J. E. Grindlay

Chandra Exposes the Core-collapsed Globular Cluster NGC 6397

We report results of the Chandra deep-imaging observation of the closest post-core-collapse globular cluster, NGC 6397. Some 25 sources are detected within 2 of the cluster center, of which ~20 are likely cluster members, with LX 3 × 1029 ergs s-1. The X-ray spectra suggest identifications with one quiescent low-mass X-ray binary (qLMXB) detected by the thermal emission from its neutron star (NS) and nine cataclysmic variables (CVs), eight of which are identified in our deep Hubble Space Telescope (HST) imaging survey (reported separately). Three (of 16) BY Dra main-sequence binary candidates identified in our earlier HST imaging study (Taylor et al.) are detected, of which one is indeed the counterpart of the eclipsing millisecond pulsar (MSP) as recently identified by Ferraro et al. Two other BY Dra candidates are also detected, whereas none of the probable He white dwarf (WD) binaries identified by Taylor et al. are, indicating they do not contain MSP primaries. The ratio of CVs to MSPs appears to be ~10 times greater than in 47 Tuc.

Chasing the heaviest black holes of jetted active galactic nuclei

We investigate the physical properties of 10 blazars at redshift greater than 2 detected in the 3-yr all-sky survey performed by the Burst Alert Telescope (BAT) on board the Swift satellite. We find that the jets of these blazars are among the most powerful known. Furthermore, the mass of their central black hole, inferred from the optical–ultraviolet bump, exceeds a few billions of solar masses, with accretion luminosities being a large fraction of the Eddington one. We compare their properties with those of the brightest blazars of the 3-month survey performed by the Large Area Telescope (LAT) on board the Fermi satellite. We find that the BAT blazars have more powerful jets, more luminous accretion discs and larger black hole masses than LAT blazars. These findings can be simply understood on the basis of the blazar sequence, which suggests that the most powerful blazars have a spectral energy distribution with a high-energy peak at MeV (or even sub-MeV) energies. This implies that the most extreme blazars can be found more efficiently in hard X-rays, rather than in the high-energy γ -ray band. We then discuss the implications of our findings for future missions, such as the New Hard X-ray Mission (NHXM) and especially the Energetic X-ray Imaging Survey Telescope (EXIST) mission which, during its planned 2-yr all-sky survey, is expected to detect thousands of blazars, with a few of them at z 6.

Discovery of a possible X-ray triple - 4U 1915-05

The optical counterpart of the 50 minute binary and X-ray burst source 4U 1915-05 has been identified with a 21st mag blue object. Extensive CCD photometric observations show that the optical modulation of the system defines a period of 50.4567 minutes with high precision and is thus probably the true orbital period, whereas the range of X-ray ray dip period values (49.7-50.1 minutes) reported previously are excluded. A model is described whereby the optical modulation is due to partial eclipse of the rim of the accretion disk by a low-mass companion star and the X-ray dips are due to absorption and scattering by blobs of material above and below the disk plane and with density proportional to the current mass transfer rate from the companion star. The 1 percent difference in periods may suggest that the system is a hierarchical triple with a third companion star of low-mass orbiting the binary ion in an about 2.5 day retrograde orbit. The possible origin of such a system by tidal capture in a globular cluster, since disrupted, is briefly discussed. 25 references.

Building large area CZT imaging detectors for a wide-field hard X-ray telescope—ProtoEXIST1

