19 research outputs found
Measuring Black Hole Spin using X-ray Reflection Spectroscopy
I review the current status of X-ray reflection (a.k.a. broad iron line)
based black hole spin measurements. This is a powerful technique that allows us
to measure robust black hole spins across the mass range, from the stellar-mass
black holes in X-ray binaries to the supermassive black holes in active
galactic nuclei. After describing the basic assumptions of this approach, I lay
out the detailed methodology focusing on "best practices" that have been found
necessary to obtain robust results. Reflecting my own biases, this review is
slanted towards a discussion of supermassive black hole (SMBH) spin in active
galactic nuclei (AGN). Pulling together all of the available XMM-Newton and
Suzaku results from the literature that satisfy objective quality control
criteria, it is clear that a large fraction of SMBHs are rapidly-spinning,
although there are tentative hints of a more slowly spinning population at high
(M>5*10^7Msun) and low (M<2*10^6Msun) mass. I also engage in a brief review of
the spins of stellar-mass black holes in X-ray binaries. In general,
reflection-based and continuum-fitting based spin measures are in agreement,
although there remain two objects (GROJ1655-40 and 4U1543-475) for which that
is not true. I end this review by discussing the exciting frontier of
relativistic reverberation, particularly the discovery of broad iron line
reverberation in XMM-Newton data for the Seyfert galaxies NGC4151, NGC7314 and
MCG-5-23-16. As well as confirming the basic paradigm of relativistic disk
reflection, this detection of reverberation demonstrates that future large-area
X-ray observatories such as LOFT will make tremendous progress in studies of
strong gravity using relativistic reverberation in AGN.Comment: 19 pages. To appear in proceedings of the ISSI-Bern workshop on "The
Physics of Accretion onto Black Holes" (8-12 Oct 2012). Revised version adds
a missing source to Table 1 and Fig.6 (IRAS13224-3809) and corrects the
referencing of the discovery of soft lags in 1H0707-495 (which were in fact
first reported in Fabian et al. 2009
Constraining the mass of dark photons and axion-like particles through black-hole superradiance
Ultralight bosons and axion-like particles appear naturally in different
scenarios and could solve some long-standing puzzles. Their detection is
challenging, and all direct methods hinge on unknown couplings to the Standard
Model of particle physics. However, the universal coupling to gravity provides
model-independent signatures for these fields. We explore here the superradiant
instability of spinning black holes triggered in the presence of such fields.
The instability taps angular momentum from and limits the maximum spin of
astrophysical black holes. We compute, for the first time, the spectrum of the
most unstable modes of a massive vector (Proca) field for generic black-hole
spin and Proca mass. The observed stability of the inner disk of stellar-mass
black holes can be used to derive \emph{direct} constraints on the mass of dark
photons in the mass range . By including also higher azimuthal modes, similar
constraints apply to axion-like particles in the mass range
.
Likewise, mass and spin distributions of supermassive BHs --~as measured
through continuum fitting, K iron line, or with the future space-based
gravitational-wave detector LISA~-- imply indirect bounds in the mass range
approximately , for both axion-like particles and dark photons. Overall,
superradiance allows to explore a region of approximately orders of
magnitude in the mass of ultralight bosons
X-ray Structure between the Innermost Disk and Optical Broad Line Region in NGC 4151
International audienceWe present an analysis of the narrow Fe Kα line in Chandra/HETGS observations of the Seyfert active galactic nucleus (AGN) NGC 4151. The sensitivity and resolution afforded by the gratings reveal asymmetry in this line. Models including weak Doppler boosting, gravitational redshifts, and scattering are generally preferred over Gaussians at the 5Ï level of confidence, and generally measure radii consistent with . Separate fits to âhigh/unobscuredâ and âlow/obscuredâ phases reveal that the line originates at smaller radii in high-flux states; model-independent tests indicate that this effect is significant at the 4â5Ï level. Some models and Ît â 2 Ă 104 s variations in line flux suggest that the narrow Fe Kα line may originate at radii as small as in high-flux states. These results indicate that the narrow Fe Kα line in NGC 4151 is primarily excited in the innermost part of the optical broad line region (BLR), or X-ray BLR. Alternatively, a warp could provide the solid angle needed to enhance Fe Kα line emission from intermediate radii, and might resolve an apparent discrepancy in the inclination of the innermost and outer disk in NGC 4151. Both warps and the BLR may originate through radiation pressure, so these explanations may be linked. We discuss our results in detail, and consider the potential for future observations with Chandra, XARM, and ATHENA to measure black hole masses and to study the intermediate disk in AGNs using narrow Fe Kα emission lines
A hard look at local, optically-selected, obscured Seyfert galaxies
International audienceWe study the X-ray spectra of a sample of 19 obscured, optically selected Seyfert galaxies (Sy 1.8, 1.9, and 2) in the local universe (d â€Â 175 Mpc), drawn from the CfA Seyfert sample. Our analysis is driven by the high sensitivity of NuSTAR in the hard X-rays, coupled with soft X-ray spectra using XMM-Newton, Chandra, Suzaku, and Swift/XRT. We also analyze the optical spectra of these sources in order to obtain accurate mass estimates and Eddington fractions. We employ four different models to analyze the X-ray spectra of these sources, which all provide consistent results. We find that 79%â90% of the sources are heavily obscured with line-of-sight column density N H > 1023 cmâ2. We also find a Compton-thick (N H > 1024 cmâ2) fraction of 37%â53%. These results are consistent with previous estimates based on multiwavelength analyses. We find that the fraction of reprocessed to intrinsic emission is positively correlated with N H and negatively correlated with the intrinsic, unabsorbed X-ray luminosity (in agreement with the IwasawaâTaniguchi effect). Our results support the hypothesis that radiation pressure regulates the distribution of the circumnuclear material
The nature of the broadband X-ray variability in the dwarf Seyfert galaxy NGC 4395
International audienceWe present a flux-resolved X-ray analysis of the dwarf Seyfert 1.8 galaxy NGC 4395, based on three archival and one archival observations. The source is known to harbor a low mass black hole () and shows strong variability in the full X-ray range during these observations. We model the flux-resolved spectra of the source assuming three absorbing layers: neutral, mildly ionized, and highly ionized (, , and , respectively. The source also shows intrinsic variability by a factor of , on short timescales, due to changes in the nuclear flux, assumed to be a power law (). Our results show a positive correlation between the intrinsic flux and the absorbers' ionization parameter. The covering fraction of the neutral absorber varies during the first observation, which could explain the pronounced soft X-ray variability. However, the source remains fully covered by this layer during the other two observations, largely suppressing the soft X-ray variability. This suggests an inhomogeneous and layered structure in the broad line region. We also find a difference in the characteristic timescale of the power spectra between different energy ranges and observations. We finally show simulated spectra with , , and , which will allow us to characterize the different absorbers, study their dynamics, and will help us identify their locations and sizes
Iron K alpha line of Proca stars
X-ray reflection spectroscopy can be a powerful tool to test the nature of astrophysical black holes. Extending previous work on Kerr black holes with scalar hair [1] and on boson stars [2], here we study whether astrophysical black hole candidates may be horizonless, self-gravitating, vector Bose-Einstein condensates, known as Proca stars [3]. We find that observations with current X-ray missions can only provide weak constraints and rule out solely Proca stars with low compactness. There are two reasons. First, at the moment we do not know the geometry of the corona, and therefore the uncertainty in the emissivity pro file limits the ability to constrain the background metric. Second, the photon number count is low even in the case of a bright black hole binary, and we cannot have a precise measurement of the spectrum