A Search for the Optical Counterpart of the Luminous X-Ray Source in NGC 6652
Abstract
We examine images of the field of X1832-330, the luminous (L_X ~ 10^36 ergs s^-1) X-ray burst source near the center of the globular cluster NGC 6652, in order to identify the optical counterpart for further study. U and B ground-based images allow us to set a limit M_B_0 >~ 3.5 for the counterpart at the time of those observations, provided that the color is (U - B)_0 ~ -1, similar to the sources known in other clusters. Archival Hubble Space Telescope observations survey most, but not all, of the 1 sigma X-ray error circle and allow us to set limits M_B_0 > 5.9 and M_B_0 > 5.2 in the Wide Field Planetary Camera 1 (WF/PC-1) and Wide Field Planetary Camera 2 (WFPC2) regions, respectively. In the WF/PC-1 images we do weakly detect a faint object with UV excess, but it is located 11.7" from the ROSAT X-ray position. This considerable (2.3 sigma) discrepancy in position suggests that this candidate be treated with caution, but it remains the only reasonable one advanced thus far. We measure for this star m_439 = 20.2 +/- 0.2 and m_336 - m_439 = -0.5 +/- 0.2 and estimate M_B_0 = 5.5 and (U - B)_0 = -0.9, similar to other known optical counterparts. If this candidate is not the identification, our limits imply that the true counterpart, not yet identified, is probably the optically faintest cluster source yet known or, alternatively, that it did not show significant UV excess at the time of these observations. Finally, we assess the outlook for the identification of the remaining luminous globular cluster X-ray sources. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
- Publication:
-
The Astronomical Journal
- Pub Date:
- September 1998
- DOI:
- 10.1086/300512
- arXiv:
- arXiv:astro-ph/9805280
- Bibcode:
- 1998AJ....116.1301D
- Keywords:
-
- GALAXY: GLOBULAR CLUSTERS: INDIVIDUAL: NGC NUMBER: NGC 6652;
- STARS: NEUTRON;
- ULTRAVIOLET EMISSION;
- X-RAYS;
- Astrophysics
- E-Print:
- 15 pages including 5 figures and no tables. Accepted for publication in The Astronomical Journal