Andrew F. Ptak

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.

Detection of Nuclear X-Ray Sources in Nearby Galaxies with Chandra

We report preliminary results from an arcsecond-resolution X-ray survey of nearby galaxies using the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. The total sample consists of 41 low-luminosity active galactic nuclei (AGNs), including Seyfert galaxies, LINERs, and LINER/H II transition objects. In the initial subsample of 24 objects observed thus far, we detect in ~62% of the objects a compact, pointlike source astrometrically coincident with either the optical or radio position of the nucleus. The high detection rate strongly suggests that the majority of the objects do contain weakly active, AGN-like cores, presumably powered by central massive black holes. The 2-10 keV luminosities of the nuclear sources range from less than 1038 to 1041 ergs s-1, with a median value of 2 × 1038 ergs s-1. Our detection limit corresponds to LX(2-10 keV) ≈ 8 × 1037 ergs s-1 for the typical sample distance of 12 Mpc; this limit is 2 orders of magnitude fainter than the weakest sources of this kind previously studied using ASCA or BeppoSAX. The new data extend toward lower luminosities the known linear correlation between hard X-ray and Hα luminosity for broad-line AGNs. Many narrow-line objects do contain X-ray cores, consistent with either weak AGNs or X-ray binary systems, but they have X-ray luminosities a factor of 10 below the LX-LHα relation of the broad-line sources. Their distributions of photon energies show no indication of exceptionally high absorption. The optical line emission in these nuclei is likely powered, at least in part, by stellar processes.

Old and Young X-Ray Point Source Populations in Nearby Galaxies

We have analyzed Chandra ACIS observations of 32 nearby spiral and elliptical galaxies and present the results of 1441 X-ray point sources that were detected in these galaxies. The total point-source X-ray (0.3-8.0 keV) luminosity LXP is well correlated with the B-band, K-band, and FIR+UV luminosities of spiral host galaxies and is well correlated with the B-band and K-band luminosities of elliptical galaxies. This suggests an intimate connection between LXP and both the old and young stellar populations, for which K and FIR+UV luminosities are reasonable proxies for the galaxy mass M and star formation rate SFR. We derive proportionality constants ? = 1.3 ? 1029 ergs s-1 M and ? = 0.7 ? 1039 ergs s-1 (M? yr-1)-1, which can be used to estimate the old and young components from M and SFR, respectively. The cumulative X-ray luminosity functions for the point sources have significantly different slopes. For the spiral and starburst galaxies, ? ? 0.6-0.8, and for the elliptical galaxies, ? ? 1.4. This implies that the most luminous point sources?those with LX 1038 ergs s-1?dominate LXP for the spiral and starburst galaxies. Most of the point sources have X-ray colors that are consistent with soft-spectrum (photon index ? ~ 1-2) low-mass X-ray binaries, accretion-powered black hole high-mass X-ray binaries (BH HMXBs), or ultraluminous X-ray sources (ULXs, also known as IXOs). We rule out hard-spectrum neutron star HMXBs (e.g., accretion-powered X-ray pulsars) as contributing much to LXP. Thus, for spirals, LXP is dominated by ULXs and BH HMXBs. We find no discernible difference between the X-ray colors of ULXs (LX ? 1039 ergs s-1) in spiral galaxies and point sources with LX ? 1038-1039 ergs s-1. We estimate that 20% of all ULXs found in spirals originate from the older (Population II) stellar populations, indicating that many of the ULXs that have been found in spiral galaxies are in fact Population II ULXs, like those in elliptical galaxies. We find that LXP depends linearly (within uncertainties) on both M and SFR for our sample galaxies (M 1011 M? and SFR 10 M? yr-1).

THE 4 Ms CHANDRA DEEP FIELD-SOUTH NUMBER COUNTS APPORTIONED BY SOURCE CLASS: PERVASIVE ACTIVE GALACTIC NUCLEI AND THE ASCENT OF NORMAL GALAXIES

This article presents cumulative and differential number-count measurements for the recently completed 4 Ms Chandra Deep Field-South survey.

X-ray Binary Evolution Across Cosmic Time

High redshift galaxies permit the study of the formation and evolution of X-ray binary populations on cosmological timescales, probing a wide range of metallicitie s and star-formation rates. In this paper, we present results from a large scale population synthesis study that m odels the X-ray binary populations from the first galaxies of the universe until today. We use as input to our modeling the Millennium II Cosmological Simulation and the updated semi-analytic galaxy catalog by Guo et al. (2011) to self-consistently account for the star formation history and metallicity evolution of the uni verse. Our modeling, which is constrained by the observed X-ray properties of local galaxies, gives predict ions about the global scaling of emission from X-ray binary populations with properties such as star-formation rate and stellar mass, and the evolution of these relations with redshift. Our simulations show that the X-ray luminosity density (X-ray luminosity per unit volume) from X-ray binaries in our Universe today is dominated by low-mass X-ray binaries, and it is only at z & 2.5 that high-mass X-ray binaries become dominant. We also find t hat there is a delay of � 1.1 Gyr between the peak of X-ray emissivity from low-mass Xray binaries (at z � 2.1) and the peak of star-formation rate density (at z � 3.1). The peak of the X-ray luminosity from high-mass X-ray binaries (at z � 3.9), happens � 0.8 Gyr before the peak of the star-formation rate density, which is due to the metallicity evolution of the Universe. Subject headings:stars: binaries: close, stars: evolution, X-rays: binarie s, galaxies, diffuse background, galaxies: stellar content

