We have searched for optical identifications for 79 Chandra X-ray sources that lie within the halfmass radius of the nearby, core-collapsed globular cluster NGC 6397, using deep Hubble Space Telescope Advanced Camera for Surveys Wide Field Channel imaging in Hα, R, and B. Photometry of these images allows us to classify candidate counterparts based on color-magnitude diagram location. In addition to recovering nine previously detected cataclysmic variables (CVs), we have identified six additional faint CV candidates, a total of 42 active binaries (ABs), two millisecond pulsars (MSPs), one candidate active galactic nucleus, and one candidate interacting galaxy pair. Of the 79 sources, 69 have a plausible optical counterpart.The 15 likely and possible CVs in NGC 6397 mostly fall into two groups: a brighter group of six for which the optical emission is dominated by contributions from the secondary and accretion disk, and a fainter group of seven for which the white dwarf dominates the optical emission. There are two possible transitional objects that lie between these groups. The faintest CVs likely lie near the minimum of the CV period distribution, where an accumulation is expected. The spatial distribution of the brighter CVs is much more centrally concentrated than those of the fainter CVs and the active binaries. This may represent the result of an evolutionary process in which CVs are produced by dynamical interactions, such as exchange reactions, near the cluster center and are scattered to larger orbital radii, over their lifetimes, as they age and become fainter.

We report the discovery of peculiar X-ray spectral variability in the binary radio millisecond pulsar PSR J0024À7204W in the globular cluster 47 Tuc. The observed emission consists of a dominant nonthermal component, which is eclipsed for a portion of the orbit, and a thermal component, which appears to be persistent. We propose that the nonthermal X-rays originate in a relativistic intrabinary shock, formed due to interaction between the relativistic particle wind from the pulsar and matter from the main-sequence companion star, while the thermal photons are from the heated magnetic polar caps of the millisecond pulsar. At optical wavelengths, the emission exhibits large-amplitude variations at the orbital period, which can be attributed to heating of one side of the tidally locked secondary star by the pulsar wind. The observed X-ray and optical properties of PSR J0024À7204W are remarkably similar to those of the low-mass X-ray binary and X-ray millisecond pulsar SAX J1808.4À3658 in quiescence. This supports the conjecture that the nonthermal X-ray emission and optical modulations seen in the SAX J1808.4À3658 system in a quiescent state are due to interaction between the wind from a reactivated rotationpowered pulsar and matter from the companion star. The striking similarities between the two systems provide support for the long-sought connection between millisecond radio pulsars and accreting neutron star systems.

We describe the X-ray analysis procedure of the on-going Chandra Multiwavelength Plane (ChaMPlane) survey and report the initial results from the analysis of 15 selected anti-Galactic center observations (90 • < l < 270 • ). We describe the X-ray analysis procedures for ChaMPlane using custom-developed analysis tools appropriate for Galactic sources but also of general use: optimum photometry in crowded fields using advanced techniques for overlapping sources, rigorous astrometry and 95% error circles for combining X-ray images or matching to optical/IR images, and application of quantile analysis for spectral analysis of faint sources. We apply these techniques to 15 anti-Galactic center observations (of 14 distinct fields) in which we have detected 921 X-ray point sources. We present logN-logS distributions and quantile analysis to show that in the hard band (2 -8 keV) active galactic nuclei dominate the sources. Complete analysis of all ChaMPlane anti-Galactic center fields will be given in a subsequent paper, followed by papers on sources in the Galactic center and Bulge regions.

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. 6 For an up-to-date list of all known globular cluster MSPs see

We report the discovery of two spectroscopic binaries in the field of the old open cluster M67-S1063 and S1113-whose positions in the color-magnitude diagram place them %1 mag below the subgiant branch. A ROSAT study of M67 independently discovered these stars to be X-ray sources. Both have proper-motion membership probabilities greater than 97%; precise center-of-mass velocities are consistent with the cluster mean radial velocity. S1063 is also projected within one core radius of the cluster center. It is a single-lined binary with a period of 18.396 days and an orbital eccentricity of 0.206. S1113 is a double-lined system with a circular orbit having a period of 2.823094 days. The primary stars of both binaries are subgiants. The secondary of S1113 is likely a 0.9 M main-sequence star, which implies a 1.3 M primary star. We have been unable to explain securely the low apparent luminosities of the primary stars. The colors of S1063 suggest 0.15 mag higher reddening than found for either M67 or through the entire Galaxy in the direction of M67. S1063 could be explained as an extincted M67 subgiant, although the origin of such enhanced extinction is unknown. The photometric properties of S1113 are well modeled by a cluster binary with a 0.9 M mainsequence secondary star. However, the low composite luminosity requires a small (2.0 R ) primary star that would be supersynchronously rotating, in contrast to the short synchronization timescales, the circular orbit, and the periodic photometric variability with the orbital period. Geometric arguments based on a tidally relaxed system suggest a larger (4.0 R ) primary star in a background binary, but such a large star violates the observed flux ratio. Thus, we have not been able to find a compelling solution for the S1113 system. We speculate that S1063 and S1113 may be the products of close stellar encounters involving binaries in the cluster environment and may define alternative stellar evolutionary tracks associated with mass transfer episodes, mergers, and/or dynamical stellar exchanges.

