1,075 research outputs found
Narrow-Line Seyfert 1 Galaxies
I provide a short review of the properties of Narrow-line Seyfert 1 (NLS1)
galaxies across the electromagnetic spectrum and of the models to explain them.
Their continuum and emission-line properties manifest one extreme form of
Seyfert activity. As such, NLS1 galaxies may hold important clues to the key
parameters that drive nuclear activity. Their high accretion rates close to the
Eddington rate provide new insight into accretion physics, their low black hole
masses and perhaps young ages allow us to address issues of black hole growth,
their strong optical FeII emission places strong constraints on FeII and
perhaps metal formation models and physical conditions in these emission-line
clouds, and their enhanced radio quiteness permits a fresh look at causes of
radio loudness and the radio-loud radio-quiet bimodality in AGN.Comment: To appear in the proceedings of "The nuclear region, host galaxy and
environment of AGN", E. Benitez, I. Cruz-Gonzalez & Y. Krongold (eds),
RevMexA
Recoiling black holes: electromagnetic signatures, candidates, and astrophysical implications
Supermassive black holes (SMBHs) may not always reside right at the centers
of their host galaxies. This is a prediction of numerical relativity
simulations, which imply that the newly formed single SMBH, after binary
coalescence in a galaxy merger, can receive kick velocities up to several 1000
km/s due to anisotropic emission of gravitational waves. Long-lived
oscillations of the SMBHs in galaxy cores, and in rare cases even SMBH
ejections from their host galaxies, are the consequence. Observationally,
accreting recoiling SMBHs would appear as quasars spatially and/or
kinematically off-set from their host galaxies. The presence of the "kicks" has
a wide range of astrophysical implications which only now are beginning to be
explored, including consequences for black hole and galaxy assembly at the
epoch of structure formation, black hole feeding, and unified models of Active
Galactic Nuclei (AGN). Here, we review the observational signatures of
recoiling SMBHs and the properties of the first candidates which have emerged,
including follow-up studies of the candidate recoiling SMBH of
SDSSJ092712.65+294344.0.Comment: 10 pages, Advances in Astronomy 2012, one chapter of the special
issue "Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus
Supermassive Black Holes
Tidal disruption of stars by supermassive black holes: Status of observations
Stars in the immediate vicinity of supermassive black holes (SMBHs) can be
ripped apart by the tidal forces of the black hole. The subsequent accretion of
the stellar material causes a spectacular flare of electromagnetic radiation.
Here, we provide a review of the observations of tidal disruption events
(TDEs), with an emphasis on the important contributions of Swift to this field.
TDEs represent a new probe of matter under strong gravity, and have opened up a
new window into studying accretion physics under extreme conditions. The events
probe relativistic effects, provide a new means of measuring black hole spin,
and represent signposts of intermediate-mass BHs, binary BHs and recoiling BHs.
Luminous, high-amplitude X-ray flares, matching key predictions of the tidal
disruption scenario, have first been discovered with ROSAT, and more recently
with other missions and in other wavebands. The Swift discovery of two
gamma-ray emitting, jetted TDEs, never seen before, has provided us with a
unique probe of the early phases of jet formation and evolution, and
SwiftJ1644+75 has the best covered lightcurve of any TDE to date. Further,
Swift has made important contributions in providing well-covered lightcurves of
TDEs discovered with other instruments, setting constraints on the physics that
govern the TDE evolution, and including the discovery of the first candidate
binary SMBH identified from a TDE lightcurve.Comment: Review, 12 pages, to appear in Journal of High-Energy Astrophysics.
Proceedings (review contributions) of "Swift: 10 years of discovery
Follow-Up Chandra Observations of Three Candidate Tidal Disruption Events
Large-amplitude, high-luminosity soft X-ray flares were detected by the ROSAT
All-Sky Survey in several galaxies with no evidence of Seyfert activity in
their ground-based optical spectra. These flares had the properties predicted
for a tidal disruption of a star by a central supermassive black hole. We
report Chandra observations of three of these galaxies taken a decade after
their flares that reveal weak nuclear X-ray sources that are from 240 to 6000
times fainter than their luminosities at peak, supporting the theory that these
were special events and not ongoing active galactic nucleus (AGN) variability.
