19 research outputs found
A systematic analysis of the X-ray emission in optically selected tidal disruption events: observational evidence for the unification of the optically and X-ray selected populations
We present a systematic analysis of the X-ray emission of a sample of 17
optically selected, X-ray-detected tidal disruption events (TDEs) discovered
between 2014 and 2021. The X-ray light curves show a diverse range of temporal
behaviors, with most sources not following the expected power-law decline. The
X-ray spectra are mostly extremely soft and consistent with thermal emission
from the inner region of an accretion disk that cools as the accretion rate
decreases. Three sources show the formation of a hard X-ray corona around 200
days after the UV/optical peak. The shape of the spectral energy distribution,
traced by the ratio () between the UV/optical and X-ray
luminosities, shows a wide range at
early-times, and converges to disk-like values at late-times. The evolution of the derived physical parameters favors a
decrease in the optical depth of a reprocessing layer instead of delayed disk
formation to explain the late-time X-ray brightening found in several sources.
We estimate the fraction of optically discovered TDEs with erg s to be at least , and find that the X-ray loudness
is independent of black hole mass. We combine our sample with those from X-ray
surveys to construct an X-ray luminosity function, best fitted by a broken
power-law with a brake at erg s. We show that there is no
dichotomy between optically and X-ray selected TDEs; instead, there is a
continuum of early time , at least as wide as , with optical/X-ray surveys selecting
preferentially, but not exclusively, from the higher/lower end of the
distribution. Our findings are consistent with an orientation-dependent and
time-evolving reprocessing layer, and support viewing-angle unification models.Comment: 38 pages, 23 figures, submitted to Ap
A Comprehensive Investigation of Gamma-Ray Burst Afterglows Detected by TESS
Gamma-ray bursts produce afterglows that can be observed across the
electromagnetic spectrum and can provide insight into the nature of their
progenitors. While most telescopes that observe afterglows are designed to
rapidly react to trigger information, the Transiting Exoplanet Survey Satellite
(TESS) continuously monitors sections of the sky at cadences between 30 minutes
and 200 seconds. This provides TESS with the capability of serendipitously
observing the optical afterglow of GRBs. We conduct the first extensive search
for afterglows of known GRBs in archival TESS data reduced with the TESSreduce
package, and detect 11 candidate signals that are temporally coincident with
reported burst times. We classify 3 of these as high-likelihood GRB afterglows
previously unknown to have been detected by TESS, one of which has no other
afterglow detection reported on the Gamma-ray Coordinates Network. We classify
5 candidates as tentative and the remainder as unlikely. Using the afterglowpy
package, we model each of the candidate light curves with a Gaussian and a top
hat model to estimate burst parameters; we find that a mean time delay of
s between the explosion and afterglow onset is required to perform
these fits. The high cadence and large field of view make TESS a powerful
instrument for localising GRBs, with the potential to observe afterglows in
cases when no other backup photometry is possible.Comment: 17 pages, 7 figures, 5 table
The Eddington ratio-dependent âchanging lookâ events in NGC 2992
We present an analysis of historical multiwavelength emission of the `Changing Lookâ (CL) Active Galactic Nucleu (AGN) in NGC 2992, covering epochs ranging from 1978 to 2021, as well as new X-ray and optical spectra. The galaxy presents multiple Seyfert type transitions from Type 2 to intermediate-type, losing and regaining its Hα broad emission lines (BEL) recurrently. In X-rays, the source shows intrinsic variability with the absorption corrected luminosity varying by a factor of ⌠40. We rule-out tidal disruption events or variable obscuration as causes of the type transitions, and show that the presence and the flux of th Hα BEL is directly correlated with the 2â10 keV X-ray luminosity (L2â10): the component disappears at L2â10 †2.6 Ă 1042 erg cmâ2 sâ1; this luminosity value translates into an Eddington ratio (λEdd) of ⌠1 per cent. The λEdd in which the BEL transitions occur is the same as the critical value at which a state transition between a radiatively inefficient accretion flow and a thin accretion disk is expected, such similarity suggests that the AGN is operating at the threshold mass accretion rate between the two accretion modes. We find a correlation between the narrow Fe Kα flux and λEdd, and an anticorrelation between full-width at half maximum of Hα BEL and λEdd, in agreement with theoretical predictions. Two possible scenarios for type transitions are compatible with our results: either the dimming of the AGN continuum, which reduces the supply of ionizing photons available to excite the gas in the Broad Line Region (BLR), or the fading of the BLR structure itself occurs as the low accretion rate is not able to sustain the required cloud flow rate in a disc-wind BLR model
Exploring the AGN-Merger Connection in Arp 245 I: Nuclear Star Formation and Gas Outflow in NGC 2992
Galaxy mergers are central to our understanding of galaxy formation,
especially within the context of hierarchical models. Besides having a large
impact on the star formation history, mergers are also able to influence gas
motions at the centre of galaxies and trigger an Active Galactic Nucleus (AGN).
