arXiv subjects

Jonathan E. Grindlay

Publications and source records attributed to Jonathan E. Grindlay.

At least 73 records · Page 4Linked to original sources

arXiv2003

An Extensive Census of HST Counterparts to Chandra X-ray Sources in the Globular Cluster 47 Tucanae. I. Astrometry and Photometry

We report the largest number of optical identifications of X-ray sources yet obtained in a single globular cluster. Using deep Chandra/ACIS-I imaging and extensive HST studies with WFPC2, we have detected optical counterparts to at least 22 cataclysmic variables (CVs) and 29 chromospherically active binaries (BY Dras and RS CVns) in 47 Tuc. These identifications are all based on tight astrometric matches between X-ray sources and objects with unusual (non main sequence) optical colors and/or optical variability. In the U vs U-V color magnitude diagram (CMD), the CVs all show evidence for blue colors compared to the main sequence, but most of them fall close to the main sequence in the V vs V-I CMD, showing that the secondary stars dominate the optical light. The X-ray detected active binaries have magnitude offsets above the main sequence that are indistinguishable from those of the much larger sample of optical variables (eclipsing and contact binaries and BY Dras) detected in the WFPC2 studies of Albrow et al. (2001). We also present the results of a new, deeper search for optical companions to MSPs. One possible optical companion to an MSP (47 Tuc T) was found, adding to the two optical companions already known. Finally, we study several blue stars with periodic variability from Albrow et al. (2001) that show little or no evidence for X-ray emission. The optical colors of these objects differ from those of 47 Tuc (and field) CVs.

astro-ph↗

arXiv2003

An Extensive Census of HST Counterparts to Chandra X-ray Sources in the Globular Cluster 47 Tucanae. II. Time Series and Analysis

We report time series and variability information for the optical identifications of X-ray sources in 47 Tuc reported in Paper I. The eight CVs with secure orbital periods have fainter accretion disks, in the V band, than field CVs with similar periods. These faint disks and the faint absolute magnitudes (Mv) of the 47 Tuc CVs suggests they have low accretion rates. The median Fx/Fopt value for the 47 Tuc CVs is higher than that of all known classes of field CV, because of both the faint Mv values and the relatively high X-ray luminosities (Lx). The latter are only seen in DQ Her systems in the field, but the 47 Tuc CVs are much fainter optically than most field DQ Hers. Some combination of magnetic behavior and low accretion rates may be able to explain our observations, but the results at present are ambiguous, since no class of field CV has distributions of both Lx and Mv that are consistent with those of the 47 Tuc CVs. The radial distribution of the X-ray detected active binaries is indistinguishable from that of the much larger sample of optical variables detected in previous WFPC2 studies. The X-ray properties of these objects are consistent with those of active binaries found in field studies. One reasonable and one marginal candidate for optical identification of a quiescent LMXB was found (one is already known). Finally, we study the blue variables showing little or no evidence for X-ray emission. Their generally long periods and the absence of flickering suggests they are not CVs. At present we have no satisfactory explanation for these objects, although some may be detached WD-main sequence star binaries.

astro-ph↗

arXiv2002

Background Measurements from Balloon-Borne CZT Detectors

We report detector characteristics and background measurements from two prototype imaging CZT detectors flown on a scientific balloon payload in May 2001. The detectors are both platinum-contact 10mm x 10mm x 5mm CZT crystals, each with a 4 $\times$ 4 array of pixels tiling the anode. One is made from IMARAD horizontal Bridgman CZT, the other from eV Products high-pressure Bridgman material. Both detectors were mounted side-by-side in a flip-chip configuration and read out by a 32-channel IDE VA/TA ASIC preamp/shaper. We enclosed the detectors in the same 40deg field-of-view collimator (comprisinga graded passive shield and plastic scintillator) used in our previously-reported September 2000 flight. I-V curves for the detectors are diode-like, and we find that the platinum contacts adhere significantly better to the CZT surfaces than gold to previous detectors. The detectors and instrumentation performed well in a 20-hour balloon flight on 23/24 May 2001. Although we discovered a significant instrumental background component in flight, it was possible to measure and subtract this component from the spectra. The resulting IMARAD detector background spectrum (from 30 keV to ~450 keV) reaches ~5 x 10^{-3}$ counts/cm^2 -sec-keV at 100 keV and has a power-law index of ~2 at high energies. The eV Products detector has a similar spectrum, although there is more uncertainty in the energy scale because of calibration complications.

