Christopher S. Reynolds

The Swift gamma-ray burst mission

The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr � 1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z >10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a newgeneration wide-field gamma-ray (15‐150 keV) detector that will detect bursts, calculate 1 0 ‐4 0 positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5 00 positions and perform spectroscopy in the 0.2‐10 keV band; and a narrow-field UV/optical telescope that will operate in the 170‐ 600 nm band and provide 0B3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of � 1m crab (� 2;10 � 11 ergs cm � 2 s � 1 in the 15‐150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of

The relation between accretion rate and jet power in X-ray luminous elliptical galaxies

Using Chandra X-ray observations of nine nearby, X-ray luminous elliptical galaxies with good optical velocity dispersion measurements, we show that a tight correlation exists between the Bondi accretion rates calculated from the observed gas temperature and density profiles and estimated black hole masses, and the power emerging from these systems in relativistic jets. The jet powers, which are inferred from the energies and timescales required to inflate cavities observed in the surrounding X-ray emitting gas, can be related to the accretion rates using a power law model of the form log(PBondi/10 43 ergs 1 ) = A + B log(Pjet/10 43 ergs 1 ), with A = 0.65± 0.16 and B = 0.77± 0.20. Our results show that a significant fraction of the energy associated with the rest mass of material entering the Bondi accretion radius (2.2 +1.0 0.7 per cent, for Pjet = 10 43 ergs 1 ) eventually emerges in the relativistic jets. The data also hint that this fraction may rise slightly with increasing jet power. Our results have significant implications for studies of accretion, jet formation and galaxy formation. The observed tight correlation suggests that the Bondi formulae provide a reasonable description of the accretion process in these systems, despite the likely presence of magnetic pressure and angular momentum in the accreting gas. The similarity of the PBondi and Pjet values argues that a significant fraction of the matter entering the accretion radius flows down to regions close to the black holes, where the jets are presumably formed. The tight correlation between PBondi and Pjet also suggests that the accretion flows are approximately stable over timescales of a few million years. Our results show that the black hole ‘engines’ at the hearts of large elliptical galaxies and groups can feed back sufficient energy to stem cooling and s formation, leading naturally to the observed exponential cut off at the bright end of the galaxy luminosity function.

Constraining Black Hole Spin Via X-ray Spectroscopy

We present an analysis of the observed broad iron line feature and putative warm absorber in the long 2001 XMM-Newton observation of the Seyfert 1.2 galaxy MCG -06-30-15. The new kerrdisk model we have designed for simulating line emission from accretion disk systems allows black hole spin to be a free parameter in the fit, enabling the user to formally constrain the angular momentum of a black hole, among other physical parameters of the system. In an important extension of previous work, we derive constraints on the black hole spin in MCG -06-30-15 using a self-consistent model for X-ray reflection from the surface of the accretion disk while simultaneously accounting for absorption by dusty photoionized material along the line of sight (the warm absorber). Even including these complications, the XMM-Newton EPIC pn data require extreme relativistic broadening of the X-ray reflection spectrum; assuming no emission from within the radius of marginal stability, we derive a formal constraint on the dimensionless black hole spin parameter of a = 0.989 at 90% confidence. The principal unmodeled effect that can significantly reduce the inferred black hole spin is powerful emission from within the radius of marginal stability. Although significant theoretical developments are required to fully understand this region, we argue that the need for a rapidly spinning black hole is robust to physically plausible levels of emission from within the radius of marginal stability. In particular, we show that a nonrotating black hole is strongly ruled out.

Broad line emission from iron K- and L-shell transitions in the active galaxy 1H 0707-495

Since the 1995 discovery of the broad iron K-line emission from the Seyfert galaxy MCG–6-30-15 (ref. 1), broad iron K lines have been found in emission from several other Seyfert galaxies, from accreting stellar-mass black holes and even from accreting neutron stars. The iron K line is prominent in the reflection spectrum created by the hard-X-ray continuum irradiating dense accreting matter. Relativistic distortion of the line makes it sensitive to the strong gravity and spin of the black hole. The accompanying iron L-line emission should be detectable when the iron abundance is high. Here we report the presence of both iron K and iron L emission in the spectrum of the narrow-line Seyfert 1 galaxy 1H 0707-495. The bright iron L emission has enabled us to detect a reverberation lag of about 30 s between the direct X-ray continuum and its reflection from matter falling into the black hole. The observed reverberation timescale is comparable to the light-crossing time of the innermost radii around a supermassive black hole. The combination of spectral and timing data on 1H 0707-495 provides strong evidence that we are witnessing emission from matter within a gravitational radius, or a fraction of a light minute, from the event horizon of a rapidly spinning, massive black hole.

