73 research outputs found
The Host Galaxy and Rapidly Evolving Broad-line Region in the Changing-look Active Galactic Nucleus 1ES 1927+654
Changing-look active galactic nuclei (AGNs) present an important laboratory
to understand the origin and physical properties of the broad-line region
(BLR). We investigate follow-up optical spectroscopy spanning days
after the outburst of the changing-look AGN 1ES\,1927+654. The emission lines
displayed dramatic, systematic variations in intensity, velocity width,
velocity shift, and symmetry. Analysis of optical spectra and multi-band images
indicate that the host galaxy contains a pseudobulge and a total stellar mass
of . Enhanced continuum radiation
from the outburst produced an accretion disk wind, which condensed into BLR
clouds in the region above and below the temporary eccentric disk. Broad Balmer
lines emerged days after the outburst, together with an unexpected,
additional component of narrow-line emission. The newly formed BLR clouds then
traveled along a similar eccentric orbit (). The Balmer
decrement of the BLR increased by a factor of as a result of secular
changes in cloud density. The drop in density at late times allowed the
production of \hei\ and \heii\ emission. The mass of the black hole cannot be
derived from the broad emission lines because the BLR is not virialized.
Instead, we use the stellar properties of the host galaxy to estimate
. The nucleus
reached near or above its Eddington limit during the peak of the outburst. We
discuss the nature of the changing-look AGN 1ES\,1927+654 in the context of
other tidal disruption events.Comment: 29 pages, 13 figures, published in the Astrophysical Journa
Nebular-Phase Spectra of Nearby Type Ia Supernovae
We present late-time spectra of eight Type Ia supernovae (SNe Ia) obtained at
days after peak brightness using the Gemini South and Keck telescopes.
All of the SNe Ia in our sample were nearby, well separated from their host
galaxy's light, and have early-time photometry and spectroscopy from the Las
Cumbres Observatory (LCO). Parameters are derived from the light curves and
spectra such as peak brightness, decline rate, photospheric velocity, and the
widths and velocities of the forbidden nebular emission lines. We discuss the
physical interpretations of these parameters for the individual SNe Ia and the
sample in general, including comparisons to well-observed SNe Ia from the
literature. There are possible correlations between early-time and late-time
spectral features that may indicate an asymmetric explosion, so we discuss our
sample of SNe within the context of models for an offset ignition and/or white
dwarf collisions. A subset of our late-time spectra are uncontaminated by host
emission, and we statistically evaluate our nondetections of H emission
to limit the amount of hydrogen in these systems. Finally, we consider the
late-time evolution of the iron emission lines, finding that not all of our SNe
follow the established trend of a redward migration at days after
maximum brightness.Comment: 20 pages, 8 figures, 9 tables; accepted to MNRA
Limits on Simultaneous and Delayed Optical Emission from Well-Localized Fast Radio Bursts
We present the largest compilation to date of optical observations during and
following fast radio bursts (FRBs). The data set includes our dedicated
simultaneous and follow-up observations, as well as serendipitous archival
survey observations, for a sample of 15 well-localized FRBs, including 8
repeating and 7 one-off sources. Our simultaneous (and nearly simultaneous with
a -sec delay) optical observations of 7 (1) bursts from the repeating FRB
20220912A provide the deepest such limits to date for any extragalactic FRB,
reaching a luminosity limit of erg s
( erg s); these observations are also the
deepest to date in terms of optical flux to radio fluence ratio of ms ( ms),
and place a limit on the flux ratio of
on a msec timescale or () on a sec
timescale. These simultaneous limits provide useful constraints in the context
of FRB emission models, such as the pulsar magnetosphere and pulsar nebula
models. Interpreting all available optical limits in the context of the FRB
synchrotron maser model, we find that they constrain the flare energies to
erg (depending on the distances of the various
repeating FRBs, with erg for SGR 1935+2154). These limits
are generally at least an order of magnitude larger than the energies inferred
from the FRBs themselves, although in the case of FRB 20220912A our
simultaneous and rapid follow-up observations severely restrict the model
parameter space. We conclude by exploring the potential of future rapid
response and simultaneous observations with large optical telescopes.Comment: 22 pages, 8 figures, submitted to ApJL. Please let us know if we
missed any optical and/or radio observations of the FRB sampl
Probing the Sub-Parsec Dust of a Supermassive Black Hole with the Tidal Disruption Event AT 2020mot
AT 2020mot is a typical UV/optical tidal disruption event (TDE) with no radio
or X-ray signatures in a quiescent host. We find an i-band excess and
re-brightening along the decline of the light curve which could be due to two
consecutive dust echoes from a TDE. We model our observations following van
Velzen et al. (2016) and find that the near-infrared light curve can be
explained by concentric rings of thin dust within 0.1 parsecs of a 6e6
M supermassive black hole (SMBH), among the smallest scales at which
dust has been inferred near SMBHs. We find dust covering factors of order fc
2%, much lower than found for dusty tori of active galactic nuclei.