Abstract We have constructed a moderately large area ( 32 cm 2 ), fine pixel (2.5xa0mm pixel, 5xa0mm thick) CZT imaging detector which constitutes the first section of a detector module ( 256 cm 2 ) developed for a balloon-borne wide-field hard X-ray telescope, ProtoEXIST1 . ProtoEXIST1 is a prototype for the High Energy Telescope (HET) in the Energetic X-ray imaging Survey Telescope ( EXIST ), a next generation space-borne multi-wavelength telescope. We have constructed a large (nearly gapless) detector plane through a modularization scheme by tiling of a large number of 2 cm × 2 cm CZT crystals. Our innovative packaging method is ideal for many applications such as coded-aperture imaging, where a large, continuous detector plane is desirable for the optimal performance. Currently we have been able to achieve an energy resolution of 3.2xa0keV (FWHM) at 59.6xa0keV on average, which is exceptional considering the moderate pixel size and the number of detectors in simultaneous operation. We expect to complete two modules ( 512 cm 2 ) within the next few months as more CZT becomes available. We plan to test the performance of these detectors in a near space environment in a series of high altitude balloon flights, the first of which is scheduled for Fall 2009. These detector modules are the first in a series of progressively more sophisticated detector units and packaging schemes planned for ProtoEXIST2 & 3, which will demonstrate the technology required for the advanced CZT imaging detectors (0.6xa0mm pixel, 4.5 m 2 area) required in EXIST/HET .

Interstellar extinction and the distribution of stellar populations in the direction of the ultra-deep Chandra Galactic field

We studied the stellar population in the central 6.6x6.6arcmin,region of the ultra-deep (1Msec) Chandra Galactic field - the Chandra bulge field (CBF) approximately 1.5 degrees away from the Galactic Center - using the Hubble Space Telescope ACS/WFC blue (F435W) and red (F625W) images. We mainly focus on the behavior of red clump giants - a distinct stellar population, which is known to have an essentially constant intrinsic luminosity and color. By studying the variation in the position of the red clump giants on a spatially resolved color-magnitude diagram, we confirm the anomalous total-to-selective extinction ratio, as reported in previous work for other Galactic bulge fields. We show that the interstellar extinction in this area is = 4 on average, but varies significantly between ~3-5 on angular scales as small as 1 arcminute. Using the distribution of red clump giants in an extinction-corrected color-magnitude diagram, we constrain the shape of a stellar-mass distribution model in the direction of this ultra-deep Chandra field, which will be used in a future analysis of the population of X-ray sources. We also show that the adopted model for the stellar density distribution predicts an infrared surface brightness in the direction of the Chandra bulge field in good agreement (i.e. within ~15%) with the actual measurements derived from the Spitzer/IRAC observations.

Probing the very high redshift Universe with gamma-ray bursts: prospects for observations with future X-ray instruments

Gamma-ray bursts (GRBs) are the most violent explosions in the Universe. Long-duration GRBs are associated with the collapse of massive stars, rivalling their host galaxies in luminosity. The discovery of the most distant spectroscopically confirmed object in the Universe, GRB090423, opened a new window on the high-redshift Universe, making it possible to study the cosmic reionization epoch and the preceding dark ages, as well as the generation of the first stars (Population III) using GRBs. Obviously this enables a wealth of new studies using the near-infrared (NIR) characteristics of GRB afterglows. Here we explore a different path, focusing on the next generation of X-ray missions with large-area-focusing telescopes and fast-repointing capabilities. We found that X-ray data can complement NIR observations and for the brightest GRBs can provide an accurate and independent redshift determination. Metallicity studies can also be carried out profitably once the redshift is known. Finally we discuss observational signatures of GRBs arising from Population III stars in the X-ray band.

EXIST'S Gamma-Ray Burst Sensitivity

We use semianalytic techniques to evaluate the burst sensitivity of designs for the EXIST hard X-ray survey mission. Applying these techniques to the mission design proposed for the Beyond Einstein program, we find that with its very large field of view and faint gamma-ray burst detection threshold, EXIST will detect and localize approximately two bursts per day, a large fraction of which may be at high redshift. We estimate that EXISTs maximum sensitivity will be ~4 times greater than that of Swifts Burst Alert Telescope. Bursts will be localized to better than 40 at threshold, with a burst position as good as a few arcseconds for strong bursts. EXISTs combination of three different detector systems will provide spectra from 3 keV to more than 10 MeV. Thus, EXIST will enable a major leap in the understanding of bursts, their evolution, environment, and utility as cosmological probes.