Fluorescent iron lines as a probe of astrophysical black hole systems

Abstract With most physicists and astrophysicists in agreement that black holes do indeed exist, the focus of astrophysical black hole research has shifted to the detailed properties of these systems. Nature has provided us with an extremely useful probe of the region very close to an accreting black hole—X-ray irradiation of relatively cold material in the vicinity of the black hole can imprint characteristic features into the X-ray spectra of black hole systems, most notably the Kα fluorescent line of iron. Detailed X-ray spectroscopy of these features can be used to study Doppler and gravitational redshifts, thereby providing key information on the location and kinematics of the cold material. This is a powerful tool that allows us to probe within a few gravitational radii, or less, of the event horizon. Here, we present a comprehensive review of relativistic iron line studies for both accreting stellar mass black holes (i.e., galactic black hole candidate systems, GBHCs), and accreting supermassive black holes (i.e., active galactic nuclei, AGN). We begin with a pedagogical introduction to astrophysical black holes, GBHCs, AGN, and accretion disks (including a brief discussion of recent work on the magnetohydrodynamical properties of accretion disks). We then discuss studies of relativistic iron lines in the AGN context, and show how differences between classes of AGN can be diagnosed using X-ray spectroscopy. Furthermore, through a detailed discussion of one particular object (MCG–6-30-15), we illustrate how the exotic physics of black hole spin, such as the Penrose and Blandford-Znajek processes, are now open to observational study. We proceed to discuss GBHCs, which turn out to possess rather more complicated X-ray spectra, making robust conclusions more difficult to draw. However, even in these cases, modern X-ray observatories are now providing convincing evidence for relativistic effects. We conclude by discussing the science that can be addressed by future X-ray observatories.With most physicists and astrophysicists in agreement that black holes do indeed exist, the focus of astrophysical black hole research has shifted to the detailed properties of these systems. Nature has provided us with an extremely useful probe of the region very close to an accreting black hole—X-ray irradiation of relatively cold material in the vicinity of the black hole can imprint characteristic features into the X-ray spectra of black hole systems, most notably the K 5uorescent line of iron. Detailed X-ray spectroscopy of these features can be used to study Doppler and gravitational redshifts, thereby providing key information on the location and kinematics of the cold material. This is a powerful tool that allows us to probe within a few gravitational radii, or less, of the event horizon. Here, we present a comprehensive review of relativistic iron line studies for both accreting stellar mass black holes (i.e., galactic black hole candidate systems, GBHCs), and accreting supermassive black holes (i.e., active galactic nuclei, AGN). We begin with a pedagogical introduction to astrophysical black holes, GBHCs, AGN, and accretion disks (including a brief discussion of recent work on the magnetohydrodynamical properties of accretion disks). We then discuss studies of relativistic iron lines in the AGN context, and show how di<erences between classes of AGN can be diagnosed using X-ray spectroscopy. Furthermore, through a detailed discussion of one particular object (MCG–6-30-15), we illustrate how the exotic physics of black hole spin, such as the Penrose and Blandford-Znajek processes, are now open to observational study. We proceed to discuss GBHCs, which turn out to possess rather more complicated X-ray spectra, making robust conclusions more di@cult to draw. However, even in these cases, modern X-ray observatories are now providing convincing evidence for relativistic e<ects. We conclude by discussing the science that can be addressed by future X-ray observatories. c

The Variable iron k emission line in MCG-6-30-15

We report on the variability of the iron K emission line in the Seyfert 1 galaxy MCG--6-30-15 during a four-day ASCA observation. The line consists of a narrow core at an energy of about 6.4 keV, and a broad red wing extending to below 5 keV, which are interpreted as line emission arising from the inner parts of an accretion disk. The narrow core correlates well with the continuum flux whereas the broad wing weakly anti-correlates. When the source is brightest, the line is dominated by the narrow core, whilst during a deep minimum, the narrow core is very weak and a huge red tail appears. However, at other times when the continuum shows rather rapid changes, the broad wing is more variable than the narrow core, and shows evidence for correlated changes contrary to its long time scale behaviour. The peculiar line profile during the deep minimum spectrum suggests that the line emitting region is very close to a central spinning (Kerr) black hole where enormous gravitational effects operate.

