The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 2012 June 13, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 to 79 keV, extending the sensitivity of focusing far beyond the ~10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: (1) probe obscured active galactic nucleus (AGN) activity out to the peak epoch of galaxy assembly in the universe (at z ≾ 2) by surveying selected regions of the sky; (2) study the population of hard X-ray-emitting compact objects in the Galaxy by mapping the central regions of the Milky Way; (3) study the non-thermal radiation in young supernova remnants, both the hard X-ray continuum and the emission from the radioactive element ^(44)Ti; (4) observe blazars contemporaneously with ground-based radio, optical, and TeV telescopes, as well as with Fermi and Swift, to constrain the structure of AGN jets; and (5) observe line and continuum emission from core-collapse supernovae in the Local Group, and from nearby Type Ia events, to constrain explosion models. During its baseline two-year mission, NuSTAR will also undertake a broad program of targeted observations. The observatory consists of two co-aligned grazing-incidence X-ray telescopes pointed at celestial targets by a three-axis stabilized spacecraft. Deployed into a 600 km, near-circular, 6° inclination orbit, the observatory has now completed commissioning, and is performing consistent with pre-launch expectations. NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of 10 yr, we anticipate proposing a guest investigator program, to begin in late 2014.

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THE NUCLEAR SPECTROSCOPIC TELESCOPE ARRAY (NuSTAR) HIGH-ENERGY X-RAY MISSION

Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe

We examine the disc-jet connection in stellar mass and supermassive black holes by investigating the properties of their compact emission in the X-ray and radio bands. We compile a sample of ∼100 active galactic nuclei with measured masses, 5-GHz core emission, and 2-10 keV luminosities, together with eight galactic black holes with a total of ∼50 simultaneous observations in the radio and X-ray bands. Using this sample, we study the correlations between the radio (L R ) and the X-ray (L X ) luminosity and the black hole mass (M). We find that the radio luminosity is correlated with both M and L X , at a highly significant level. In particular, we show that the sources define a 'Fundamental Plane' in the three-dimensional (log L R , log L X , log M) space, given by log L R = (0.60 + 0 . 1 1 -0.11) log L X + (0.78 + 0 . 1 1 -0.09) log M + 7.33 + 4 . 0 5 -4.07, with a substantial scatter of σ R = 0.88. We compare our results to the theoretical relations between radio flux, black hole mass, and accretion rate derived by Heinz & Sunyaev. Such relations depend only on the assumed accretion model and on the observed radio spectral index. Therefore, we are able to show that the X-ray emission from black holes accreting at less than a few per cent of the Eddington rate is unlikely to be produced by radiatively efficient accretion, and is marginally consistent with optically thin synchrotron emission from the jet. On the other hand, models for radiatively inefficient accretion flows seem to agree well with the data.

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A Fundamental plane of black hole activity

We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity parameter $\alpha$. For $\dot M<\dot M_{crit}$ we show that there are three equilibrium branches of solutions. One of the branches corresponds to a cool optically thick flow which is the well-known thin disk solution of Shakura \& Sunyaev (1973). Another branch corresponds to a hot optically thin flow, discovered originally by Shapiro, Lightman \& Eardley (1976, SLE). This solution is thermally unstable. The third branch corresponds to our new advection-dominated solution. This solution is hotter and more optically thin than the SLE solution, but is viscously and thermally stable. It is related to the ion torus model of Rees et al. (1982) and may potentially explain the hard X-ray and $\gamma$-ray emission from X-ray binaries and active galactic nuclei.

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Advection-Dominated Accretion: Underfed Black Holes and Neutron Stars

Neutron Stars 1: Equation of State and Structure

Based on Chandra and ASCA observations of nearby starburst galaxies and RXTE/ASM, ASCA and MIR-KVANT/TTM studies of high-mass X-ray binary (HMXB) populations in the Milky Way and Magellanic Clouds, we propose that the number and/or the collective X-ray luminosity of HMXBs can be used to measure the star formation rate (SFR) of a galaxy. We show that, within the accuracy of the presently available data, a linear relation between HMXB number and star formation rate exists. The relation between SFR and collective luminosity of HMXBs is non-linear in the low-SFR regime, L X ∝ SFR ∼1.7 , and becomes linear only for a sufficiently

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High mass x-ray binaries as a star formation rate indicator in distant galaxies

