The near-IR counterpart of IGR J17480-2446 in Terzan 5

TL;DR

This paper identifies a near-infrared counterpart to the 11 Hz pulsar IGR J17480-2446 in Terzan 5 using adaptive optics, providing insights into the donor star and the system's evolution.

Contribution

First detection of the likely near-IR counterpart of IGR J17480-2446 in Terzan 5, combining AO observations and archival data to analyze its properties.

Findings

The counterpart is located at the blue edge of the turn-off in the cluster's color-magnitude diagram.

Luminosity increases by a factor of 2.5 from quiescence to outburst.

The donor star's properties are consistent with the double stellar population of Terzan 5.

Abstract

Some globular clusters in our Galaxy are noticeably rich in low-mass X-ray binaries. Terzan 5 has the richest population among globular clusters of X- and radio-pulsars and low-mass X-ray binaries. The detection and study of optical/IR counterparts of low-mass X-ray binaries is fundamental to characterizing both the low-mass donor in the binary system and investigating the mechanisms of the formation and evolution of this class of objects. We aim at identifying the near-IR counterpart of the 11 Hz pulsar IGRJ17480-2446 discovered in Terzan 5. Adaptive optics (AO) systems represent the only possibility for studying the very dense environment of GC cores from the ground. We carried out observations of the core of Terzan 5 in the near-IR bands with the ESO-VLT NAOS-CONICA instrument. We present the discovery of the likely counterpart in the Ks band and discuss its properties both in outburst and in quiescence. Archival HST observations are used to extend our discussion to the optical bands. The source is located at the blue edge of the turn-off area in the color-magnitude diagram of the cluster. Its luminosity increase from quiescence to outburst, by a factor 2.5, allows us to discuss the nature of the donor star in the context of the double stellar generation population of Terzan 5 by using recent stellar evolution models.