expectations during an X-ray outburst. The centroid of this object well agrees with the position, in the archival images, of a star located in the Turn-Off/Sub Giant Branch region of Terzan 5. This supports the scenario that the companion should has recently filled its Roche Lobe. Such a system represents the pre-natal stage of a millisecond pulsar, an evolutionary phase during which heavy mass accretion on the compact object occurs, thus producing X-ray outbursts and re-accelerating the neutron star.