Jonathan E. Grindlay

The Nuclear Spectroscopic Telescope Array (NuSTAR) High-Energy X-Ray Mission

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a National Aeronautics and Space Administration (NASA) Small Explorer mission that carried the first focusing hard X-ray (6-79 keV) telescope into orbit. It was launched on a Pegasus rocket into a low-inclination Earth orbit on June 13, 2012, from Reagan Test Site, Kwajalein Atoll. NuSTAR will carry out a two-year primary science mission. The NuSTAR observatory is composed of the X-ray instrument and the spacecraft. The NuSTAR spacecraft is three-axis stabilized with a single articulating solar array based on Orbital Sciences Corporations LEOStar-2 design. The NuSTAR science instrument consists of two co-aligned grazing incidence optics focusing on to two shielded solid state CdZnTe pixel detectors. The instrument was launched in a compact, stowed configuration, and after launch, a 10-meter mast was deployed to achieve a focal length of 10.15 m. The NuSTAR instrument provides sub-arcminute imaging with excellent spectral resolution over a 12-arcminute field of view. The NuSTAR observatory will be operated out of the Mission Operations Center (MOC) at UC Berkeley. Most science targets will be viewed for a week or more. The science data will be transferred from the UC Berkeley MOC to a Science Operations Center (SOC) located at the California Institute of Technology (Caltech). In this paper, we will describe the mission architecture, the technical challenges during the development phase, and the post-launch activities.

On searches for pulsed emission with application to four globular cluster X-ray sources - NGC 1851, 6441, 6624, and 6712

We present the results of searches for periodic pulsations in the X-ray emission from four globular cluster sources, NGC 1851, NGC 6441, NGC 6624, and NGC 6712. The data were obtained by the Monitor Proportional Counter aboard HEAO 2 (Einstein Observatory). The methods of analysis are presented in some detail because we have correctly accounted for several effects which have been routinely overlooked by others. The periods searched cover the range from approx.1 ms to approx.500 s. No pulsed emission was detected, and the (90% confidence) upper limits for the pulsed fraction are presented.

Advection-dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center

Sagittarius A*, which is located at the Galactic center, is a puzzling source. It has a mass of M = (2.5 ± 0.4) × 106 M☉, which makes it an excellent black hole candidate. Observations of stellar winds and other gas flows in its vicinity suggest a mass accretion rate of few × 10−6 M☉ yr-1. However, such an accretion rate would imply a luminosity greater than 1040 ergs-1 if the radiative efficiency is the usual 10%, whereas observations indicate a bolometric luminosity less than 1037 ergs-1. The spectrum of Sgr A* is unusual, with emission extending over many decades of wavelength. We present a model of Sgr A* that is based on a two-temperature optically thin advection-dominated accretion flow. The model is consistent with the estimated M and and fits the observed fluxes in the centimeter/millimeter and X-ray bands, as well as upper limits in the submillimeter and infrared bands; the fit is less good in the radio spectrum below 86 GHz and in γ-rays above 100 MeV. The very low luminosity of Sgr A* is explained naturally in the model by means of advection. Most of the viscously dissipated energy is advected into the central mass by the accreting gas, and therefore the radiative efficiency is extremely low, ~5 × 10-6. A critical element of the model is the presence of an event horizon at the center that swallows the advected energy. The success of the model could thus be viewed as confirmation that Sgr A* is a black hole.

A relativistic type Ibc supernova without a detected γ-ray burst

Long duration γ-ray bursts (GRBs) mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. They are distinguished by the production of an energetic and collimated relativistic outflow powered by a central engine (an accreting black hole or neutron star). Observationally, this outflow is manifested in the pulse of γ-rays and a long-lived radio afterglow. Until now, central-engine-driven supernovae have been discovered exclusively through their γ-ray emission, yet it is expected that a larger population goes undetected because of limited satellite sensitivity or beaming of the collimated emission away from our line of sight. In this framework, the recovery of undetected GRBs may be possible through radio searches for type Ibc supernovae with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. A comparison with our radio survey of type Ibc supernovae reveals that the fraction harbouring central engines is low, about one per cent, measured independently from, but consistent with, the inferred rate of nearby GRBs. Independently, a second mildly relativistic supernova has been reported.

