Eric W. Deutsch

Empirical uncertainty estimators for astrometry from digital databases

In order to understand the positional uncertainties of arbitrary objects in several of the current major databases containing astrometric information, a sample of extragalactic radio sources with precise positions in the International Celestial Reference Frame is compared with the available positions of their optical counterparts. The discrepancies between the radio and various optical positions are used to derive empirical uncertainty estimators for the USNO-A2.0 and USNO-A1.0 catalogs, the Guide Star Selection System images, and the first and second Digitized Sky Surveys. In addition, an estimate of the uncertainty when the USNO-A2.0 catalog is transferred to different image data is provided. These optical astrometric frame uncertainties can in some cases be the dominant error term when cross-identifying sources at different wavelengths.

Ultracompact X-Ray Binaries in Globular Clusters: Variability of the Optical Counterpart of X1832–330 in NGC 6652*

Evidence is emerging that the luminous X-ray sources in the cores of globular clusters may often consist of, or perhaps even as a class be dominated by, ultracompact (P less, similar1 hr) binary stars. To the two such systems already known, in NGC 6624 and NGC 6712, we now add evidence for two more. We detect large-amplitude variability in the candidate optical counterpart for the X-ray source in the core of NGC 6652. Although the available observations are relatively brief, the existing Hubble Space Telescope data indicate a strong 43.6 minute periodic modulation of the visible flux of semiamplitude 30%. Further, although the orbital period of the source in NGC 1851 is not yet explicitly measured, we demonstrate that previous correlations of optical luminosity with X-ray luminosity and accretion disk size, strengthened by recent data, strongly imply that the period of that system is also less than 1 hr. Thus, currently there is evidence that four of the seven globular cluster X-ray sources with constrained periods are ultracompact, a fraction far greater than that found in X-ray binaries the field.

Faint High-Latitude Carbon Stars Discovered by the Sloan Digital Sky Survey: Methods and Initial Results

We report the discovery of 39 faint high-latitude carbon stars (FHLCs) from Sloan Digital Sky Survey (SDSS) commissioning data. The objects, each selected photometrically and verified spectroscopically, range over 16.6 < r* < 20.0 and show a diversity of temperatures as judged by both colors and NaD line strengths. Although a handful of these stars were previously known, these objects are, in general, too faint and too warm to be effectively identified in other modern surveys such as the Two Micron All Sky Survey, nor are their red/near-IR colors particularly distinctive. The implied surface density of FHLCs in this magnitude range is uncertain at this preliminary stage of the survey because of completeness corrections but is clearly greater than 0.05 deg-2. At the completion of the Sloan survey, there will be many hundred homogeneously selected and observed FHLCs in this sample. We present proper-motion measures for each object, indicating that the sample is a mixture of extremely distant (greater than 100 kpc) halo giant stars, useful for constraining halo dynamics, and members of the recently recognized exotic class of very nearby dwarf carbon (dC) stars. The broadband colors of the two populations are indistinguishable. Motions, and thus dC classification, are inferred for 40%–50% of the sample, depending on the level of statistical significance invoked. The new list of dC stars presented here, although selected from only a small fraction of the final SDSS, doubles the number of such objects found by all previous methods. The observed kinematics suggest that the dwarfs occupy distinct halo and disk populations. The coolest FHLCs with detectable proper motions in our sample also display multiple CaH bands in their spectra. It may be that CaH is another long-sought, low-resolution, spectroscopic luminosity discriminant between dCs and distant faint giants, at least for the cooler stars.

Time-Resolved Ultraviolet Observations of the Globular Cluster X-Ray Source in NGC 6624: The Shortest Known Period Binary System

Using the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope (HST), we have obtained the first time-resolved spectra of the King et al. (1993) ultraviolet-bright counterpart to the 11 minute binary X-ray source in the core of the globular cluster NGC 6624. This object cannot be readily observed in the visible, even from HST, because of a much brighter star superposed less than 01 away. Our FOS data show a highly statistically significant ultraviolet flux modulation with a period of 11.46 ± 0.04 minutes, very similar to the 685 s period of the known X-ray modulation, definitively confirming the association between the King et al. (1993) ultraviolet counterpart and the intense X-ray source. The ultraviolet amplitude is very large compared with the observed X-ray oscillations: X-ray variations are generally reported as 2%-3% peak to peak, whereas our data show an amplitude of about 16% in the 126-251 nm range. A model for the system by Arons & King (1993) predicts periodic ultraviolet fluctuations in this shortest known period binary system, because of the cyclically changing aspect of the X-ray heated face of the secondary star (perhaps a very low mass helium degenerate). However, prior to our observations, this predicted modulation has not been detected. Employing the Arons & King (1993) formalism, which invokes a number of different physical assumptions, we infer a system orbital inclination 35° i 50°. Among the three best-studied ultraviolet/optical counterparts to the intense globular cluster X-ray sources, two are now thought to consist of exotic double-degenerate ultrashort-period binary systems.

