The Optical Counterpart to the Accreting Millisecond X-Ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440
- Dipartimento di Fisica e Astronomia, Università di Bologna, Via P. Gobetti 93/2, I-40129 Bologna (Italy)
- INAF—Osservatorio Astronomico di Bologna, Via P. Gobetti 93/3, I-40129 Bologna (Italy)
- Leiden Observatory, Leiden University, Neils Bohrweg 2, 2333 CA, Leiden (Netherlands)
We used a combination of deep optical and Hα 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 J1748.9-2021 during quiescence. A strong Hα 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. 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--0.83 M{sub ⊙}, a radius of 0.88±0.02 R{sub ⊙}, and a superficial temperature of 5250 ± 80 K. These parameters, combined with the orbital characteristics of the binary, suggest that the system is observed at a very low inclination angle (∼8{sup ∘}--14{sup ∘}) and that the star is filling or even overflowing its Roche lobe. This, together with the EW of the Hα emission (∼20 Å), suggests possible ongoing mass transfer. The possible presence of such an ongoing 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.
- OSTI ID:
- 22876001
- Journal Information:
- Astrophysical Journal, Vol. 844, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
Similar Records
DISCOVERY OF A 205.89 Hz ACCRETING MILLISECOND X-RAY PULSAR IN THE GLOBULAR CLUSTER NGC 6440
Discovery of the optical counterparts to four energetic Fermi millisecond pulsars