702 research outputs found
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
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
A Huge Drop in X-ray Luminosity of the Non-Active Galaxy RXJ1242.6-1119A, and First Post-Flare Spectrum - Testing the Tidal Disruption Scenario
It has been suggested that an unavoidable consequence of the existence of
supermassive black holes, and the best diagnostic of their presence in
non-active galaxies, would be occasional tidal disruption of stars captured by
the black holes. These events manifest themselves in form of luminous flares
powered by accretion of debris from the disrupted star into the black hole.
Candidate events among optically non-active galaxies emerged in the past few
years. For the first time, we have looked with high spatial and spectral
resolution at one of these most extreme variability events ever recorded among
galaxies. Here, we report measuring a factor ~200 drop in luminosity of the
X-ray source RXJ 1242-1119 with the X-ray observatories Chandra and XMM-Newton,
and perform key tests of the favored outburst scenario, tidal disruption of a
star by a supermassive black hole. We show that the detected `low-state'
emission has properties such that it must still be related to the flare. The
power-law shaped post-flare X-ray spectrum indicates a `hardening' compared to
outburst. The inferred black hole mass, the amount of liberated energy, and the
duration of the event favor an accretion event of the form expected from the
(partial or complete) tidal disruption of a star (abstract abbreviated).Comment: to appear in March 1 issue of ApJ Letters (submitted Nov. 10,
accepted in Dec. 2003); background information available at
http://www.xray.mpe.mpg.de/~skomossa
Seyfert galaxies with Swift: giant flares, rapid drops, and other surprises
Swift has initiated a new era of understanding the extremes of active
galactic nuclei (AGN) variability, their drivers and underlying physics. This
is based on its rapid response, high sensitivity, good spatial resolution, and
its ability to collect simultaneously X--ray-to-optical SEDs. Here, we present
results from our recent monitoring campaigns with Swift of highly variable AGN,
including outbursts, deep low states, and unusual long-term trends in several
Seyfert galaxies including Mrk 335, WPVS007, and RXJ2314.9+2243. We also report
detection of a new X-ray and optical outburst of IC 3599 and our Swift
follow-ups. IC 3599 was previously known as one of the AGN with the
highest-amplitude outbursts. We briefly discuss implications of this second
outburst of IC 3599 for emission scenarios including accretion-disk
variability, repeat tidal disruption events, and the presence of a binary
supermassive black hole.Comment: to appear in "Swift: 10 years of discovery", Proceedings of Scienc
Narrow-line Seyfert 1 Galaxies and the M_BH - sigma Relation
We have studied the location of narrow-line Seyfert 1 (NLS1) galaxies and
broad-line Seyfert 1 (BLS1) galaxies on the M_BH - sigma relation of non-active
galaxies. We find that NLS1 galaxies as a class - as well as the BLS1 galaxies
of our comparison sample - do follow the M_BH-sigma relation of non-active
galaxies if we use the width of the [SII]6716,6731 emission lines as surrogate
for stellar velocity dispersion, sigma_*. We also find that the width of
[OIII]5007 is a good surrogate for sigma_*, but only after (a) removal of
asymmetric blue wings, and, more important, after (b) excluding core [OIII]
lines with strong blueshifts (i.e., excluding galaxies which have their [OIII]
velocity fields dominated by radial motions, presumably outflows). The same
galaxies which are extreme outliers in [OIII] still follow the M_BH - sigma
relation in [SII]. We confirm previous findings that NLS1 galaxies are
systematically off-set from the M_BH - sigma relation if the full [OIII]
profile is used to measure sigma. We systematically investigate the influence
of several parameters on the NSL1 galaxies' location on the M_BH - sigma plane:
[OIII]_core blueshift, L/L_Edd, intensity ratio FeII/H_beta, NLR density, and
absolute magnitude. Implications for NLS1 models and for their evolution along
the M_BH - sigma relation are discussed.Comment: ApJ Letters, in press (3 figures, one in colour
Discovery of superstrong, fading, iron line emission and double-peaked Balmer lines of the galaxy SDSSJ0952+2143 - the light echo of a huge flare
We report the discovery of superstrong, fading, high-ionization iron line
emission in the galaxy SDSSJ095209.56+214313.3 (SDSSJ0952+2143 hereafter),
which must have been caused by an X-ray outburst of large amplitude.