These results highlight the potential of TDEs for uncovering the environments
around black holes when including near-infrared observations in high-cadence
transient studies
The Structure of Tidal Disruption Event Host Galaxies on Scales of Tens to Thousands of Parsecs
We explore the galaxy structure of four tidal disruption event (TDE) host
galaxies on 30 pc to kpc scales using HST WFC3 multi-band imaging. The star
formation histories of these hosts are diverse, including one post-starburst
galaxy (ASASSN-14li), two hosts with recent weak starbursts (ASASSN-14ae and
iPTF15af), and one early type (PTF09ge). Compared to early type galaxies of
similar stellar masses, the TDE hosts have higher central surface brightnesses
and stellar mass surface densities on 30-100 pc scales. The TDE hosts do not
show the large, kpc-scale tidal disruptions seen in some post-starburst
galaxies; the hosts have low morphological asymmetries similar to those of
early type galaxies. The lack of strong asymmetries are inconsistent with a
recent major (~1:1 mass) merger, although minor (1:3) mergers are
possible. Given the time elapsed since the end of the starbursts in the three
post-burst TDE hosts and the constraints on the merger mass ratios, it is
unlikely that a bound supermassive black hole binary (SMBHB) has had time to
coalesce. The TDE hosts have low central (<140 pc) ellipticities compared to
early type galaxies. The low central ellipticities disfavor a strong radial
anisotropy as the cause for the enhanced TDE rate, although we cannot rule out
eccentric disks at the scale of the black hole gravitational radius of
influence (~1 pc). These observations suggest that the high central stellar
densities are a more important driver than SMBHBs or radial anisotropies in
increasing the TDE rate in galaxies with recent starbursts.Comment: Accepted for publication in ApJ. 22 pages, 12 figure
Millimeter Observations of the Type II SN2023ixf: Constraints on the Proximate Circumstellar Medium
We present 1.3 mm (230 GHz) observations of the recent and nearby Type II
supernova, SN2023ixf, obtained with the Submillimeter Array (SMA) at 2.6-18.6
days after explosion. The observations were obtained as part the SMA Large
Program POETS (Pursuit of Extragalactic Transients with the SMA). We do not
detect any emission at the location of SN2023ixf, with the deepest limits of
erg s Hz at 2.7
and 7.7 days, and erg
s Hz at 18.6 days. These limits are about a factor of 2 times
dimmer than the mm emission from SN2011dh (IIb), about an order of magnitude
dimmer compared to SN1993J (IIb) and SN2018ivc (IIL), and about 30 times dimmer
than the most luminous non-relativistic SNe in the mm-band (Type IIb/Ib/Ic).
Using these limits in the context of analytical models that include synchrotron
self-absorption and free-free absorption we place constraints on the proximate
circumstellar medium around the progenitor star, to a scale of cm, excluding the range
M yr (for a wind velocity, km s, and ejecta
velocity, km s). These results are
consistent with an inference of the mass loss rate based on optical
spectroscopy ( M yr for km
s), but are in tension with the inference from hard X-rays ( M yr for km s). This tension
may be alleviated by a non-homogeneous and confined CSM, consistent with
results from high-resolution optical spectroscopy.Comment: Submitte
Evolution of A Peculiar Type Ibn Supernova SN 2019wep
We present a high-cadence short term photometric and spectroscopic monitoring
campaign of a type Ibn SN 2019wep, which is one of the rare SN Ibn after SNe
2010al and 2019uo to display signatures of flash ionization (\ion{He}{2},
\ion{C}{3}, \ion{N}{3}). We compare the decline rates and rise time of SN
2019wep with other SNe Ibn and fast transients. The post-peak decline in all
bands (0.1 mag d) are consistent with SNe Ibn but less than the fast
transients. On the other hand, the m values are slightly lower
than the average values for SNe Ibn but consistent with the fast transients.