Flight performance of an advanced CZT imaging detector in a balloon-borne wide-field hard X-ray telescope—ProtoEXIST1

Abstract We successfully carried out the first high-altitude balloon flight of a wide-field hard X-ray coded-aperture telescope ProtoEXIST1 , which was launched from the Columbia Scientific Balloon Facility at Ft. Sumner, New Mexico on October 9, 2009. ProtoEXIST1 is the first implementation of an advanced CdZnTe (CZT) imaging detector in our ongoing program to establish the technology required for next generation wide-field hard X-ray telescopes such as the High Energy Telescope (HET) in the Energetic X-ray Imaging Survey Telescope ( EXIST ). The CZT detector plane in ProtoEXIST1 consists of an 8×8 array of closely tiled 2xa0cm×2xa0cm×0.5xa0cm thick pixellated CZT crystals, each with 8×8 pixels, mounted on a set of readout electronics boards and covering a 256xa0cm 2 active area with 2.5xa0mm pixels. A tungsten mask, mounted at 90xa0cm above the detector provides shadowgrams of X-ray sources in the 30–600xa0keV band for imaging, allowing a fully coded field of view of 9°×9° (and 19°×19° for 50% coding fraction) with an angular resolution of 20 ′ . In order to reduce the background radiation, the detector is surrounded by semi-graded (Pb/Sn/Cu) passive shields on the four sides all the way to the mask. On the back side, a 26xa0cm×26xa0cm×2xa0cm CsI(Na) active shield provides signals to tag charged particle induced events as well as ≳ 100 keV background photons from below. The flight duration was only about 7.5xa0h due to strong winds (60 knots) at float altitude (38–39xa0km). Throughout the flight, the CZT detector performed excellently. The telescope observed Cyg X-1, a bright black hole binary system, for ∼ 1 h at the end of the flight. Despite a few problems with the pointing and aspect systems that caused the telescope to track about 6.4° off the target, the analysis of the Cyg X-1 data revealed an X-ray source at 7.2 σ in the 30–100xa0keV energy band at the expected location from the optical images taken by the onboard daytime star camera. The success of this first flight is very encouraging for the future development of the advanced CZT imaging detectors ( ProtoEXIST2 , with 0.6xa0mm pixels), which will take advantage of the modularization architecture employed in ProtoEXIST1 .

A Dwarf Nova in the Globular Cluster M13

Dwarf novae (DNe) in globular clusters (GCs) seem to be rare with only 13 detections in the 157 known Galactic GCs. We report the identification of a new DN in M13, the 14th DN identified in a GC to date. Using the 2 m Faulkes Telescope North, we conducted a search for stars in M13 that show variability over a year (2005–2006) on timescales of days and months. This led to the detection of one DN showing several outbursts. A Chandra X-ray source is coincident with this DN and shows both a spectrum and variability consistent with that expected from a DN, thus supporting the identification. We searched for a counterpart in Hubble Space Telescope Advanced Camera for Surveys/Wide Field Camera archived images and found at least 11 candidates, of which we could characterize only the 7 brightest, including one with a 3σ Hα excess and a faint blue star. The detection of one DN when more could have been expected likely indicates that our knowledge of the global Galactic population of cataclysmic variables is too limited. The proportion of DNe may be lower than found in catalogs, or they may have a much smaller mean duty cycle (∼1%) as proposed by some population synthesis models and recent observations in the field.

EXIST: A high sensitivity hard x-ray imaging sky survey mission for ISS

A deep all-sky imaging hard x-ray survey and wide-field monitor is needed to extend soft (ROSAT) and medium (ABRIXAS2) x-ray surveys into the 10–100 keV band at comparable sensitivity (~0.05 mCrab). This would enable discovery and study of ≳3000 obscured AGN, which probably dominate the hard x-ray background; detailed study of spectra and variability of accreting black holes and a census of BHs in the Galaxy; Gamma-ray bursts and associated massive star formation (PopIII) at very high redshift and Soft Gamma-ray Repeaters throughout the Local Group; and a full galactic survey for obscured supernova remnants. The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed array of 8×1 m^2 coded aperture telescopes fixed on the International Space Station (ISS) with 160°×40° field of view which images the full sky each 90 min orbit. EXIST has been included in the most recent NASA Strategic Plan as a candidate mission for the next decade. An overview of the science goals and mission concept is presented.