The decline of RX J1624.9+7554 by a factor of 6000 is consistent with the
(t-t_D)^(-5/3) decay predicted for the fall-back phase of a tidal disruption
event, but only if ROSAT was lucky enough to catch the event exactly at its
peak in 1990 October. RX J1242.6-1119A has declined by a factor of 240, also
consistent with (t-t_D)^(-5/3). In the H II galaxy NGC 5905 we find only
resolved, soft X-ray emission that is undoubtedly associated with starburst
activity. When accounting for the starburst component, the ROSAT observations
of NGC 5905, as well as the Chandra upper limit on its nuclear flux, are
consistent with a (t-t_D)^(-5/3) decay by at least a factor of 1000. Although
we found weak Seyfert~2 emission lines in Hubble Space Telescope spectra of NGC
5905, indicating that a low-luminosity AGN was present prior to the X-ray
flare, we favor a tidal disruption explanation for the flare itself.Comment: 17 pages, 4 figures, to appear in ApJ April 1 200
Narrow double-peaked emission lines of SDSS J131642.90+175332.5: signature of a single or a binary AGN in a merger, jet-cloud interaction, or unusual narrow-line region geometry
We present an analysis of the active galaxy SDSS J131642.90+175332.5, which
is remarkable because all of its narrow emission lines are double-peaked, and
because it additionally shows an extra broad component (FHWM ~ 1400 km/s) in
most of its forbidden lines, peaking in between the two narrow systems. The
peaks of the two narrow systems are separated by 400--500 km/s in velocity
space. The spectral characteristics of double-peaked [O III] emission have
previously been interpreted as a signature of dual or binary active galactic
nuclei (AGNs), among other models. In the context of the binary scenario, SDSS
J131642.90+175332.5 is a particularly good candidate because not just one line
but all of its emission lines are double-peaked. However, we also discuss a
number of other scenarios which can potentially account for double-peaked
narrow emission lines, including projection effects, a two-sided outflow,
jet-cloud interactions, special narrow-line region (NLR) geometries (disks,
bars, or inner spirals), and a galaxy merger with only one AGN illuminating two
NLRs. We argue that the similarity of the emission-line ratios in both systems,
and the presence of the very unusual broad component at intermediate velocity,
makes a close pair of unrelated AGNs unlikely, and rather argues for processes
in a single galaxy or merger. We describe future observations which can
distinguish between these remaining possibilities.Comment: ApJ Letters, 705, L20-L24, 2009 (November 1 issue); incl. 2 colour
figure
Mkn 1239: A highly polarized NLS1 with a steep X-ray spectrum and strong NeIX emission
We report the results of an XMM-Newton observation of the Narrow-Line Seyfert
1 galaxy Mkn 1239. This optically highly polarized AGN has one of the steepest
X-ray spectra found in AGN with alpha-X = +3.0 based on ROSAT PSPC data. The
XMM-Newton EPIC PN and MOS data confirm this steep X-ray spectrum. The PN data
are best-fit by a powerlaw with a partial covering absorption model suggesting
two light paths between the continuum source and the observer, one indirect
scattered one which is less absorbed and a highly absorbed direct light path.
This result agrees with the wavelength dependent degree of polarization in the
optical/UV band. Residuals in the X-ray spectra of all three XMM-Newton EPIC
detectors around 0.9 keV suggest the presence of an emission line feature, most
likely the Ne IX triplet. The detection of NeIX and the non-detection of
OVII/OVIII suggest a super-solar Ne/O ratio.Comment: Submitted to Aj, 11 pages, 8 figue
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