The Ultraluminous X-Ray Sources NGC 1313 X-1 and X-2: A Broadband Study with NuSTAR and XMM-Newton

We present the results of NuSTAR and XMM-Newton observations of the two ultraluminous X-ray sources: NGC 1313 X-1 and X-2. The combined spectral bandpass of the two satellites enables us to produce the first spectrum of X-1 between 0.3 and 30 keV, while X-2 is not significantly detected by NuSTAR above 10 keV. The NuSTAR data demonstrate that X-1 has a clear cutoff above 10 keV, whose presence was only marginally detectable with previous X-ray observations. This cutoff rules out the interpretation of X-1 as a black hole in a standard low/hard state, and it is deeper than predicted for the downturn of a broadened iron line in a reflection-dominated regime. The cutoff differs from the prediction of a single-temperature Comptonization model. Further, a cold disk-like blackbody component at ~0.3 keV is required by the data, confirming previous measurements by XMM-Newton only. We observe a spectral transition in X-2, from a state with high luminosity and strong variability to a lower-luminosity state with no detectable variability, and we link this behavior to a transition from a super-Eddington to a sub-Eddington regime.

X-RAY CONSTRAINTS ON ACCRETION AND STARBURST PROCESSES IN GALACTIC NUCLEI. I. SPECTRAL RESULTS

The results of the analysis of 0.4-10.0 keV ASCA spectral analysis of a sample of low-luminosity active galactic nuclei (AGNs) (LLAGNs; M51, NGC 3147, and NGC 4258), low-ionization nuclear emission-line regions (LINERs; NGC 3079, NGC 3310, NGC 3998, NGC 4579, and NGC 4594), and starburst galaxies (M82, NGC 253, NGC 3628, and NGC 6946) are presented. In spite of the heterogeneous optical classifications of these galaxies, the X-ray spectra are fitted well by a canonical model consisting of an optically thin Raymond-Smith plasma soft component with T ~ 7 ? 106 K and a hard component that can be modeled by either a power law with a photon index ? ~ 1.7 or a thermal bremsstrahlung with T ~ 6 ? 107 K. The soft-component absorption is typically less than 1021 cm-2, while the hard component is typically absorbed by an additional column on the order of 1022 cm-2. The soft-component 0.4-10 keV intrinsic luminosities tend to be on the order 1039?40 ergs s-1, while the hard-component luminosities tend to be on the order of 1040?41 ergs s-1. The abundances inferred from the fits to the soft component are significantly subsolar. The Fe abundance can be measured independently of the other elemental abundances (dominated by ?-process elements) in M51, M82, NGC 253, and NGC 4258. In these galaxies the Fe abundance relative to ?-process elements is also (statistically) significantly subsolar. There is some indication (at a low statistical significance) that the abundance properties of starburst emission from starburst galaxies differs from the starburst emission from low-luminosity AGNs. However, these results on abundances are model dependent. Significant Fe K line emission is observed in M51, M82, NGC 3147, NGC 4258, and NGC 4579. An analysis of the short-term variability properties was given in Ptak et al. and detailed interpretation of these results will be given in Paper II in this series.

The X-ray-derived cosmological star formation history and the galaxy X-ray luminosity functions in the chandra deep fields north and south

The cosmological star formation rate in the combined Chandra Deep Fields North and South is derived from our X-ray luminosity function for galaxies in these deep fields. Mild evolution is seen up to redshift order unity with star formation rate ~ (1 + z)2.7. This is the first directly observed normal star-forming galaxy X-ray luminosity function (XLF) at cosmologically interesting redshifts (z > 0). This provides the most direct measure yet of the X-ray-derived cosmic star formation history of the universe. We make use of Bayesian statistical methods to classify the galaxies and the two types of active galactic nuclei (AGNs), finding the most useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and X-ray to optical flux ratio. There is some residual AGN contamination in the sample at the bright end of the luminosity function. Incompleteness slightly flattens the XLF at the faint end of the luminosity function. The XLF has a lognormal distribution and agrees well with the radio and infrared luminosity functions. However, the XLF does not agree with the Schechter luminosity function for the Hα LF, indicating that, as discussed in the text, additional and different physical processes may be involved in the establishment of the lognormal form of the XLF. The agreement of our star formation history points with the other star formation determinations in different wavebands (IR, radio, Hα) gives an interesting constraint on the initial mass function (IMF). The X-ray emission in the Chandra band is most likely due to binary stars, although X-ray emission from nonstellar sources (e.g., intermediate-mass black holes and/or low-luminosity AGNs) remains a possibility. Under the assumption that it is due to binary stars, the overall consistency and correlations between single-star effects and binary-star effects indicate that not only is the one-parameter IMF (M) constant but also the bivariate IMF(M1, M2) must be constant, at least at the high-mass end. Another way to put this, quite simply, is that X-ray observations may be measuring directly the binary-star formation history of the universe. X-ray studies will continue to be useful for probing the star formation history of the universe by avoiding problems of obscuration. Star formation may therefore be measured in more detail by deep surveys with future X-ray missions.