In this paper, we present a case study of the Seyfert galaxy NGC 2992, which
together with NGC 2993 forms the early-stage merger system Arp 245. Using
Gemini Multi-Object Spectrograph (GMOS) integral field unit (IFU) data from the
inner 1.1 kpc of the galaxy we were able to spatially resolve the stellar
populations, the ionisation mechanism and kinematics of ionised gas. From full
spectral synthesis, we found that the stellar population is primarily composed
by old metal-rich stars (t 1.4 Gyr, \zsun), with a
contribution of at most 30 per cent of the light from a young and metal-poor
population (t 100 Myr, \zsun). We detect \halpha and \hbeta
emission from the Broad Line Region (BLR) with a Full Width at Half Maximum
(FWHM) of 2000\kms. The Narrow Line Region (NLR) kinematics presents two
main components: one from gas orbiting the galaxy disk and a blueshifted
(velocity -200\kms) outflow, possibly correlated with the radio
emission, with mass outflow rate of 2 M yr and a
kinematic power of 2 erg s (\Eout/\Lbol
0.2 per cent). We also show even though the main ionisation mechanism
is the AGN radiation, ionisation by young stars and shocks may also contribute
to the emission line ratios presented in the innermost region of the galaxy.Comment: 20 pages, 13 Figs, Accepted for publication to the MNRA
Exploring the AGN-merger connection in Arp 245 : I: nuclear star formation and gas outflow in NGC 2992
Galaxy mergers are central to our understanding of galaxy formation, especially within the context of hierarchical models. Besides having a large impact on the star formation history, mergers are also able to influence gas motions at the centre of galaxies and trigger an active galactic nucleus (AGN). In this paper, we present a case study of the Seyfert galaxy NGC 2992, which together with NGC 2993 forms the early-stage merger system Arp 245. Using Gemini Multi-Object Spectrograph integral field unit data from the inner 1.1 kpc of the galaxy, we were able to spatially resolve the stellar populations, the ionization mechanism, and kinematics of ionized gas. From full spectral synthesis, we found that the stellar population is primarily composed by old metal-rich stars (t â„ 1.4 Gyr, Z â„ 2.0âZâ), with a contribution of at most 30 per cent of the light from a young and metal-poor population (t †100 Myr, Z †1.0âZâ). We detect Hâα and HâÎČ emission from the broad-line region with a full width at half-maximum of âŒ2000 kmsâ1â . The narrow-line region kinematics presents two main components: one from gas orbiting the galaxy disc and a blueshifted (velocity â â200 kmsâ1â ) outflow, possibly correlated with the radio emission, with mass outflow rate of âŒ2 Mâ yrâ1 and a kinematic power of âŒ2 Ă 1040 erg sâ1 (â EËout/Lbol â 0.2 per cent). We also show even though the main ionization mechanism is the AGN radiation, ionization by young stars and shocks may also contribute to the emission line ratios presented in the innermost region of the galaxy
The first systematically identified repeating partial tidal disruption event
Tidal disruption events (TDEs) occur when a star enters the tidal radius of a
supermassive black hole (SMBH). If the star only grazes the tidal radius, a
fraction of the stellar mass will be accreted in a partial TDE (pTDE). The
remainder can continue orbiting and may re-disrupted at pericenter, causing a
repeating pTDE. pTDEs may be as or more common than full TDEs (fTDEs), yet few
are known. In this work, we present the discovery of the first repeating pTDE
from a systematically-selected sample, AT\,2020vdq. AT\,2020vdq was originally
identified as an optically- and radio-flaring TDE. Around years after its
discovery, it rebrightened dramatically and rapidly in the optical. The optical
flare was remarkably fast and luminous compared to previous TDEs. It was
accompanied by extremely broad () optical/UV spectral features and
faint X-ray emission (\,erg\,s), but no new
radio-emitting component. Based on the transient optical/UV spectral features
and the broadband light curve, we show that AT\,2020vdq is a repeating pTDE. We
then use it to constrain TDE models; in particular, we favor a star originally
in a very tight binary system that is tidally broken apart by the Hills
mechanism. We also constrain the repeating pTDE rate to be to
yr galaxy, with uncertainties dominated by the unknown
distribution of pTDE repeat timescales. In the Hills framework, this means the
binary fraction in the galactic nucleus is of the order few percent.Comment: 24 pages, 13 figures, submitted to Ap
X-ray eruptions every 22 days from the nucleus of a nearby galaxy
Galactic nuclei showing recurrent phases of activity and quiescence have
recently been discovered, with recurrence times as short as a few hours to a
day -- known as quasi-periodic X-ray eruption (QPE) sources -- to as long as
hundreds to a thousand days for repeating nuclear transients (RNTs). Here we
report the discovery of Swift J023017.0+283603 (hereafter Swift J0230+28), a
source that exhibits X-ray quasi-periodic eruptions from the nucleus of a
previously unremarkable galaxy at 165 Mpc, with a recurrence time of
approximately 22 days, an intermediary timescale between known RNTs and QPE
sources. We also report transient radio emission from the source, which is
likely associated with the X-ray eruptions. Such recurrent soft X-ray eruptions
from a low-mass black hole, with no accompanying UV/optical emission are
strikingly similar to QPE sources. However, in addition to having a recurrence
time that is times longer than the longest-known QPE source, Swift