A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae

Current X-ray missions are providing high-quality X-ray spectra from neutron stars (NSs) in quiescent low-mass X-ray binaries (qLMXBs). This has motivated us to calculate new hydrogen atmosphere models, including opacity due to free-free absorption and Thomson scattering, thermal electron conduction, and self-irradiation by photons from the compact object. We have constructed a self-consistent grid of neutron star models covering a wide range of surface gravities, as well as effective temperatures, which we make available to the scientific community. We present multiepoch Chandra X-ray observations of the qLMXB X7 in the globular cluster 47 Tuc, which is remarkably nonvariable on timescales from minutes to years. Its high-quality X-ray spectrum is adequately fitted by our hydrogen atmosphere model without any hard power-law component or narrow spectral features. If a mass of 1.4 M☉ is assumed, our spectral fits require that its radius be in the range Rns = 14.5 km (90% confidence), which is larger than that expected from currently preferred models of NS interiors. If its radius is assumed to be 10 km, then a mass of Mns = 2.20 M☉ is required. Using models with the appropriate surface gravity for each value of the mass and radius becomes important for interpretation of the highest quality data.

Discovery of intense X-ray bursts from the globular cluster NGC 6624

A new type of time variation of cosmic X-ray sources has been found from the Astronomical Netherlands Satellite (ANS) observations of the source 3U 1820--30 associated with the globular cluster NGC 6624. Two bursts in the approx.1--30 keV X-ray intensity of this source are reported. Each displayed a rapid rise in flux (approximately-less-than1 s) by a factor of 20--30 followed by a approx.8 s exponential decay. These bursts appear to be qualitatively different from short time variations previously reported from X-ray sources. Analysis for further source variability, energy spectra, and position is presented. The characteristics of these events may imply the existence of a collapsed core in the globular cluster. (AIP)

Luminosity Differences Between Black Holes and Neutron Stars

We compare the X-ray (1-20 keV) and hard X-ray (20-200 keV) luminosities of black hole binaries (BHBs; i.e., binaries for which the mass of the compact object is known to exceed 3 M☉) and X-ray bursters (neutron star binaries, NSBs). We discuss two ways of distinguishing a BHB from a NSB: (1) If the X-ray luminosity exceeds ~ 1037 ergs s–1, the hard X-ray luminosity of BHBs is relatively unaffected, whereas the hard X-ray luminosity of NSBs decreases drastically; and (2) the hard X-ray luminosity of BHBs is commonly in the range 1037 -6 × 1037 ergs s–1, whereas for NSBs it is 1037 ergs s–1. We show that late in their decays transient BHBs (e.g., GRS 1124-68) have X-ray and hard X-ray luminosities comparable to those observed for NSBs. Thus BHBs can be distinguished from NSBs only at relatively high luminosities.We also compare NSBs with the so-called black hole candidates (BHCs; i.e., systems with similar spectral/temporal properties to BHBs). The X-ray and hard X-ray luminosities of LMC X-1, GRO J0422 + 32, GRS 1915 + 105, 4U 1543-47, and 4U 1630-47 are much larger than the maximum luminosities observed from NSBs, which supports the idea that they contain black holes. Three other BHCs, namely GRS 1716-249, 1E 1740.7-2942, and GRS 1758-258 (which all lack an ultrasoft spectral component), have hard X-ray luminosities at least a factor of ~ 2-3 above the maximum observed from NSBs, which suggests that these objects also contain black hole primaries. The case of GX 339-4 remains very uncertain because of the large uncertainty in its distance estimates (from 1.3 to 4 kpc). Assuming the larger distance, the X-ray and hard X-ray luminosities of the source, and its luminosity-related spectral changes which are similar to transient BHBs (e.g., GRS 1124-68), support the idea that it contains a black hole. Finally, the X-ray and hard X-ray luminosities of the puzzling X-ray source 4U 1957 + 11 are in the range of those observed for NSBs, consistent with the idea that it might contain a neutron star. If 4U 1957 + 11 is, in fact, a neutron star system, this would establish that the combination of a power-law tail and an ultrasoft component (which is present in the spectrum of 4U 1957+ 11) is not a unique spectral signature of an accreting black hole.