The rise time is typically shorter than SNe Ibn but longer than fast
transients. SN 2019wep lies at the fainter end of SNe Ibn but possesses an
average luminosity amongst the fast transients sample. The peculiar color
evolution places it between SNe Ib and the most extreme SNe Ibn. The bolometric
light curve modelling shows resemblance with SN 2019uo with ejecta masses
consistent with SNe Ib. SN 2019wep belongs to the "P cygni" sub-class of SNe
Ibn and shows faster evolution in line velocities as compared to the "emission"
sub-class. The post-maximum spectra show close resemblance with ASASSN-15ed
hinting it to be of SN Ib nature. The low \ion{He}{1} CSM velocities and
residual H further justifies it and gives evidence of an intermittent
progenitor between WR and LBV star.Comment: 19 pages, 14 figures, 2 Tables, Accepted for publication in ApJ main
journa
From Discovery to the First Month of the Type II Supernova 2023ixf: High and Variable Mass Loss in the Final Year Before Explosion
We present the discovery of Type II supernova (SN) 2023ixf in M101, among the
closest core-collapse SNe in the last several decades, and follow-up
photometric and spectroscopic observations in the first month of its evolution.
The light curve is characterized by a rapid rise ( days) to a
luminous peak ( mag) and plateau ( mag)
extending to days with a smooth decline rate of mag
day. During the rising phase, color shows blueward evolution,
followed by redward evolution in the plateau phase. Prominent flash features of
hydrogen, helium, carbon, and nitrogen dominate the spectra up to
days after first light, with a transition to a higher ionization state in the
first days. Both the color and flash ionization states suggest
a rise in the temperature, indicative of a delayed shock-breakout inside dense
circumstellar material (CSM). From the timescales of CSM interaction, we
estimate its compact radial extent of cm. We then
construct numerical light-curve models based on both continuous and eruptive
mass-loss scenarios shortly before explosion. For the continuous mass-loss
scenario, we infer a range of mass-loss history with in the final years before explosion, with a potentially
decreasing mass loss of in
years towards the explosion. For the eruptive mass-loss scenario, we favor
eruptions releasing of the envelope at about a year before
explosion, which result in CSM with mass and extent similar to the continuous
scenario. We discuss the implications of the available multi-wavelength
constraints obtained thus far on the progenitor candidate and SN 2023ixf to our
variable CSM models.Comment: 15 pages, 5 figures, submitted to ApJ
A Multi-Wavelength View on the Rapidly-Evolving Supernova 2018ivc: An Analog of SN IIb 1993J but Powered Primarily by Circumstellar Interaction
SN 2018ivc is an unusual type II supernova (SN II). It is a variant of SNe
IIL, which might represent a transitional case between SNe IIP with a massive
H-rich envelope, and IIb with only a small amount of the H-rich envelope.
However, SN 2018ivc shows an optical light curve evolution more complicated
than canonical SNe IIL. In this paper, we present the results of prompt
follow-up observations of SN 2018ivc with the Atacama Large
Millimeter/submillimeter Array (ALMA). Its synchrotron emission is similar to
that of SN IIb 1993J, suggesting that it is intrinsically an SN IIb-like
explosion of a He star with a modest (~0.5 - 1 Msun) extended H-rich envelope.
Its radio, optical, and X-ray light curves are explained primarily by the
interaction between the SN ejecta and the circumstellar material (CSM); we thus
suggest that it is a rare example (and the first involving the `canonical' SN
IIb ejecta) for which the multi-wavelength emission is powered mainly by the
SN-CSM interaction. The inner CSM density, reflecting the progenitor activity
in the final decade, is comparable to that of SN IIb 2013cu that showed a flash
spectral feature. The outer CSM density, and therefore the mass-loss rate in
the final ~200 years, is larger than that of SN 1993J by a factor of ~5. We
suggest that SN 2018ivc represents a missing link between SNe IIP and IIb/Ib/Ic
in the binary evolution scenario.Comment: 31 pages, 14 figures, 3 tables. Accepted for publication in Ap
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