Chandra Observation of the Globular Cluster NGC 6440 and the Nature of Cluster X-ray Luminosity Functions

As part of our campaign to determine the nature of the various source populations of the low-luminosity globular cluster X-ray sources, we have obtained a Chandra X-ray Observatory ACIS-S3 image of the globular cluster NGC 6440. We detect 24 sources to a limiting luminosity of ~2 times 10^31 erg/s (0.5-2.5keV) inside the cluster's half-mass radius, all of which lie within ~2 core radii of the cluster center. We also find excess emission in and around the core which could be due to unresolved point sources. Based upon X-ray luminosities and colors, we conclude that there are 4-5 likely quiescent low-mass X-ray binaries and that most of the other sources are cataclysmic variables. We compare these results to Chandra results from other globular clusters and find the X-ray luminosity functions differ among the clusters.Comment: 9 pages, 4 figures, accepted by ApJ, minor changes, added table of clusters' physical parameter

Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440

After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in August 2001, as noted with the RXTE All-Sky Monitor. We carried out a short target of opportunity observation with the Chandra X-ray Observatory and are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in July 2000 with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter

Optical Identification of Multiple Faint X-ray Sources in the Globular Cluster NGC 6752: Evidence for Numerous Cataclysmic Variables

We report on the Chandra ACIS-S3 imaging observation of the globular cluster NGC 6752. We detect 6 X-ray sources within the 10.5" core radius and 13 more within the 115" half-mass radius down to a limiting luminosity of Lx approx 10^{30} erg/s for cluster sources. We reanalyze archival data from the Hubble Space Telescope and the Australia Telescope Compact Array and make 12 optical identifications and one radio identification. Based on X-ray and optical properties of the identifications, we find 10 likely cataclysmic variables (CVs), 1-3 likely RS CVn or BY Dra systems, and 1 or 2 possible background objects. Of the 7 sources for which no optical identifications were made, we expect that ~2-4 are background objects and that the rest are either CVs or some or all of the five millisecond pulsars whose radio positions are not yet accurately known. These and other Chandra results on globular clusters indicate that the dozens of CVs per cluster expected by theoretical arguments are being found. The findings to date also suggest that the ratio of CVs to other types of X-ray sources is remarkably similar in clusters of very different structural parameters.Comment: 15 pages, 6 figures, accepted by ApJ, minor changes in response to referee's comment

An Overabundance of Transient X-ray Binaries within 1 pc of the Galactic Center

During five years of Chandra observations, we have identified seven X-ray transients located within 23 pc of Sgr A*. These sources each vary in luminosity by more than a factor of 10, and have peak X-ray luminosities greater than 5e33 erg/s, which strongly suggests that they are accreting black holes or neutron stars. The peak luminosities of the transients are intermediate between those typically considered outburst and quiescence for X-ray binaries. Remarkably four of these transients lie within only 1 pc of Sgr A*. This implies that, compared to the numbers of similar systems located between 1 and 23 pc, transients are over-abundant by a factor of 20 per unit stellar mass within 1 pc of Sgr A*. It is likely that the excess transient X-ray sources are low-mass X-ray binaries that were produced, as in the cores of globular clusters, by three-body interactions between binary star systems and either black holes or neutron stars that have been concentrated in the central parsec through dynamical friction. Alternatively, they could be high-mass X-ray binaries that formed among the young stars that are present in the central parsec.Comment: 4 pages, including 2 figures (one color). Submitted to ApJ Letter

A Search for the Optical Counterpart of the Luminous X-ray Source in NGC 6652

We examine images of the field of X1832-330, the luminous (Lx ~ 10^36 erg/s) 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 > 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 > 5.9 and M_B > 5.2 in the WF/PC and WFPC2 regions, respectively. In the WF/PC 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, (m_336 - m_439) = -0.5 +- 0.2, and estimate M_B = 5.5, (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.Comment: 15 pages including 5 figures and no tables. Accepted for publication in The Astronomical Journal; to appear in Volume 116, September 1998. A preprint with full resolution figures may be downloaded from http://www.astro.washington.edu/deutsch/pubs

Bondi-Hoyle-Lyttleton Accretion Model for Low-luminosity X-ray Sources in Globular Clusters