We report the results of 19 years of Arecibo timing for two pulsars in the globular cluster NGC 5904 (M5), PSR B1516+02A (M5A) and PSR B1516+02B (M5B). This has resulted in the measurement of the proper motions of these pulsars and, by extension, that of the cluster itself. M5B is a 7.95 ms pulsar in a binary system with a >0.13 M companion and an orbital period of 6.86 days. In deep HST images, no optical counterpart is detected within $2.5 of the position of the pulsar, implying that the companion is either a white dwarf or a low-mass main-sequence star. The eccentricity of the orbit (e ¼ 0:14) has allowed a measurement of the rate of advance of periastron:! ¼ 0:0142 AE 0:0007 yr À1 . We argue that it is very likely that this periastron advance is due to the effects of general relativity, the total mass of the binary system then being 2:29 AE 0:17 M . The small measured mass function implies, in a statistical sense, that a very large fraction of this total mass is contained in the pulsar: M p ¼ 2:08 AE 0:19 M (1 ); there is a 5% probability that the mass of this object is <1.72 M and a 0.77% probability that 1:2 M M p 1:44 M . Confirmation of the median mass for this neutron star would exclude most ''soft'' equations of state for dense neutron matter. Millisecond pulsars (MSPs) appear to have a much wider mass distribution than is found in double neutron star systems; about half of these objects are significantly more massive than 1.44 M . A possible cause is the much longer episode of mass accretion necessary to recycle a MSP, which in some cases corresponds to a much larger mass transfer.

We report the detection of steady radio emission from the known X-ray source X9 in the globular cluster 47 Tuc. With a double-peaked C IV emission line in its ultraviolet spectrum providing a clear signature of accretion, this source had been previously classified as a cataclysmic variable. In deep ATCA imaging from 2010 and 2013, we identified a steady radio source at both 5.5 and 9.0 GHz, with a radio spectral index (defined as S ν ∝ ν α ) of α = −0.4 ± 0.4. Our measured flux density of 42 ± 4 µJy beam −1 at 5.5 GHz implies a radio luminosity (νL ν ) of 5.8 × 10 27 erg s −1 , significantly higher than any previous radio detection of an accreting white dwarf. Transitional millisecond pulsars, which have the highest radio-to-X-ray flux ratios among accreting neutron stars (still a factor of a few below accreting black holes at the same L X ), show distinctly different patterns of X-ray and radio variability than X9. When combined with archival X-ray measurements, our radio detection places 47 Tuc X9 very close to the radio/X-ray correlation for accreting black holes, and we explore the possibility that this source is instead a quiescent stellar-mass black hole X-ray binary. The nature of the donor star is uncertain; although the luminosity of the optical counterpart is consistent with a low-mass main sequence donor star, the mass transfer rate required to produce the high quiescent X-ray luminosity of 10 33 erg s −1 suggests the system may instead be ultracompact, with an orbital period of order 25 minutes. This is the fourth quiescent black hole candidate discovered to date in a Galactic globular cluster, and the only one with a confirmed accretion signature from its optical/ultraviolet spectrum.

We present a database of BV RI time-series photometry of the Orion Nebula Cluster obtained with two ground-based telescopes at different longitudes to provide simultaneous coverage with the 13-d Chandra observation of the cluster. The resulting database of simultaneous optical and X-ray light curves for some 800 pre-main-sequence (PMS) stars represents, by a factor of hundreds, the largest synoptic, multi-wavelength-regime, time-series study of young stars to date. This database will permit detailed analyses of the relationship between optical and X-ray variability among a statistically significant ensemble of PMS stars, with the goal of elucidating the origins of PMS X-ray production. In this first paper, we present the optical observations, describe the combined X-ray/optical database, and perform an analysis of time-correlated variability in the optical and X-ray light curves. We identify 40 stars (representing 5% of our study sample) with possible time-correlated optical and X-ray variability. Examples of both positive and negative time-correlations are found, possibly representing X-ray flares and persistent coronal features associated with both cool and hot surface spots (i.e. magnetically active regions and accretion shocks). We also find two possible examples of "white-light" flares coincident with X-ray flares; these may correspond to the impulsive heating phase in solar-analog flares. However, though interesting, these represent unusual cases. More generally, we find very little evidence to suggest a direct causal link between the sources of optical and X-ray variability in PMS stars. The conclusion that accretion is a primary driver of X-ray production in PMS stars is not supported by our findings.