astro-ph↗

arXiv2002

A Millisecond Pulsar Optical Counterpart with Large-Amplitude Variability in the Globular Cluster 47 Tucanae

Using extensive HST imaging, combined with Chandra X-ray and Parkes radio data, we have detected the optical binary companion to a second millisecond pulsar (MSP) in the globular cluster 47 Tucanae. This faint (V=22.3) blue (V-I=0.7) star shows a large amplitude (60-70%) sinusoidal variation in both V and I. The period (3.19066 hr) and phase of the variation match those of the MSP 47 Tuc W (which does not have an accurate radio timing position) to within 0.5 seconds and 1.2 minutes respectively, well within the ~1sigma errors. The phase dependence of the intensity and color implies that heating of a tidally locked companion causes the observed variations. The eclipsing nature of this MSP in the radio, combined with the relatively large companion mass (> 0.13 solar masses) and the companion's position in the color-magnitude diagram, suggest that the companion is a main sequence star, a rare circumstance for an MSP companion. This system is likely to have had a complex evolution and represents an interesting case study in MSP irradiation of a close companion. We present evidence for another optical variable with similar properties to the companion of 47 Tuc W. This variable may also be an MSP companion, although no radio counterpart has yet been detected.

astro-ph↗

arXiv2002

A two-dimensional Kolmogorov-Smirnov test for crowded field source detection: ROSAT sources in NGC 6397

We present a two-dimensional version of the classical one-dimensional Kolmogorov-Smirnov (K-S) test, extending an earlier idea due to Peacock (1983) and an implementation proposed by Fasano & Franceschini (1987). The two-dimensional K-S test is used to optimise the goodness of fit in an iterative source-detection scheme for astronomical images. The method is applied to a ROSAT/HRI x-ray image of the post core-collapse globular cluster NGC 6397 to determine the most probable source distribution in the cluster core. Comparisons to other widely-used source detection methods, and to a Chandra image of the same field, show that our iteration scheme is superior in measuring statistics-limited sources in severely crowded fields.

astro-ph↗

arXiv2002

A Chandra observation of the globular cluster Terzan 1: the neutron-star X-ray transient X 1732-304 in quiescence

We present a short (~3.6 ks) Chandra/HRC-I observation of the globular cluster Terzan 1. This cluster is known to contain the bright neutron star low-mass X-ray binary X 1732-304 which was active during the 1980's and most of the 1990's. But a BeppoSAX observation performed in 1999 only showed a very weak source indicating that the source had become quiescent. During our Chandra observation, we detect one source with a 0.5-10 keV luminosity of approximately 1 - 2 x 10^{33} erg/s (for an assumed distance of 5.2 kpc). However, its position is not consistent with that of X 1732-304. We do not conclusively detect X 1732-304 with a 0.5-10 keV luminosity upper limit of 0.5 - 1 x 10^{33} erg/s. This limit is consistent with the luminosities observed for several neutron-star X-ray transients in our Galaxy when they are quiescent, strongly suggesting that X 1732-304 was still quiescent during our Chandra observation. If the quiescent emission in neutron star X-ray transients is due to the thermal emission from the neutron star, then it is expected that the quiescent luminosity depends on the time-averaged accretion rate of the source. However, the upper limit on the quiescent luminosity of X 1732-304, combined with its very long accretion episode prior to the current quiescent episode, indicates that the quiescent episodes of the source have to be longer than ~200 years. This would be the second system after KS 1731-260 for which quiescent episodes longer than several hundreds of years have been inferred. We discuss this possibility and alternative quiescent models to explain our results.

