649 research outputs found
Rapid accretion state transitions following the tidal disruption event AT2018fyk
Following a tidal disruption event (TDE), the accretion rate can evolve from
quiescent to near-Eddington levels and back over months - years timescales.
This provides a unique opportunity to study the formation and evolution of the
accretion flow around supermassive black holes (SMBHs). We present two years of
multi-wavelength monitoring observations of the TDE AT2018fyk at X-ray, UV,
optical and radio wavelengths. We identify three distinct accretion states and
two state transitions between them. These appear remarkably similar to the
behaviour of stellar-mass black holes in outburst. The X-ray spectral
properties show a transition from a soft (thermal-dominated) to a hard
(power-law dominated) spectral state around Lfew L, and the strengthening of the corona over time
100--200 days after the UV/optical peak. Contemporaneously, the spectral
energy distribution (in particular, the UV-to-X-ray spectral slope
) shows a pronounced softening as the outburst progresses. The
X-ray timing properties also show a marked change, initially dominated by
variability at long (day) timescales while a high frequency (10
Hz) component emerges after the transition into the hard state. At late times
(500 days after peak), a second accretion state transition occurs, from
the hard into the quiescent state, as identified by the sudden collapse of the
bolometric (X-ray+UV) emission to levels below 10 L. Our
findings illustrate that TDEs can be used to study the scale (in)variance of
accretion processes in individual SMBHs. Consequently, they provide a new
avenue to study accretion states over seven orders of magnitude in black hole
mass, removing limitations inherent to commonly used ensemble studies.Comment: Accepted version following referee comments. 2 new figures compared
to previous arxiv version (Figs 9 and 10). Data will be available from the
journal webpages, or upon request to the author
High-Resolution X-ray and Ultraviolet Spectroscopy of the Complex Intrinsic Absorption in NGC 4051 with Chandra and HST
We present the results from simultaneous observations of the Narrow-Line
Seyfert 1 galaxy NGC 4051 with the Chandra High Energy Transmission Grating
Spectrometer and the HST Space Telescope Imaging Spectrograph. The X-ray
grating spectrum reveals absorption and emission lines from hydrogen-like and
helium-like ions of O, Ne, Mg and Si. We resolve two distinct X-ray absorption
systems: a high-velocity blueshifted system at -2340+/-130 km/s and a
low-velocity blueshifted system at -600+/-130 km/s. In the UV spectrum we
detect strong absorption, mainly from C IV, N V and Si IV, that is resolved
into as many as nine different intrinsic absorption systems with velocities
between -650 km/s and 30 km/s. Although the low-velocity X-ray absorption is
consistent in velocity with many of the UV absorption systems, the
high-velocity X-ray absorption seems to have no UV counterpart. In addition to
the absorption and emission lines, we also observe rapid X-ray variability and
a state of low X-ray flux during the last ~15 ks of the observation. NGC 4051
has a soft X-ray excess which we fit in both the high and low X-ray flux
states. The high-resolution X-ray spectrum directly reveals that the soft
excess is not composed of narrow emission lines and that it has significant
spectral curvature. A power-law model fails to fit it, while a blackbody
produces a nearly acceptable fit. We compare the observed spectral variability
with the results of previous studies of NGC 4051.Comment: 16 pages, 13 figures included, LaTeX emulateapj5.sty, accepted for
publication in The Astrophysical Journal (this version is the same as the
first version
Recommended from our members
The high energy Universe at ultra-high resolution: the power and promise of X-ray interferometry
We propose the development of X-ray interferometry (XRI), to reveal the Universe at high energies with ultra-high spatial resolution. With baselines which can be accommodated on a single spacecraft, XRI can reach 100 ÎŒ as resolution at 10 Ă
(1.2 keV) and 20 ÎŒ as at 2 Ă
(6 keV), enabling imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby accreting supermassive black holes (SMBH) and the SMBH âshadowâ; SMBH accretion flows and outflows; X-ray binary winds and orbits; stellar coronae within âŒ100 pc and many exoplanets which transit across them. For sufficiently luminous sources XRI will resolve sub-pc scales across the entire observable Universe, revealing accreting binary SMBHs and enabling trigonometric measurements of the Hubble constant with X-ray light echoes from quasars or explosive transients. A multi-spacecraft âconstellationâ interferometer would resolve well below 1 ÎŒ as, enabling SMBH event horizons to be resolved in many active galaxies and the detailed study of the effects of strong field gravity on the dynamics and emission from accreting gas close to the black hole
Intensive disc-reverberation mapping of Fairall 9: First year of Swift and LCO monitoring