J0230+28's eruptions exhibit slightly distinct shapes and temperature evolution
than the known QPE sources. The observed properties disfavor disk instability
models, and instead favor scenarios involving extreme mass ratio inspirals. Our
discovery reveals a new timescale for repeating extragalactic transients and
highlights the need for a wide-field, time-domain X-ray mission, which would
enable the exploration of the parameter space of recurring X-ray transients.Comment: Under review on Nature Astronomy. Main Section: 14 pages, 3 figures
and 1 Table. Methods: 32 pages, 11 Figures, 4 Table
The Tidal Disruption Event AT2021ehb : Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk-Corona System
© 2022. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/We present X-ray, UV, optical, and radio observations of the nearby ( Mpc) tidal disruption event (TDE) AT2021ehb/ZTF21aanxhjv during its first 430 days of evolution. AT2021ehb occurs in the nucleus of a galaxy hosting a black hole ( inferred from host galaxy scaling relations). High-cadence Swift and NICER monitoring reveals a delayed X-ray brightening. The spectrum first undergoes a gradual transition and then suddenly turns soft again within 3 days at days during which the X-ray flux drops by a factor of ten. In the joint NICER+NuSTAR observation ( days, harder state), we observe a prominent non-thermal component up to 30 keV and an extremely broad emission line in the iron K band. The bolometric luminosity of AT2021ehb reaches a maximum of when the X-ray spectrum is the hardest. During the dramatic X-ray evolution, no radio emission is detected, the UV/optical luminosity stays relatively constant, and the optical spectra are featureless. We propose the following interpretations: (i) the transition may be caused by the gradual formation of a magnetically dominated corona; (ii) hard X-ray photons escape from the system along solid angles with low scattering optical depth (a few) whereas the UV/optical emission is likely generated by reprocessing materials with much larger column density -- the system is highly aspherical; (iii) the abrupt X-ray flux drop may be triggered by the thermal-viscous instability in the inner accretion flow leading to a much thinner disk.Peer reviewe
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A Transient âChanging-lookâ Active Galactic Nucleus Resolved on Month Timescales from First-year Sloan Digital Sky Survey V Data
We report the discovery of a new âchanging-lookâ active galactic nucleus (CLAGN) event, in the quasar SDSS J162829.17+432948.5 at z = 0.2603, identified through repeat spectroscopy from the fifth Sloan Digital Sky Survey (SDSS-V). Optical photometry taken during 2020-2021 shows a dramatic dimming of Îg â 1 mag, followed by a rapid recovery on a timescale of several months, with the âČ2 month period of rebrightening captured in new SDSS-V and Las Cumbres Observatory spectroscopy. This is one of the fastest CLAGN transitions observed to date. Archival observations suggest that the object experienced a much more gradual dimming over the period of 2011-2013. Our spectroscopy shows that the photometric changes were accompanied by dramatic variations in the quasar-like continuum and broad-line emission. The excellent agreement between the pre- and postdip photometric and spectroscopic appearances of the source, as well as the fact that the dimmest spectra can be reproduced by applying a single extinction law to the brighter spectral states, favor a variable line-of-sight obscuration as the driver of the observed transitions. Such an interpretation faces several theoretical challenges, and thus an alternative accretion-driven scenario cannot be excluded. The recent events observed in this quasar highlight the importance of spectroscopic monitoring of large active galactic nucleus samples on weeks-to-months timescales, which the SDSS-V is designed to achieve
A Transient "Changing-look'' Active Galactic Nucleus Resolved on Month Timescales from First-year Sloan Digital Sky Survey V Data
We report the discovery of a new ``changing-look'' active galactic nucleus
(CLAGN) event, in the quasar SDSS J162829.17+432948.5 at z=0.2603, identified
through repeat spectroscopy from the fifth Sloan Digital Sky Survey (SDSS-V).
Optical photometry taken during 2020--2021 shows a dramatic dimming of
g1 mag, followed by a rapid recovery on a timescale of
several months, with the 2 month period of rebrightening captured
in new SDSS-V and Las Cumbres Observatory spectroscopy. This is one of the
fastest CLAGN transitions observed to date. Archival observations suggest that
the object experienced a much more gradual dimming over the period of
2011--2013. Our spectroscopy shows that the photometric changes were
accompanied by dramatic variations in the quasar-like continuum and broad-line
emission. The excellent agreement between the pre- and postdip photometric and
spectroscopic appearances of the source, as well as the fact that the dimmest
spectra can be reproduced by applying a single extinction law to the brighter
spectral states, favor a variable line-of-sight obscuration as the driver of
the observed transitions. Such an interpretation faces several theoretical
challenges, and thus an alternative accretion-driven scenario cannot be
excluded. The recent events observed in this quasar highlight the importance of
spectroscopic monitoring of large active galactic nucleus samples on
weeks-to-months timescales, which the SDSS-V is designed to achieve.Comment: Published in ApJ