The XMM-Newton/Chandra monitoring campaign of the Galactic center region Description of the program and preliminary results

We present the first results of our X-ray monitoring campaign on a 1.7 square degree region centered on Sgr A* using the satellites XMM-Newton and Chandra. The purpose of this campaign is to monitor the behavior (below 10 keV) of X-ray sources (both persistent and transient) which are too faint to be detected by monitoring instruments aboard other satellites currently in orbit (e.g., Rossi X-ray Timing Explorer; INTEGRAL). Our first monitoring observations (using the HRC-I aboard Chandra) were obtained on June 5, 2005. Most of the sources detected could be identified with foreground sources, such as X-ray active stars. In addition we detected two persistent X-ray binaries (1E 1743.1-2843; 1A 1742-294), two faint X-ray transients (GRS 1741.9-2853; XMM J174457-2850.3), as well as a possible new transient source at a luminosity of a few times 1034 erg s-1. We report on the X-ray results on these systems and on the non-detection of the transients in follow-up radio data using the Very Large Array. We discuss how our monitoring campaign can help to improve our understanding of the different types of X-ray transients (i.e., the very faint ones).

Chandra X-ray observations of 19 millisecond pulsars in the globular cluster 47 Tucanae

We present spectral and long-timescale variability analyses of Chandra X-Ray Observatory ACIS-S observations of the 19 millisecond pulsars (MSPs) with precisely known positions in the globular cluster 47 Tucanae. The X-ray emission of the majority of these MSPs is well described by a thermal (blackbody or neutron star hydrogen atmosphere) spectrum with a temperature Teff ~ (1-3) × 106 K, emission radius Reff ~ 0.1-3 km, and luminosity LX ~ 1030-1031 ergs s-1. For several MSPs, there are indications that a second thermal component is required, similar to what is seen in some nearby field MSPs. The observed radiation most likely originates from the heated magnetic polar caps of the MSPs. The small apparent scatter in LX is consistent with thermal emission from the polar caps of a global dipole field, although the small emission areas may imply either a more complex small-scale magnetic field configuration near the neutron star surface or nonuniform polar cap heating. The radio eclipsing binary MSPs 47 Tuc J, O, and W show a significant nonthermal (power-law) component, with spectral photon index Γ ~ 1-1.5, which most likely originates in an intrabinary shock formed due to interaction between the relativistic pulsar wind and matter from the stellar companion. We reexamine the X-ray-spin-down luminosity relation (LX-Ė relation) and find that for the MSPs with thermal spectra LX ∝ Ėβ, where β ~ 0.2 ± 1.1. Due to the large uncertainties in both parameters, the result is consistent with both the linear LX-Ė relation and the flatter LX ∝ Ė0.5 predicted by polar cap heating models. In terms of X-ray properties, we find no clear systematic differences between MSPs in globular clusters and in the field of the Galaxy. We discuss the implications of these results on the present understanding of the X-ray emission properties of MSPs.

Chandra X-Ray Observatory observations of the globular cluster M28 and its millisecond pulsar PSR B1821-24

We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). 46 X-ray sources are detected, of which 12 lie within one core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources for which we determine the X-ray luminosity function down to a limit of about 6xE30 erg/s. For the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 is measured and found to be best described by a power law with photon index ~ 1.2. Marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum is found, which could be interpreted as cyclotron emission from a corona above the pulsars polar cap if the the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0 keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightest unidentified sources is presented. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T_eff^\infty = 90^{+30}_{-10} eV and the radius R_NS^\infty = 14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core of M28. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). We detect 46 X-ray sources, of which 12 lie within 1 core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources, for which we determine the X-ray luminosity function down to a limit of about 6 � 10 30 ergs s � 1 . We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius of rc;X � 11 00 . We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05 ms pulsar B1821� 24 and find that it is best described by a power law with photon index � ’ 1:2. We find marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar’s polar cap if the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5–8.0 keV band is � 3:5 � 10 � 13 ergs s � 1 cm � 2 . We present spectral analyses of the five brightest unidentified sources. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T 1 eff ¼ 90 þ30 � 10 eV and the radius R 1 ¼ 14:5 þ6:9 � 3:8 km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts. Subject headings: globular clusters: individual (M28) — pulsars: general — stars: neutron — X-rays: stars