89 research outputs found
O VII and O VIII absorption by hot gas in the vicinity of the Galaxy
(abridged) We searched for evidence of soft X-ray absorption by hot gas in
the vicinity of the Galaxy in a small sample of fifteen type I AGN observed
with the high resolution X-ray gratings on board Chandra. We find that around
half of the sight lines in our sample exhibit absorption due to local H- or
He-like Oxygen (or both) at confidence levels ranging from >90% to >3sigma.
Depending on the sight line, the absorption can be identified with hot gas in
particular local structures, the Local Group (LG) or the putative local hot
intergalactic medium (IGM). Several sight lines in our sample coincide with
sight lines in a study of O VI absorption by local gas, so an assumption of
collisional ionization equilibrium (CIE) allows us to constrain the temperature
of the local hot gas. We show that some portion of the hot absorbing outflows
apparently detected in the spectra of NGC 4051, PDS 456 and PG 1211+143
respectively could actually correspond to absorption by hot local gas since the
outflow velocity from each of these AGN coincides with the respective
cosmological recession velocity of the AGN.Comment: 8 pages. Modified discussion of Fe-K band absorption features in PDS
456 and PG 1211+14
Effective two-body scatterings around a massive object
Two-body scatterings under the potential of a massive object are very common
in astrophysics. If the massive body is far enough away that the two small
bodies are in their own gravitational sphere of influence, the gravity of the
massive body can be temporarily ignored. However, this requires the scattering
process to be fast enough that the small objects do not spend too much time at
distances near the surface of the sphere of influence. In this paper, we derive
the validation criteria for effective two-body scattering and establish a
simple analytical solution for this process, which we verify through numerical
scattering experiments. We use this solution to study star-black hole
scatterings in the disks of Active Galactic Nuclei and planet-planet
scatterings in planetary systems, and calculate their one-dimensional
cross-section analytically. Our solution will be valuable in reducing
computational time when treating two-body scatterings under the potential of a
much more massive third body, provided that the problem settings are in the
valid parameter space region identified by our study.Comment: Comments are Welcom
A new physical interpretation of optical and infrared variability in quasars
Changing-look quasars are a recently identified class of active galaxies in
which the strong UV continuum and/or broad optical hydrogen emission lines
associated with unobscured quasars either appear or disappear on timescales of
months to years. The physical processes responsible for this behaviour are
still debated, but changes in the black hole accretion rate or accretion disk
structure appear more likely than changes in obscuration. Here we report on
four epochs of spectroscopy of SDSS J110057.70-005304.5, a quasar at a redshift
of whose UV continuum and broad hydrogen emission lines have faded,
and then returned over the past 20 years. The change in this quasar
was initially identified in the infrared, and an archival spectrum from 2010
shows an intermediate phase of the transition during which the flux below
rest-frame 3400\AA\ has decreased by close to an order of magnitude.
This combination is unique compared to previously published examples of
changing-look quasars, and is best explained by dramatic changes in the
innermost regions of the accretion disk. The optical continuum has been rising
since mid-2016, leading to a prediction of a rise in hydrogen emission line
flux in the next year. Increases in the infrared flux are beginning to follow,
delayed by a 3 year observed timescale. If our model is confirmed, the
physics of changing-look quasars are governed by processes at the innermost
stable circular orbit (ISCO) around the black hole, and the structure of the
innermost disk. The easily identifiable and monitored changing-look quasars
would then provide a new probe and laboratory of the nuclear central engine.Comment: 13 pages, 4 figures, 3 tables. Published in MNRAS. All code and data
links on GitHub, https://github.com/d80b2t/WISE_L
The first high-redshift changing-look quasars
We report on three redshift quasars with dramatic changes in their C IV
emission lines, the first sample of changing-look quasars (CLQs) at high
redshift. This is also the first time the changing-look behaviour has been seen
in a high-ionisation emission line. SDSS J1205+3422, J1638+2827, and J2228+2201
show interesting behaviour in their observed optical light curves, and
subsequent spectroscopy shows significant changes in the C IV broad emission
line, with both line collapse and emergence being displayed on rest-frame
timescales of 240-1640 days. These are rapid changes, especially when
considering virial black hole mass estimates of
for all three quasars. Continuum and emission line measurements from the three
quasars show changes in the continuum-equivalent width plane with the CLQs seen
to be on the edge of the full population distribution, and showing indications
of an intrinsic Baldwin effect. We put these observations in context with
recent state-change models, and note that even in their observed low-state, the
C IV CLQs are generally above 5\% in Eddington luminosity.Comment: 12 pages, 7 figures, 4 tables. All data, analysis code and text are
fully available at: github.com/d80b2t/CIV_CLQs. Comments, questions and
suggestions welcome and encourage
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