astro-ph↗

arXiv2002

HST-NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core

We report results from HST-NICMOS imaging of the extremely dense core of the globular cluster Terzan 5. This highly obscured bulge cluster has been estimated to have one of the highest collision rates of any galactic globular cluster, making its core a particularly conducive environment for the production of interacting binary systems. We have reconstructed high-resolution images of the central 19"x19" region of Terzan 5 by application of the drizzle algorithm to dithered NIC2 images in the F110W, F187W, and F187N near-infrared filters. We have used a DAOPHOT/ALLSTAR analysis of these images to produce the deepest color-magnitude diagram (CMD) yet obtained for the core of Terzan 5. We have also analyzed the parallel 11"X11" NIC1 field, centered 30" from the cluster center and imaged in F110W and F160W, and an additional NIC2 field that is immediately adjacent to the central field. This imaging results in a clean detection of the red-giant branch and horizontal branch in the central NIC2 field, and the detection of these plus the main-sequence turnoff and the upper main sequence in the NIC1 field. We have constructed an H versus J-H CMD for the NIC1 field. We obtain a new distance estimate of 8.7 kpc, which places Terzan 5 within less than 1 kpc of the galactic center. We have also determined a central surface-density profile which results in a maximum likelihood estimate of 7.9" +/- 0.6" for the cluster core radius. We discuss the implications of these results for the dynamical state of Terzan 5.

astro-ph↗

arXiv2001

Detection of a Quiescent Low Mass X-Ray Binary Companion in 47 Tucanae

We present the results of a search for optical counterparts to the two quiescent low mass X-ray binaries (X5 and X7) in the globular cluster 47 Tucanae, using high quality Chandra and HST images. A faint blue (V=21.7; U-V=0.9) star within 0.03" of the eclipsing system X5 shows variability on both short and long timescales, and is the counterpart of the X-ray source. The colors and variability of this object are consistent with the combination of light from an accretion disk and a red main sequence star (possibly somewhat larger than a normal MS star with similar luminosity). No evidence is found for a star showing either variability or unusual colors near the position of X7, but a probable chance superposition of a star with V=20.25 limits the depth of our search.

astro-ph↗

arXiv2001

Optical Detection of a Variable Millisecond Pulsar Companion in 47 Tucanae

Using results from radio and X-ray observations of millisecond pulsars in 47 Tucanae, and extensive HST U, V, I imaging of the globular cluster core, we have derived a common astrometric solution good to < 0.1". A close positional coincidence is found for 47 Tuc U, a 4.3 ms pulsar in a 0.429 day orbit, detected in radio and X-rays, with an m_V = 20.9 blue star. Analysis of extensive time series data for this optical candidate shows a 0.004 magnitude semi-amplitude variation at the period and phase expected from the radio ephemeris, and the optical variations are spatially coincident with the candidate. This provides secure optical detection of the white dwarf companion to the millisecond pulsar, the first such detection in a globular cluster, allowing for comparisons to recent models for such companions with dependencies on mass and age.

astro-ph↗

arXiv2001

High Resolution X-ray Imaging of a Globular Cluster Core: Compact Binaries in 47Tuc

We have obtained high resolution (<1") deep x-ray images of the globular cluster 47Tucanae (NGC 104) with the Chandra X-ray Observatory to study the population of compact binaries in the high stellar density core. A 70 kilosec exposure of the cluster reveals a centrally concentrated population of faint (Lx ~10^{30-33} erg/s) x-ray sources, with at least 108 located within the central 2arcmin X 2.5arcmin and at least half with Lx <10^{30.5} erg/s. All 15 millisecond pulsars (MSPs) recently located precisely by radio observations are identified, though two are unresolved by Chandra. The x-ray spectral and temporal characteristics, as well as initial optical identifications with the Hubble Space Telescope, suggest that >~50% are millisecond pulsars, ~30% are accreting white dwarfs, ~15% are main sequence binaries in flare outbursts and only 2 to 3 are quiescent low mass x-ray binaries containing neutron stars, the conventional progenitors of MSPs. An approximate upper limit of ~470Msun for the mass of an accreting central black hole in the cluster is derived. These observations provide the first x-ray "color-magnitude" diagram for a globular cluster and census of its compact object and binary population.

astro-ph↗

arXiv1999

Magnetic CVs in Globular Clusters

Cataclysmic variables (CVs) are tracers of dynamical evolution in globular clusters. A significant sample has been found by the relatively shallow \Halpha surveys conducted with HST thus far in just three globulars: NGC 6397 (3), NGC 6752 (2), and $ω$-Cen (2). These are identified with the low luminosity x-ray sources discovered in these globulars with similarly moderate-depth ROSAT surveys. Follow-up spectroscopy of the initial 3 CV candidates in NGC 6397 with the FOS on HST suggests they may be magnetic CVs of the intermediate polar (IP) type, and follow-up deep imaging with both HST and ROSAT yield a 4th confirmed and 5th strong candidate CV near the center of this collapsed core cluster. The optical vs. x-ray properties of the 4 confirmed IP candidates in NGC 6397 are compared with disk IPs, and arguments for and against this interpretation are presented. The possible strong excess of magnetic CVs in globulars will be tested with much deeper HST/AXAF surveys.

