Discovery of a candidate quiescent low-mass X-ray binary in the globular cluster NGC 6553

This paper reports the search for quiescent low-mass X-ray binaries (qLMXBs) in the globular cluster (GC) NGC 6553 using an XMM-Newton observation designed specifically for that purpose. We spectrally identify one candidate qLMXB in the core of the cluster, based on the consistency of the spectrum with a neutron star H-atmosphere model at the distance of NGC 6553. Specifically, the best-fit radius found using the three XMM European Photon Imaging Camera spectra is R_NS=6.3(+2.3)(-0.8) km (for M_NS=1.4 Msun) and the best-fit temperature is kTeff=136 (+21)(-34) eV. Both physical parameters are in accordance with typical values of previously identified qLMXBs in GC and in the field, i.e., R_NS~5-20 km and kTeff~50-150 eV. A power-law (PL) component with a photon index Gamma=2.1(+0.5)(-0.8) is also required for the spectral fit and contributes to ~33% of the total flux of the X-ray source. A detailed analysis supports the hypothesis that the PL component originates from nearby sources in the core, unresolved with XMM. The analysis of an archived Chandra observation provides marginal additional support to the stated hypothesis. Finally, a catalog of all the sources detected within the XMM field of view is presented here.Comment: 10 pages, 5 figures, 3 tables. Accepted to ApJ (to be published in August 2011

A Chandra X-ray Observatory Study of PSR J1740--5340 and Candidate Millisecond Pulsars in the Globular Cluster NGC 6397

We present a deep Chandra X-ray Observatory study of the peculiar binary radio millisecond pulsar PSR J1740--5340 and candidate millisecond pulsars (MSPs) in the globular cluster NGC 6397. The X-rays from PSR J1740--5340 appear to be non-thermal and exhibit variability at the binary period. These properties suggest the presence of a relativistic intrabinary shock formed due to interaction of a relativistic rotation-powered pulsar wind and outflow from the unusual "red-straggler/sub-subgiant" companion. We find the X-ray source U18 to show similar X-ray and optical properties to those of PSR J1740--5340, making it a strong MSP candidate. It exhibits variability on timescales from hours to years, also consistent with an intrabinary shock origin of its X-ray emission. The unprecedented depth of the X-ray data allows us to conduct a complete census of MSPs in NGC 6397. Based on the properties of the present sample of X-ray--detected MSPs in the Galaxy we find that NGC 6397 probably hosts no more than 6 MSPs.Comment: 10 pages, 6 figures, 3 tables; accepted for publication in The Astrophysical Journa

Discovery of a Second Transient Low-Mass X-ray Binary in the Globular Cluster NGC 6440

We have identified a new transient luminous low-mass X-ray binary, NGC 6440 X-2, with Chandra/ACIS, RXTE/PCA, and Swift/XRT observations of the globular cluster NGC 6440. The discovery outburst (July 28-31, 2009) peaked at L_X~1.5*10^36 ergs/s, and lasted for <4 days above L_X=10^35 ergs/s. Four other outbursts (May 29-June 4, Aug. 29-Sept. 1, Oct. 1-3, and Oct. 28-31 2009) have been observed with RXTE/PCA (identifying millisecond pulsations, Altamirano et al. 2009a) and Swift/XRT (confirming a positional association with NGC 6440 X-2), with similar peak luminosities and decay times. Optical and infrared imaging did not detect a clear counterpart, with best limits of V>21, B>22 in quiescence from archival HST imaging, g'>22 during the August outburst from Gemini-South GMOS imaging, and J>~18.5$ and K>~17 during the July outburst from CTIO 4-m ISPI imaging. Archival Chandra X-ray images of the core do not detect the quiescent counterpart, and place a bolometric luminosity limit of L_{NS}< 6*10^31 ergs/s (one of the lowest measured) for a hydrogen atmosphere neutron star. A short Chandra observation 10 days into quiescence found two photons at NGC 6440 X-2's position, suggesting enhanced quiescent emission at L_X~6*10^31 ergs/s . NGC 6440 X-2 currently shows the shortest recurrence time (~31 days) of any known X-ray transient, although regular outbursts were not visible in the bulge scans before early 2009. Fast, low-luminosity transients like NGC 6440 X-2 may be easily missed by current X-ray monitoring.Comment: 13 pages (emulateapj), 8 (color) figures, ApJ in press. Revised version adds 5th outburst (Oct./Nov. 2009), additional discussion of possible causes of short outburst recurrence time