X-RAY SPECTRAL PROPERTIES OF THE BAT AGN SAMPLE

The 9-month Swift Burst Alert Telescope (BAT) catalog provides the first unbiased (NH < 1024 cm?2) look at local (z = 0.03) active galactic nuclei (AGNs). In this paper, we present the collected X-ray properties (0.3-12 keV) for the 153 AGNs detected. In addition, we examine the X-ray properties for a complete sample of nonbeamed sources, above the Galactic plane (b ? 15?). Of these, 45% are best fit by simple power law models, while 55% require the more complex partial covering model. One of our goals was to determine the fraction of hidden AGNs, which we define as sources with scattering fractions less than or equal to 0.03 and ratios of soft to hard X-ray flux less than or equal to 0.04. We found that hidden AGNs constitute a high percentage of the sample (24%), proving that they are a very significant portion of local AGNs. Further, we find that the fraction of absorbed sources does increase at lower unabsorbed 2-10 keV luminosities, as well as accretion rates. This suggests that the unified model requires modification to include luminosity dependence, as suggested by models such as the receding torus model. Some of the most interesting results for the BAT AGN sample involve the host galaxy properties. We found that 33% are hosted in peculiar/irregular galaxies and only 5/74 are hosted in ellipticals. Further, 54% are hosted in interacting/merger galaxies. Finally, we present both the average X-ray spectrum (0.1-10 keV) and log N-log S in the 2-10 keV band. With our average spectrum, we have the remarkable result of reproducing the measured CXB X-ray power law slope of ? 1.4. From the log N-log S relationship, we show that we are complete to log S ? ?11 in the 2-10 keV band. Below this value, we are missing as many as 3000 sources at log S = ?12. Both the collected X-ray properties of our uniform sample and the log N-log S relationship will now provide valuable input to X-ray background models for z 0.

Broad iron L line and X-ray reverberation in 1H0707-495

A detailed analysis of a long XMM-Newton observation of the narrow-line type 1 Seyfert galaxy 1H0707-495 is presented, including spectral fitting, spectral variability and timing studies. The two main features in the spectrum are the drop at ?7?keV and a complex excess below 1?keV. These are well described by two broad, K and L, iron lines. Alternative models based on absorption, although they may fit the high-energy drop, cannot account for the 1?keV complexity and the spectrum as a whole. Spectral variability shows that the spectrum is composed of at least two components, which are interpreted as a power law dominating between 1-4?keV and a reflection component outside this range. The high count rate at the iron L energies has enabled us to measure a significant soft lag of ?30 s between 0.3-1 and 1-4?keV, meaning that the direct hard emission leads the reflected emissions. We interpret the lag as a reverberation signal originating within a few gravitational radii of the black hole.

IMPROVED REFLECTION MODELS OF BLACK-HOLE ACCRETION DISKS: TREATING THE ANGULAR DISTRIBUTION OF X-RAYS

X-ray reflection models are used to constrain the properties of the accretion disk, such as the degree of ionization of the gas and the elemental abundances. In combination with general relativistic ray tracing codes, additional parameters like the spin of the black hole and the inclination to the system can be determined. However, current reflection models used for such studies only provide angle-averaged solutions for the flux reflected at the surface of the disk. Moreover, the emission angle of the photons changes over the disk due to relativistic light bending. To overcome this simplification, we have constructed an angle-dependent reflection model with the XILLVER code and self-consistently connected it with the relativistic blurring code RELLINE. The new model, relxill, calculates the proper emission angle of the radiation at each point on the accretion disk and then takes the corresponding reflection spectrum into account. We show that the reflected spectra from illuminated disks follow a limb-brightening law highly dependent on the ionization of disk and yet different from the commonly assumed form Iln (1 + 1/μ). A detailed comparison with the angle-averaged model is carried out in order to determine the bias in the parameters obtained by fitting a typical relativistic reflection spectrum. These simulations reveal that although the spin and inclination are mildly affected, the Fe abundance can be overestimated by up to a factor of two when derived from angle-averaged models. The fit of the new model to the Suzaku observation of the Seyfert galaxy Ark 120 clearly shows a significant improvement in the constraint of the physical parameters, in particular by enhancing the accuracy in the inclination angle and the spin determinations.

X-RAY REFLECTED SPECTRA FROM ACCRETION DISK MODELS. III. A COMPLETE GRID OF IONIZED REFLECTION CALCULATIONS

We present a new and complete library of synthetic spectra for modeling the component of emission that is reflected from an illuminated accretion disk. The spectra were computed using an updated version of our code XILLVER that incorporates new routines and a richer atomic database. We offer in the form of a table model an extensive grid of reflection models that cover a wide range of parameters. Each individual model is characterized by the photon index Γ of the illuminating radiation, the ionization parameter ξ at the surface of the disk (i.e., the ratio of the X-ray flux to the gas density), and the iron abundance A Fe relative to the solar value. The ranges of the parameters covered are 1.2 ≤ Γ ≤ 3.4, 1 ≤ ξ ≤ 104, and 0.5 ≤ A Fe ≤ 10. These ranges capture the physical conditions typically inferred from observations of active galactic nuclei, and also stellar-mass black holes in the hard state. This library is intended for use when the thermal disk flux is faint compared to the incident power-law flux. The models are expected to provide an accurate description of the Fe K emission line, which is the crucial spectral feature used to measure black hole spin. A total of 720 reflection spectra are provided in a single FITS file (http://hea-www.cfa.harvard.edu/~javier/xillver/) suitable for the analysis of X-ray observations via the atable model in XSPEC. Detailed comparisons with previous reflection models illustrate the improvements incorporated in this version of XILLVER.