astro-ph↗

arXiv1998

Cataclysmic Variables and a Candidate Helium White Dwarf in the Globular Cluster NGC 6397

We have used HST/FOS to study faint UV stars in the core of the nearby globular cluster NGC 6397. We confirm the presence of a 4th cataclysmic variable (CV) in NGC 6397 (CV 4), and we use the photometry of Cool et al. (1998) to present evidence that CVs 1--4 all have faint disks and probably low accretion rates. By combining these results with new UV spectra of CV 1 and the published spectra of Grindlay et al. (1995) we present new evidence that CVs 1--3 may be DQ Her systems, and we show that CV 4 may either be a dwarf nova or another magnetic system. Another possibility is that the CVs could be old novae in hibernation between nova eruptions. We also present the first spectrum of a member of a new class of UV bright stars in NGC 6397. These faint, hot stars do not vary, unlike the CVs, and are thus denoted as ``non-flickerers'' (NFs). Like the CVs, their spatial concentration is strongly concentrated toward the cluster center. Using stellar atmosphere models we have determined log g = 6.25, and T_eff = 17,500 K for this NF. Using these line parameters and the luminosity of the NF we show that the NF spectrum is consistent with a helium WD having a mass of ~0.25 solar masses and an age between 0.1 and 0.5 Gyr (depending on the models used). The NF spectrum appears to be significantly Doppler shifted from the expected wavelength, suggesting the presence of a dark, massive companion, probably a carbon-oxygen WD.

astro-ph↗

arXiv1998

Cataclysmic Variables and a New Class of Faint UV Stars in the Globular Cluster NGC 6397

We present evidence that the globular cluster NGC 6397 contains two distinct classes of centrally-concentrated UV-bright stars. Color-magnitude diagrams constructed from U, B, V, and I data obtained with the HST/WFPC2 reveal seven UV-bright stars fainter than the main-sequence turnoff, three of which had previously been identified as cataclysmic variables (CVs). Lightcurves of these stars show the characteristic ``flicker'' of CVs, as well as longer-term variability. A fourth star is identified as a CV candidate on the basis of its variability and UV excess. Three additional UV-bright stars show no photometric variability and have broad-band colors characteristic of B stars. These non-flickering UV stars are too faint to be extended horizontal branch stars. We suggest that they could be low-mass helium white dwarfs, formed when the evolution of a red giant is interrupted, due either to Roche-lobe overflow onto a binary companion, or to envelope ejection following a common-envelope phase in a tidal-capture binary. Alternatively, they could be very-low-mass core-He-burning stars. Both the CVs and the new class of faint UV stars are strongly concentrated toward the cluster center, to the extent that mass segregation from 2-body relaxation alone may be unable to explain their distribution.

astro-ph↗

arXiv1998

Fast X-ray Transients and Gamma-ray Bursts: Constraints on Beaming

The detection of x-ray afterglows of gamma-ray burst (GRB) sources has enabled the optical discovery that GRBs are at cosmological distances, which confirms that the enormous luminosities required must produce a relativistically expanding fireball. It is not yet clear if this expansion is quasi-spherical or beamed, or if the beaming differs between the prompt GRB and afterglow, although the differences for both source emission and population models are profound. We show that relative beaming (prompt vs. afterglow) may be constrained by analysis of the "fast transient" x-ray sources catalogued by Pye and McHardy (1983) from the Ariel V survey as well as several other (e.g. HEAO-A1/A2) surveys. After removing contamination from likely stellar transients, and scaling from x-ray/$γ$-ray fluence ratios recently derived from Ginga and BeppoSAX, approximately half of the sources were possible prompt emission or afterglows from GRBs with gamma-ray fluence of >~1.2 X 10$^{-5} /erg-cm^2. Since the rate of these candidate afterglow events, 0.15 /day, is consistent with the BATSE logN-logS distribution, no difference in prompt vs. afterglow beaming is required. However more sensitive x-ray monitors or hard x-ray imaging surveys, with better temporal and spatial resolution, could provide strong constraints on GRBs and beaming.