X-ray Studies of Two Neutron Stars in 47 Tucanae: Toward Constraints on the Equation of State

We report spectral and variability analysis of two quiescent low mass X-ray binaries (X5 and X7, previously detected with the ROSAT HRI) in a Chandra ACIS-I observation of the globular cluster 47 Tuc. X5 demonstrates sharp eclipses with an 8.666+-0.01 hr period, as well as dips showing an increased N_H column. The thermal spectra of X5 and X7 are well-modeled by unmagnetized hydrogen atmospheres of hot neutron stars. No hard power law component is required. A possible edge or absorption feature is identified near 0.64 keV, perhaps an OV edge from a hot wind. Spectral fits imply that X7 is significantly more massive than the canonical 1.4 \Msun neutron star mass, with M>1.8 \Msun for a radius range of 9-14 km, while X5's spectrum is consistent with a neutron star of mass 1.4 \Msun for the same radius range. Alternatively, if much of the X-ray luminosity is due to continuing accretion onto the neutron star surface, the feature may be the 0.87 keV rest-frame absorption complex (O VIII & other metal lines) intrinsic to the neutron star atmosphere, and a mass of 1.4 \Msun for X7 may be allowed.Comment: 16 pages, 7 figures, accepted by Ap

The Unusual X-ray Binaries of the Globular Cluster NGC 6652

Our 5 ks Chandra ACIS-S observation of the globular cluster NGC 6652 detected 7 X-ray sources, 3 of which are previously unidentified. This cluster hosts a well-known bright low-mass X-ray binary, source A (or XB 1832-330). Source B shows unusual rapid flaring variability, with an average L_X(0.5-10 keV) ~2*10^{34} ergs/s, but with minutes-long flares up to L_X=9*10^{34} ergs/s. Its spectrum can be fit by an absorbed power-law of photon index Gamma~1.24, and hardens as the countrate decreases. This suggests that part or all of the variation might be due to obscuration by the rim of a highly inclined accretion disk. Sources C and D, with L_X ~10^{33} ergs/s, have soft and unusual spectra. Source C requires a very soft component, with a spectrum peaking at 0.5 keV, which might be the hot polar cap of a magnetically accreting polar cataclysmic variable. Source D shows a soft spectrum (fit by a power-law of photon index ~2.3) with marginal evidence for an emission line around 1 keV; its nature is unclear. The faint new sources E, F, and G have luminosities of 1-2*10^{32} ergs/s, if associated with the cluster (which is likely). E and F have relatively hard spectra (consistent with power-laws with photon index ~1.5). G lacks soft photons, suggesting absorption with N_H>10^{22} cm^{-2}.Comment: ApJ, in press. 9 pages, 9 figures (3 in color

Constraints on Thermal X-ray Radiation from SAX J1808.4-3658 and Implications for Neutron Star Neutrino Emission

Thermal X-ray radiation from neutron star soft X-ray transients in quiescence provides the strongest constraints on the cooling rates of neutron stars, and thus on the interior composition and properties of matter in the cores of neutron stars. We analyze new (2006) and archival (2001) XMM-Newton observations of the accreting millisecond pulsar SAX J1808.4-3658 in quiescence, which provide the most stringent constraints to date. The X-ray spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a power-law of photon index 1.83\pm0.17, without requiring the presence of a blackbody-like component from a neutron star atmosphere. Our 2006 observation shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of 68^{+15}_{-13}% that inferred from the 2001 observation. Simultaneous fitting of all available XMM data allows a constraint on the quiescent neutron star (0.01-10 keV) luminosity of L_{NS}<1.1*10^{31} erg/s. This limit excludes some current models of neutrino emission mediated by pion condensates, and provides further evidence for additional cooling processes, such as neutrino emission via direct Urca processes involving nucleons and/or hyperons, in the cores of massive neutron stars.Comment: 5 pages, 2 figures; slight revisions, accepted by Ap