SDSSJ0952+2143 is unique in its strong multiwavelength variability; such a
broadband emission-line and continuum response has not been observed before.
The strong iron line emission is accompanied by unusual Balmer line emission
with a broad base, narrow core and double-peaked narrow horns, and strong HeII
emission. These lines, while strong in the SDSS spectrum taken in 2005, have
faded away significantly in new spectra taken in December 2007. Comparison of
SDSS, 2MASS, GALEX and follow-up GROND photometry reveals variability in the
NUV, optical and NIR band. Taken together, these unusual observations can be
explained by a giant outburst in the EUV--X-ray band, detected even in the
optical and NIR. The intense and variable iron, Helium and Balmer lines
represent the ``light echo'' of the flare, as it traveled through circumnuclear
material. The outburst may have been caused by the tidal disruption of a star
by a supermassive black hole. Spectroscopic surveys such as SDSS are well
suited to detect emission-line light echoes of such rare flare events.
Reverberation-mapping of these light echoes can then be used as a new and
efficient probe of the physical conditions in the circumnuclear material in
non-active or active galaxies.Comment: ApJ Letters, 678, L13 (May 1 issue); incl. 4 colour figures. This and
related papers on tidal disruption flares also available at
http://www.xray.mpe.mpg.de/~skomossa
Candidate tidal disruption events from the XMM-Newton Slew Survey
In recent years, giant amplitude X-ray flares have been observed from a
handful of non-active galaxies. The most plausible scenario of these unusual
phenomena is tidal disruption of a star by a quiescent supermassive black hole
at the centre of the galaxy. Comparing the XMM-Newton Slew Survey Source
Catalogue with the ROSAT PSPC All-Sky Survey five galaxies have been detected a
factor of up to 88 brighter in XMM-Newton with respect to ROSAT PSPC upper
limits and presenting a soft X-ray colour. X-ray luminosities of these sources
derived from slew observations have been found in the range 10^41-10^44 erg
s^-1, fully consistent with the tidal disruption model. This model predicts
that during the peak of the outburst, flares reach X-ray luminosities up to
10^45 erg s^-1, which is close to the Eddington luminosity of the black hole,
and afterwards a decay of the flux on a time scale of months to years is
expected. Multi-wavelength follow-up observations have been performed on these
highly variable objects in order to disentangle their nature and to investigate
their dynamical evolution. Here we present sources coming from the XMM-Newton
Slew Survey that could fit in the paradigm of tidal disruption events. X-ray
and optical observations revealed that two of these objects are in full
agreement with that scenario and three other sources that, showing signs of
optical activity, need further investigation within the transient galactic
nuclei phenomena.Comment: 4 pages, 4 figures, A&A accepte
A tidal disruption-like X-ray flare from the quiescent galaxy SDSS J120136.02+300305.5
SDSS J120136.02+300305.5 was detected in an XMM-Newton slew from June 2010
with a flux 56 times higher than an upper limit from ROSAT, corresponding to
Lx~3x10^44 ergs/s. It has the optical spectrum of a quiescent galaxy (z=0.146).
Overall the X-ray flux has evolved consistently with the canonical t^-5/3
model, expected for returning stellar debris from a tidal disruption event,
fading by a factor ~300 over 300 days. In detail the source is very variable
and became invisible to Swift between 27 and 48 days after discovery, perhaps
due to self-absorption. The X-ray spectrum is soft but is not the expected tail
of optically thick thermal emission. It may be fit with a Bremsstrahlung or
double-power-law model and is seen to soften with time and declining flux.
Optical spectra taken 12 days and 11 months after discovery indicate a deficit
of material in the broad line and coronal line regions of this galaxy, while a
deep radio non-detection implies that a jet was not launched during this event.Comment: Accepted for publication in Astronomy and Astrophysics. 11 pages, 11
figure
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