astro-ph↗

arXiv1998

Spin-Up of Low Luminosity Low Mass X-ray Binaries

We examine the spin-up of low luminosity, low mass X-ray binaries (LMXBs) to millisecond pulsars (MSPs). In the conventional spin-up model of the Ghosh & Lamb type, where the stellar magnetic field interacts with the Keplerian accretion disk, MSPs could be produced from LMXBs if their magnetic field B < 10^{8}({\dot M}/10^{16}g/s)^{1/2}G, where {\dot M} is the mass accretion rate. However, for {\dot M} < {\dot M}_c ~ 10^{16}g/s accretion is likely to occur via a quasi-spherical flow with a sub-Keplerian rotation. The sub-Keplerian rotation rate is smaller than the Keplerian rate by a factor ~2-10. As a consequence, the spin-up of LMXBs produces pulsars with spin periods longer by a factor ~2-10 than those with a Keplerian accretion disk. The observed MSPs could be produced only for B < 10^7G even when {\dot M}\sim {\dot M}_c ~ 10^{16}g/s. This suggests that the low luminosity LMXBs with {\dot M} < {\dot M}_c would not be able to spin-up to the observed MSPs. This rules out any undetected populations of persistent, low luminosity LMXBs and potentially a significant fraction of the atoll sources as a possible source population of the observed MSPs. If a large number of undetected, persistent, low luminosity LMXBs do exist, they could produce MSPs near the pulsar death line with intrinsic electromagnetic luminosity <~ 10^{30} erg/s. The observed MSPs could possibly arise from a population of soft x-ray transients (SXTs) containing neutron stars although this is not supported by current estimates of the outburst rate or numbers of such systems. Accretion induced collapse of low magnetic field white dwarfs remains a possible channel for MSP formation.

astro-ph↗

arXiv1998

Surveying the Hard X-ray Sky: Imaging in Space and Time

One of the few remaining astronomical bands (factor of $\sim$10 in energy range) still without an all-sky imaging survey is the hard x-ray band (10-600 keV). This is in spite of sensitive imaging all-sky surveys already conducted at soft x-ray energies (0.2-2 keV; ROSAT) and soft/hard $γ$-ray ($\sim$750 keV - 10 GeV; COMPTEL/EGRET) energies and imminent for medium x-ray energies (2- 10 keV; ABRIXAS). A Hard x-ray imaging survey conducted with wide-field coded aperture telescopes allows both high sensitivity (and spatial/spectral resolution) and broad temporal coverage. We derive a generalized survey sensitivity/temporal parameter, Q, and compare previous and planned hard x-ray surveys with the proposed EXIST mission. Key scientific objectives that could be addressed with the enhanced capability of EXIST are outlined.

astro-ph↗

arXiv1997

Simulations and Measurements of the Background Encountered by a High-Altitude Balloon-Borne Experiment for Hard X-ray Astronomy

We have modelled the hard X-ray background expected for a high-altitude balloon flight of the Energetic X-ray Telescope Experiment (EXITE2), an imaging phoswich detector/telescope for the 20--600 keV energy range. Photon and neutron-induced contributions to the background are considered. We describe the code and the results of a series of simulations with different shielding configurations. The simulated hard X-ray background for the actual flight configuration agrees reasonably well (within a factor of $\sim$ 2) with the results measured on the first flight of EXITE2 from Palestine, Texas. The measured background flux at 100 keV is $\sim$ 4 $\times$ 10$^{-4}$ counts cm$^{-2}$ s$^{-1}$ keV$^{-1}$.

astro-ph↗

arXiv1997

Balloon-Borne Hard X-Ray Imaging and Future Surveys

Several payloads for hard X-ray (20-600 keV) imaging with coded aperture telescopes have been developed for balloon flight observations of cosmic x-ray sources. We briefly review the characteristics of these, particularly the EXITE2 system. The recent NASA program to develop an extended long duration (100d) balloon flight capability employing super-pressure balloons would allow a qualitatively new hard x-ray imaging experiment: the Energetic X-ray Imaging Survey Telescope-Long Integration Time Experiment (EXIST-LITE). The longer continuous viewing times (per source) available from an LDB platform than from low earth orbit would enable both surveys and objectives complementary to the EXIST mission proposed for a MIDEX satellite. We summarize the scientific objectives of EXIST-LITE, a possible instrumentation approach incorporating a large area array of Cd-Zn-Te (CZT) detectors, and our program for the development and balloon flight testing of relatively thick (5mm) CZT detector arrays.

astro-ph↗