We study the Log(N)-Log(S ) and X-ray luminosity function in the 2-10 keV energy band, and the spatial (3-D) distribution of bright, LX 10 34 10 35 erg s 1 , X-ray binaries in the Milky Way. In agreement with theoretical expectations and earlier results we found significant dierences between the spatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. The volume density of LMXB sources peaks strongly at the Galactic Bulge whereas HMXBs tend to avoid the inner 3 4 kpc of the Galaxy. In addition HMXBs are more concentrated towards the Galactic Plane (scale heights of150 and 410 pc for HMXB and LMXB correspondingly) and show clear signatures of the spiral structure in their spatial distribution. The Log(N)-Log(S ) distributions and the X-ray luminosity functions are also noticeably dierent. LMXB sources have a flatter Log(N)-Log(S ) distribution and luminosity function. The integrated 2-10 keV luminosities of all X-ray binaries in the Galaxy, averaged over 1996-2000, are2 3 10 39 (LMXB) and2 3 10 38 (HMXB) erg s 1 . Normalised to the stellar mass and the star formation rate, respectively, these correspond to5 10 28 erg s 1 M 1 for LMXBs and5 10 37 erg s 1 /(M yr 1 ) for HMXBs. Due to the shallow slopes of the luminosity functions the integrated emission of X-ray binaries is dominated by the5-10 most luminous sources which determine the appearance of the Milky Way in the standard X-ray band for an outside observer. In particular variability of individual sources or an outburst of a bright transient source can increase the integrated luminosity of the Milky Way by as much as a factor of2. Although the average LMXB luminosity function shows a break near the Eddington luminosity for a 1.4 M neutron star, at least 12 sources showed episodes of super-Eddington luminosity during ASM observations. We provide the maps of distribution of X-ray binaries in the Milky Way in various projections, which can be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.

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The Milky Way in X-rays for an outside observer - Log(N)-Log(S) and luminosity function of X-ray binaries from RXTE/ASM data

We compare the results of Grimm, Gilfanov & Sunyaev and Ranalli, Comastri & Seti on the L X -SFR (X-ray luminosity-star formation rate) relation in normal galaxies. Based on the Lx-stellar mass dependence for low-mass X-ray binaries (LMXBs), we show that low-SFR (≤1 M ○. yr -1 ) galaxies in the Ranalli et al. sample are contaminated by the X-ray emission from LMXBs, unrelated to the current star formation activity. However, the most important conclusion from our comparison is that, after the data are corrected for the 'LMXB contamination', the two data sets become consistent with each other, despite differences in their content, variability effects, adopted source distances, X-ray fluxes and SFR determinations, and also in the cosmological parameters used in interpreting the Hubble Deep Field North (HDF-N) data. They also agree well, both in the low- and high-SFR regimes, with the predicted L X -SFR dependence derived from the parameters of the 'universal' high-mass X-ray binary (HMXB) luminosity function. This encouraging result emphasizes the potential of the X-ray luminosity as an independent SFR indicator for normal galaxies.

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Lx-SFR relation in star-forming galaxies

We study the statistical properties of the combined emission of a population of discrete sources (for example, the X-ray emission of a galaxy due to its population of X-ray binaries). Namely, we consider the dependence of their total luminosity L tot = Σ L k and of the fractional rms tot of their variability on the number of sources n or, equivalently, on the normalization of the luminosity function. We show that, as a result of small number statistics, a regime exists in which L tot grows non-linearly with n, in apparent contradiction with the seemingly obvious prediction (L tot ) = f L (dN/dL) dL n. In this non-linear regime, rms tot decreases with n significantly more slowly than expected from the rms oc 1/√n averaging law. For example, for a power-law luminosity function with a slope of a = 3/2, in the non-linear regime, L tot n 2 and rms tot does not depend at all on the number of sources n. Only in the limit of n → ∞ do these quantities behave as intuitively expected, L tot n and rms tot 1/√n. We give exact solutions and derive convenient analytical approximations for L tot and rms tot . Using the total X-ray luminosity of a galaxy due to its X-ray binary population as an example, we show that the L X -star formation rate and L X -M * relations predicted from the respective 'universal' luminosity functions of high- and low-mass X-ray binaries are in good agreement with observations. Although caused by small number statistics, the non-linear regime in these examples extends as far as SFR <∼ 4-5 M ○. yr -1 and log(M * /M ○. ) <∼ 10.0-10.5, respectively.

Statistical properties of the combined emission of a population of discrete sources: astrophysical implications

Based on recent X-ray observations of the Milky Way, Magellanic Clouds and nearby starburst galaxies we study the population of high-mass X-ray binaries (HMXBs), their connection with ultra-luminous X-ray sources (ULXs) and the relation to the star formation rate (SFR). Although more subtle SFR-dependent effects are likely to exist, the data in the log(LX)∼ 36–40.5 luminosity range are broadly consistent with the existence of a “universal” luminosity function of HMXBs, which can be roughly described as a power law with a differential slope of ∼ 1.6 and a cutoff at log(LX)∼ 40.5. The ULX sources found in many starburst galaxies occupy the high-luminosity end of this single-slope power-law distribution, whereas its low-luminosity part is composed of “ordinary” high-mass X-ray binaries, observed, e.g. in the Milky Way and Magellanic Clouds. As the normalization of the “universal” luminosity function is proportional to the star formation rate, the number and/or the collective X-ray luminosity of HMXBs can be used to measure the current value of the SFR in the host galaxy. Distant (unresolved) starburst galaxies observed by Chandra at redshifts of z∼ 0.2–1.3 obey the same LX–SFR relation as local galaxies, indicating that the ULXs at these redshifts were not significantly more luminous than those found in nearby galaxies.

H.-J. Grimm

Papers

High mass x-ray binaries as a star formation rate indicator in distant galaxies

The Milky Way in X-rays for an outside observer - Log(N)-Log(S) and luminosity function of X-ray binaries from RXTE/ASM data

Lx-SFR relation in star-forming galaxies

Statistical properties of the combined emission of a population of discrete sources: astrophysical implications

HMXB, ULX and star formation