# The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440

## TL;DR

This study identifies the optical counterpart to the X-ray pulsar SAX J1748.9-2021 in NGC 6440 using Hubble data, revealing ongoing mass transfer and providing detailed stellar parameters during quiescence.

## Contribution

The paper presents the first identification of the optical counterpart to SAX J1748.9-2021 during quiescence using HST imaging, with detailed analysis of its properties and implications for the system's state.

## Key findings

- Detected strong Halpha emission from the optical counterpart.
- Estimated stellar parameters: mass 0.70-0.83 Msun, temperature ~5250 K.
- Indications of ongoing mass transfer and low inclination angle.

## Abstract

We used a combination of deep optical and Halpha images of the Galactic globular cluster NGC 6440, acquired with the Hubble Space Telescope, to identify the optical counterpart to the accreting millisecond X-ray pulsar SAX J1748.9-2021during quiescence. A strong Halpha emission has been detected from a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15" from the nominal position of the X-ray source. The position of the star also agrees with the optical counterpart found by Verbunt et al. (2000) during an outburst. We propose this star as the most likely optical counterpart to the binary system. By direct comparison with isochrones, we estimated that COM-SAX J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun and a superficial temperature of 5250pm80 K. These parameters combined with the orbital characteristics of the binary suggest that the system is observed at a very low inclination angle (~8 deg -14 deg) and that the star is filling or even overflowing its Roche Lobe. This, together with the equivalent width of the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The possibile presence of such a on-going mass transfer during a quiescence state also suggests that the radio pulsar is not active yet and thus this system, despite its similarity with the class of redback millisecond pulsars, is not a transitional millisecond pulsar.