We present a new model for low-luminosity X-ray sources in globular clusters, with L_x < 10^34 erg/s. The model we propose is that of a single neutron star accreting from cluster gas that has accumulated as a natural product of stellar evolution. An analytic luminosity function is derived under the assumption that the speed distribution of neutron stars in the central region of a cluster is described by a Maxwellian, and that the density and temperature of the gas are uniform. Predictions of the model and implications for the gas content of globular clusters are discussed.Comment: Accepted by ApJ. Minor changes and expanded conclusions sectio

Crustal Heating and Quiescent Emission from Transiently Accreting Neutron Stars

Nuclear reactions occurring deep in the crust of a transiently accreting neutron star efficiently maintain the core at a temperature >5e7 K. When accretion halts, the envelope relaxes to a thermal equilibrium set by the flux from the hot core, as if the neutron star were newly born. For the time-averaged accretion rates typical of low-mass X-ray transients, standard neutrino cooling is unimportant and the core thermally re-radiates the deposited heat. The resulting luminosity has the same magnitude as that observed from several transient neutron stars in quiescence. Confirmation of this mechanism would strongly constrain rapid neutrino cooling mechanisms for neutron stars. Thermal emission had previously been dismissed as a predominant source of quiescent emission since blackbody spectral fits implied an emitting area much smaller than a neutron star's surface. However, as with thermal emission from radio pulsars, fits with realistic emergent spectra will imply a substantially larger emitting area. Other emission mechanisms, such as accretion or a pulsar shock, can also operate in quiescence and generate intensity and spectral variations over short timescales. Indeed, quiescent accretion may produce gravitationally redshifted metal photoionization edges in the quiescent spectra (detectable with AXAF and XMM). We discuss past observations of Aql~X-1 and note that the low luminosity X-ray sources in globular clusters and the Be star/X-ray transients are excellent candidates for future study.Comment: 5 pages, 2 ps figures, uses AASTEX macros. To appear in ApJ letters, 10 September 1998. Revised to conform with journal; minor numerical correction

ROSAT HRI observations of the globular clusters M13 and M92

We report on 40 kiloseconds of ROSAT HRI observations of the globular cluster M92 (NGC 6341) and 20 kiloseconds of observations of the globular cluster M13 (NGC 6205). We find that the low-luminosity (10^{32.5} erg/sec at 7.5 kpc) source previously observed near the core of M92 with the ROSAT PSPC remains unresolved at HRI resolution; we can identify it with M92 with 99 per cent confidence. In M13 we find that the source seen with the ROSAT PSPC lies within the core and is possibly associated with the cluster (96 per cent confidence). We find probabilities of 99.8 per cent (M92) and 98 per cent (M13) for the presence of additional unresolved emission within these globular clusters. We interpret these results in light of current theories regarding the low-luminosity sources

Cataclysmic Variables and Other Compact Binaries in the Globular Cluster NGC 362: Candidates from Chandra and HST

Highly sensitive and precise X-ray imaging from Chandra, combined with the superb spatial resolution of HST optical images, dramatically enhances our empirical understanding of compact binaries such as cataclysmic variables and low mass X-ray binaries, their progeny, and other stellar X-ray source populations deep into the cores of globular clusters. Our Chandra X-ray images of the globular cluster NGC 362 reveal 100 X-ray sources, the bulk of which are likely cluster members. Using HST color-magnitude and color-color diagrams, we quantitatively consider the optical content of the NGC 362 Chandra X-ray error circles, especially to assess and identify the compact binary population in this condensed-core globular cluster. Despite residual significant crowding in both X-rays and optical, we identify an excess population of H{\alpha}-emitting objects that is statistically associated with the Chandra X-ray sources. The X-ray and optical characteristics suggest that these are mainly cataclysmic variables, but we also identify a candidate quiescent low mass X-ray binary. A potentially interesting and largely unanticipated use of observations such as these may be to help constrain the macroscopic dynamic state of globular clusters.Comment: 6 pages, 6 figures, to appear in the proceedings of the conference "Binary Star Evolution: Mass Loss, Accretion, and Mergers," Mykonos, Greece, June 22-25, 201