We present results of time-series analysis of the first year of the Fairall 9 intensive disc-reverberation campaign. We used Swift and the Las Cumbres Observatory global telescope network to continuously monitor Fairall 9 from X-rays to near-infrared at a daily to subdaily cadence. The cross-correlation function between bands provides evidence for a lag spectrum consistent with the Ï â λ4/3 scaling expected for an optically thick, geometrically thin blackbody accretion disc. Decomposing the flux into constant and variable components, the variable componentâs spectral energy distribution is slightly steeper than the standard accretion disc prediction. We find evidence at the Balmer edge in both the lag and flux spectra for an additional bound-free continuum contribution that may arise from reprocessing in the broad-line region. The inferred driving light curve suggests two distinct components, a rapidly variable (100 d) component with an opposite lag to the reverberation signal.JVHS and KH acknowledge support from the Science and Technology Facilities Council grant ST/R000824/1. RE gratefully acknowledges support from National Aeronautics and Space Administration (NASA) Swift Key Project
grant number 80NSSC19K0153. JMG gratefully acknowledges support from NASA under the ADAP award 80NSSC17K0126. AAB, KLP, and PAE acknowledge support from the UK Space Agency. Research by AJB was supported by National Science Foundation (NSF) grant AST-1907290. EMC gratefully acknowledges support from the NSF through grant AST-1909199. MV gratefully acknowledges
financial support from the Independent Research Fund Denmark via
grant number DFF 8021-00130. The authors appreciate the hard work and dedication of the Swift Observatory staff, who created a new UVOT mode in support of this project and put in considerableeffort in scheduling this large program. This work makes use of observations from the LCO network, and of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research was made possible through the use of the AAVSO Photometric All-Sky Survey (APASS), funded by the Robert Martin Ayers Sciences Fund and
NSF AST-1412587. This research also made use of ASTROPY, a
community-developed core PYTHON package for astronomy (Astropy Collaboration et al. 2013), and MATPLOTLIB (Hunter 2007)
Luminous Thermal Flares from Quiescent Supermassive Black Holes
A dormant supermassive black hole lurking in the center of a galaxy will be
revealed when a star passes close enough to be torn apart by tidal forces, and
a flare of electromagnetic radiation is emitted when the bound fraction of the
stellar debris falls back onto the black hole and is accreted. Here we present
the third candidate tidal disruption event discovered in the GALEX Deep Imaging
Survey: a 1.6x10^{43} erg s^{-1} UV/optical flare from a star-forming galaxy at
z=0.1855. The UV/optical SED during the peak of the flare measured by GALEX and
Palomar LFC imaging can be modeled as a single temperature blackbody with
T_{bb}=1.7x10^{5} K and a bolometric luminosity of 3x10^{45} erg s^{-1},
assuming an internal extinction with E(B-V)_{gas}=0.3. The Chandra upper limit
on the X-ray luminosity during the peak of the flare, L_{X}(2-10 keV)< 10^{41}
erg s^{-1}, is 2 orders of magnitude fainter than expected from the ratios of
UV to X-ray flux density observed in active galaxies. We compare the light
curves and broadband properties of all three tidal disruption candidates
discovered by GALEX, and find that (1) the light curves are well fitted by the
power-law decline expected for the fallback of debris from a tidally disrupted
solar-type star, and (2) the UV/optical SEDs can be attributed to thermal
emission from an envelope of debris located at roughly 10 times the tidal
disruption radius of a ~10^{7} M_sun central black hole. We use the observed
peak absolute optical magnitudes of the flares (-17.5 > M_{g} > -18.9) to
predict the detection capabilities of upcoming optical synoptic surveys.
(Abridged)Comment: Accepted for Publication in ApJ, 19 pages, 10 figures, 2 tables,
emulateapj, corrections from proofs adde
Rituximab in Combination with Corticosteroids for the Treatment of Anti-Neutrophil Cytoplasmic Antibody-Associated Vasculitis: A NICE Single Technology Appraisal
As part of its single technology appraisal (STA) process, the National Institute for Health and Care Excellence (NICE) invited the manufacturer of rituximab (Roche Products) to submit evidence of the clinical and cost effectiveness of rituximab in combination with corticosteroids for treatment of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). The School of Health and Related Research Technology Appraisal Group at the University of Sheffield was commissioned to act as the independent Evidence Review Group (ERG). The ERG produced a critical review of the evidence for the clinical and cost effectiveness of the technology, based upon the manufacturerâs submission to NICE. The evidence was derived mainly from a double-blind, phase III, placebo-controlled trial of rituximab in patients with new or relapsed âsevereâ AAV, which compared a rituximab treatment regimen with an oral cyclophosphamide treatment regimen. Intravenous cyclophosphamide is also commonly used but was not included in the pivotal trial. The evidence showed that rituximab is noninferior to oral cyclophosphamide in terms of induction of remission in adults with AAV and de novo disease, and is superior to oral cyclophosphamide in terms of remission in adults who have relapsed once on cyclophosphamide. The ERG concluded that the results of the manufacturerâs economic evaluation could not be considered robust, because of errors and because the full range of relevant treatment sequences were not modelled. The ERG amended the manufacturerâs model and demonstrated that rituximab was likely to represent a cost-effective addition to the treatment sequence if given after cyclophosphamide treatment
- âŠ