41 research outputs found
Black hole masses of tidal disruption event host galaxies
The mass of the central black hole in a galaxy that hosted a tidal disruption
event (TDE) is an important parameter in understanding its energetics and
dynamics. We present the first homogeneously measured black hole masses of a
complete sample of 12 optically/UV selected TDE host galaxies (down to
22 mag and =0.37) in the Northern sky. The mass estimates
are based on velocity dispersion measurements, performed on late time optical
spectroscopic observations. We find black hole masses in the range
310 MM210 M.
The TDE host galaxy sample is dominated by low mass black holes (10
M), as expected from theoretical predictions. The blackbody peak
luminosity of TDEs with M10 M is consistent
with the Eddington limit of the SMBH, whereas the two TDEs with M10 M have peak luminosities below their SMBH
Eddington luminosity, in line with the theoretical expectation that the
fallback rate for M10 M is sub-Eddington. In
addition, our observations suggest that TDEs around lower mass black holes
evolve faster. These findings corroborate the standard TDE picture in 10
M black holes. Our results imply an increased tension between
observational and theoretical TDE rates. By comparing the blackbody emission
radius with theoretical predictions, we conclude that the optical/UV emission
is produced in a region consistent with the stream self-intersection radius of
shallow encounters, ruling out a compact accretion disk as the direct origin of
the blackbody radiation at peak brightness.Comment: 16 pages, 9 figures. Submitted to MNRAS; including minor revisions
suggested by the refere
A New Class of Changing-Look LINERs
We report the discovery of six active galactic nuclei (AGN) caught "turning
on" during the first nine months of the Zwicky Transient Facility (ZTF) survey.
The host galaxies were classified as LINERs by weak narrow forbidden line
emission in their archival SDSS spectra, and detected by ZTF as nuclear
transients. In five of the cases, we found via follow-up spectroscopy that they
had transformed into broad-line AGN, reminiscent of the changing-look LINER
iPTF 16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up HST UV and
ground-based optical spectra revealed the transformation into a narrow-line
Seyfert 1 (NLS1) with strong [Fe VII, X, XIV] and He II 4686 coronal lines.
Swift monitoring observations of this source reveal bright UV emission that
tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks
~60 days later. Spitzer follow-up observations also detect a luminous
mid-infrared flare implying a large covering fraction of dust. Archival light
curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset
of the optical nuclear flaring from a prolonged quiescent state. Here we
present the systematic selection and follow-up of this new class of
changing-look LINERs, compare their properties to previously reported
changing-look Seyfert galaxies, and conclude that they are a unique class of
transients well-suited to test the uncertain physical processes associated with
the LINER accretion state.Comment: Submitted to ApJ, 31 pages, 17 Figures (excluding Appendix due to
file size constraints but will be available in electronic version
Distinguishing Tidal Disruption Events from Impostors
Recent claimed detections of tidal disruption events (TDEs) in
multi-wavelength data have opened potential new windows into the evolution and
properties of otherwise dormant supermassive black holes (SMBHs) in the centres
of galaxies. At present, there are several dozen TDE candidates, which share
some properties and differ in others. The range in properties is broad enough
to overlap other transient types, such as active galactic nuclei (AGN) and
supernovae (SNe), which can make TDE classification ambiguous. A further
complication is that "TDE signatures" have not been uniformly observed to
similar sensitivities or even targeted across all candidates. This chapter
reviews those events that are unusual relative to other TDEs, including the
possibility of TDEs in pre-existing AGN, and summarises those characteristics
thought to best distinguish TDEs from continuously accreting AGN, strongly
flaring AGN, SNe, and Gamma-Ray Bursts (GRBs), as well as other potential
impostors like stellar collisions, "micro-TDEs," and circumbinary accretion
flows. We conclude that multiple observables should be used to classify any one
event as a TDE. We also consider the TDE candidate population as a whole,
which, for certain host galaxy or SMBH characteristics, is distinguishable
statistically from non-TDEs, suggesting that at least some TDE candidates do in