XMM-Newton observations of NGC 253: Resolving the emission components in the disk and nuclear area

We describe the rst XMM-Newton observations of the starburst galaxy NGC 253. As known from previous X-ray observations, NGC 253 shows a mixture of extended (disk and halo) and point-source emission. The high XMM-Newton throughput allows a detailed investigation of the spatial, spectral and variability properties of these components simultaneously. We characterize the brightest sources by their hardness ratios, detect a bright X-ray transient70 00 SSW of the nucleus, and show the spectrum and light curve of the brightest point source (30 00 S of the nucleus, most likely a black-hole X-ray binary, BHXRB). The unresolved emission of two disk regions can be modeled by two thin thermal plasma components (temperatures of0.13 and 0.4 keV) plus residual harder emission, with the lower temperature component originating from above the disk. The nuclear spectrum can be modeled by a three temperature plasma (0.6, 0.9, and 6 keV) with the higher temperatures increasingly absorbed. The high temperature component most likely originates from the starburst nucleus, as no non-thermal component, that would point at a signicant contribution from an active nucleus (AGN), is needed. Assuming that type IIa supernova remnants (SNRs) are mostly responsible for the E> 4 keV emission, the detection with EPIC of the 6.7 keV line allows us to estimate a supernova rate within the nuclear starburst of 0.2 yr 1 . The unprecedented combination of RGS and EPIC also sheds new light on the emission of the complex nuclear region, the X-ray plume and the disk diuse emission. In particular, EPIC images reveal that the limb- brightening of the plume is mostly seen in higher ionization emission lines, while in the lower ionization lines, and below 0.5 keV, the plume is more homogeneously structured. The plume spectrum can again be modeled by a three temperature thermal plasma containing the two low temperature nuclear components (though less absorbed) plus an unabsorbed 0.15 keV component similar to the disk spectra. This points to new interpretations as to the make up of the starburst-driven outflow.

INDICATORS OF INTRINSIC ACTIVE GALACTIC NUCLEUS LUMINOSITY: A MULTI-WAVELENGTH APPROACH

Active galactic nuclei (AGNs) consist of an accretion disk around a supermassive black hole which in turn is surrounded by an obscuring torus of dust and gas. As the resulting geometry of this system affects the observable properties, quantifying isotropic indicators of intrinsic AGN luminosity is important in selecting unbiased samples of AGNs. In this paper, we consider five such proxies: the luminosities of the [O?III]?5007 line, the [O IV]25.89??m line, the mid-infrared (MIR) continuum emission by the torus, and the radio and hard X-ray (E > 10 keV) continuum emission. We compare these different proxies using two complete samples of low-redshift Type 2 AGNs selected in a homogeneous way based on different indicators: an optically selected [O III] sample and an MIR-selected 12??m sample. To assess the relative merits of these proxies, we have undertaken two analyses. First, we examine the correlations between all five different proxies, and find better agreement for the [O IV], MIR, and [O III] luminosities than for the hard X-ray and radio luminosities. Next, we compare the ratios of the fluxes of the different proxies to their values in unobscured Type 1 AGNs. The agreement is best for the ratio of the [O IV] and MIR fluxes, while the ratios of the hard X-ray to [O III], [O IV], and MIR fluxes are systematically low by about an order of magnitude in the Type 2 AGNs, indicating that hard X-ray-selected samples do not represent the full Type 2 AGN population. In a similar spirit, we compare different optical and MIR diagnostics of the relative energetic contributions of AGN and star formation processes in our samples of Type 2 AGNs. We find good agreement between the various diagnostic parameters, such as the equivalent width of the MIR polycyclic aromatic hydrocarbon features, the ratio of the MIR [O IV]/[Ne II] emission lines, the spectral index of the MIR continuum, and the commonly used optical emission-line ratios. Finally, we test whether the presence of cold gas associated with star formation leads to an enhanced conversion efficiency of AGN ionizing radiation into [O III] or [O IV] emission. We find that no compelling evidence exists for this scenario for the luminosities represented in this sample (L bol 109 ? 8 ? 1011 L ?).