Infrared Candidates for the Intense Galactic X-Ray Source GX 17+2

We present new astrometric solutions and infrared Hubble Space Telescope observations of GX 17+2 (X1813-140), one of the brightest X-ray sources on the celestial sphere. Despite 30 years of intensive study and the existence of a strong radio counterpart with a subarcsecond position, the object remains optically unidentified. The observed X-ray characteristics strongly suggest that it is a so-called Z source, the rare but important category that includes Sco X-1 and Cyg X-2. Use of the USNO-A2.0 catalog enables us to measure the position of optical and infrared objects near the radio source to subarcsecond precision within the International Celestial Reference Frame for direct comparison with the radio position, which we also recompute using modern calibrators. With high confidence we eliminate the V ~ 17.5 star NP Ser, often listed as the probable optical counterpart of the X-ray source, as a candidate. Our HST NICMOS observations show two faint objects within our 05 radius 90% confidence error circle. Even the brighter of the two, star A, is far fainter than expected (H ≈ 19.8), given multiple estimates of the extinction in this field and our previous understanding of Z sources, but it becomes the best candidate for the counterpart of GX 17+2. The probability of a chance coincidence of an unrelated faint object on the radio position is high. However, if the true counterpart is not star A, it is fainter still, and our conclusion that the optical counterpart is surprisingly underluminous is but strengthened.

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 ~ 1036 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 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 σ X-ray error circle and allow us to set limits M > 5.9 and M > 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 117 from the ROSAT X-ray position. This considerable (2.3 σ) 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 m439 = 20.2 ± 0.2 and m336 - m439 = -0.5 ± 0.2 and estimate M = 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.

Hubble Space Telescope Imaging of Bright Galactic X-Ray Binaries in Crowded Fields

We report high spatial resolution HST imagery and photometry of three well-studied, intense Galactic X-ray binaries, X2129 + 470, CAL 87, and GX 17 + 2. All three sources exhibit important anomalies that are not readily interpreted by conventional models. Each source also lies in a severely crowded field, and in all cases the anomalies would be removed if much of the light observed from the ground in fact came from a nearby, thus far unresolved superposed companion. For V1727 Cyg (X2129] 470), we find no such companion. We also present an HST FOS spectrum and broadband photometry which is consistent with a single, normal star. The supersoft LMC X-ray source CAL 87 was already known from ground-based work to have a companion separated by 09 from the optical counterpart; our HST images clearly resolve these objects and yield the discovery of an even closer, somewhat fainter additional companion. Our photometry indicates that contamination is not severe outside eclipse, where the companions only contribute 20% of the light in V, but during eclipse more than half of the V light comes from the companions. The previously determined spectral type of the CAL 87 secondary may need to be reevaluated due to this significant contamination, with consequences on inferences of the mass of the components. We find no companions to NP Ser (= X1813–14, = GX 17 + 2). However, for this object we point out a small but possibly significant astrometric discrepancy between the position of the optical object and that of the radio source which is the basis for the identification. This discrepancy needs to be clarified.

POSITIONS AND PROPER MOTIONS OF DWARF CARBON STARS

Recent-epoch positions and proper motions of nine dwarf carbon star candidates are presented along with finding charts for each object. Measurements are obtained from digitized POSS and Quick V plate archives at the Space Telescope Science Institute, and from recent CCD images.

Optical Identification of the X-Ray Burster X1746?370 in the Globular Cluster NGC 6441

We find convincing observational evidence to confirm the optical identification of the X-ray burster X1746-370, located in the globular cluster NGC 6441. Chandra HRC-I imaging yields a much improved X-ray position for the source, which we show to be fully consistent with our rederived position of a UV-excess star, U1, in the same astrometric reference frame. In addition, the smaller Chandra X-ray error circle excludes the only other blue stars previously identified in the old Einstein circle. We have also obtained Hubble Space Telescope STIS time-resolved optical spectra of star U1. Although there are no strong line features, the flux distribution demonstrates U1 to be unusually bright in the blue and faint in the red, consistent with earlier WFPC2 photometry. More notably, the flux level of the continuum is seen to vary significantly compared with stars of similar brightness. Indeed, the light curve can plausibly be fitted by a sinusoid with period 5.73 hr, which is the period of the recurring X-ray dips seen in this source. The presence of modulations in both wavelengths strengthens the case for an orbital origin and, therefore, deepens the puzzle of the unusual energy-independent X-ray dips. Lastly, we note that X1746-370 remains the longest-period confirmed X-ray burster in a globular cluster, and the only one with a period typical of the Galactic population as a whole.

The Optical Afterglow of GRB 971214: [ITAL]R[/ITAL] and [ITAL]J[/ITAL] Photometry

We present an R-band and J-band photometry of an optical transient which is likely to be associated with the gamma-ray burst event GRB 971214. Our first measurement took place 13 hours after the gamma-ray event. The brightness decayed with a power-law exponent alpha = -1.20 +- 0.02, which is similar to those of GRB 970228 and GRB 970508 which had exponents of alpha = -1.10 +- 0.04 and alpha = -1.141 +- 0.014 respectively. The transient decayed monotonically during the first four days following the gamma-ray event in contrast with the optical transient associated with GRB 970508 which increased in brightness, peaking two days after the burst, before settling to a power-law decay.