fact arise from SMBH-disrupted stars.Comment: 57 pages, 17 figures. Accepted for publication in Springer Space
Science Reviews. Chapter in ISSI review "The Tidal Disruption of Stars by
Massive Black Holes" vol. 79. Corrected several typos from previous
submissio
Double-Peaked Balmer Emission Indicating Prompt Accretion Disk Formation in an X-Ray Faint Tidal Disruption Event
We present the multi-wavelength analysis of the tidal disruption event (TDE)
AT~2018hyz (ASASSN-18zj). From follow-up optical spectroscopy, we detect the
first unambiguous case of resolved double-peaked Balmer emission in a TDE. The
distinct line profile can be well-modelled by a low eccentricity
() accretion disk extending out to 100 and a
Gaussian component originating from non-disk clouds, though a bipolar outflow
origin cannot be completely ruled out. Our analysis indicates that in
AT~2018hyz, disk formation took place promptly after the most-bound debris
returned to pericenter, which we estimate to be roughly tens of days before the
first detection. Redistribution of angular momentum and mass transport,
possibly through shocks, must occur on the observed timescale of about a month
to create the large \Ha-emitting disk that comprises 5\% of the
initial stellar mass. With these new insights from AT~2018hyz, we infer that
circularization is efficient in at least some, if not all optically-bright,
X-ray faint TDEs. In these efficiently circularized TDEs, the detection of
double-peaked emission depends on the disk inclination angle and the relative
strength of the disk contribution to the non-disk component, possibly
explaining the diversity seen in the current sample.Comment: 24 pages, 8 figures, 6 tables. Accepted for publication in Ap
Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT2019qiz
We report the results of UV and optical photometric and spectroscopic
analysis on the tidal disruption event (TDE) AT2019qiz. Our follow-up
observations started shortly after (10 days) its optical light begin
to rise and lasted for a period of six months. Our late-time host-dominated
spectrum indicates that the host galaxy likely harbours a weak AGN. The initial
HST spectrum of AT2019qiz exhibits a iron and low-ionization broad absorption
line (FeLoBAL) system that is seen for the first time in a TDE. This spectrum
also bears a striking resemblance to that of Gaia16apd, a superluminous
supernova. Our observations provide insights into the outflow properties in
TDEs and show evidence for a connection between TDEs and engine-powered
supernovae at early phase, as originally suggested in Metzger & Stone (2016).
In a time frame of 50 days, the UV spectra of AT2019qiz started to resemble
previous TDEs with only high-ionization BALs. The change in UV spectral
signatures is accompanied by a decrease in the outflow velocity, which began at
km s and decelerated to km s. A similar
evolution in the H emission line width further supports the speculation
that the broad Balmer emission lines are formed in TDE outflows. In addition,
we detect narrow absorption features on top of the FeLoBAL signatures in the
early HST UV spectrum of AT2019qiz. The measured HI column density corresponds
to a Lyman-limit system whereas the metal absorption lines, such as NV, CIV,
FeII, and MgII, are likely probing the circumnuclear gas and interstellar
medium in the host galaxy.Comment: 24 pages, 13 figures, 3 tables. Submitted to ApJ on Nov 03 202
The Broad-lined Ic Supernova ZTF18aaqjovh (SN 2018bvw): An Optically-discovered Engine-driven Supernova Candidate with Luminous Radio Emission
We present ZTF18aaqjovh (SN 2018bvw), a high-velocity ("broad-lined") stripped-envelope (Type Ic) supernova (Ic-BL SN) discovered in the Zwicky Transient Facility one-day cadence survey. ZTF18aaqjovh shares a number of features in common with engine-driven explosions: the photospheric velocity and the shape of the optical light curve are very similar to those of the Type Ic-BL SN 1998bw, which was associated with a low-luminosity gamma-ray burst (LLGRB) and had relativistic ejecta. However, the radio luminosity of ZTF18aaqjovh is almost two orders of magnitude fainter than that of SN 1998bw at the same velocity phase, and the shock velocity is at most mildly relativistic (v = 0.06–0.4c). A search of high-energy catalogs reveals no compelling gamma-ray burst (GRB) counterpart to ZTF18aaqjovh, and the limit on the prompt GRB luminosity of L_(γ,iso)≈1.6×10⁴⁸ erg/s⁻¹ excludes a classical GRB but not an LLGRB. Altogether, ZTF18aaqjovh represents another transition event between engine-driven SNe associated with GRBs and "ordinary" Ic-BL SNe