34 research outputs found
The Tidal Disruption Event AT 2018hyz II: Light Curve Modeling of a Partially Disrupted Star
AT 2018hyz (=ASASSN-18zj) is a tidal disruption event (TDE) located in the
nucleus of a quiescent E+A galaxy at a redshift of , first
detected by the All-Sky Automated Survey for Supernovae (ASAS-SN). We present
optical+UV photometry of the transient, as well as an X-ray spectrum and radio
upper limits. The bolometric light curve of AT 2018hyz is comparable to other
known TDEs and declines at a rate consistent with a at early times,
emitting a total radiated energy of erg. An excess bump
appears in the UV light curve about 50 days after bolometric peak, followed by
a flattening beyond 250 days. The light curve shows an excess bump in the UV
about 50 days after bolometric peak lasting for at least 100 days, which may be
related to an outflow. We detect a constant X-ray source present for at least
86 days. The X-ray spectrum shows a total unabsorbed flux of erg cm s and is best fit by a blackbody plus
power-law model with a photon index of . A thermal X-ray model is
unable to account for photons keV, while the radio non-detection favors
inverse-Compton scattering rather than a jet for the non-thermal component. We
model the optical and UV light curves using the Modular Open-Source Fitter for
Transients (MOSFiT) and find a best fit for a black hole of
M partially disrupting a M star (stripping a mass of
M for the inferred impact parameter, ). The low
optical depth implied by the small debris mass may explain how we are able to
see hydrogen emission with disk-like line profiles in the spectra of AT 2018hyz
(see our companion paper, Short et al.~2020).Comment: 10 pages, 10 figures, published in MNRA
Luminous Type II Short-Plateau Supernovae 2006Y, 2006ai, and 2016egz: A Transitional Class from Stripped Massive Red Supergiants
The diversity of Type II supernovae (SNe II) is thought to be driven mainly
by differences in their progenitor's hydrogen-rich (H-rich) envelope mass, with
SNe IIP having long plateaus ( days) and the most massive H-rich
envelopes. However, it is an ongoing mystery why SNe II with short plateaus
(tens of days) are rarely seen. Here we present optical/near-infrared
photometric and spectroscopic observations of luminous Type II short-plateau
SNe 2006Y, 2006ai, and 2016egz. Their plateaus of about -- days and
luminous optical peaks ( mag) indicate significant pre-explosion
mass loss resulting in partially-stripped H-rich envelopes and early
circumstellar material (CSM) interaction. We compute a large grid of
MESA+STELLA single-star progenitor and light-curve models with various
progenitor zero-age main-sequence (ZAMS) masses, mass-loss efficiencies,
explosion energies, Ni masses, and CSM densities. Our model grid shows a
continuous population of SNe IIP--IIL--IIb-like light-curve morphology in
descending order of H-rich envelope mass. With large Ni masses
(), short-plateau SNe II lie in a confined parameter
space as a transitional class between SNe IIL and IIb. For SNe 2006Y, 2006ai,
and 2016egz, our findings suggest high-mass red supergiant (RSG) progenitors
(--) with small H-rich envelope masses
() that experience enhanced mass
loss () for the last few
decades before the explosion. If high-mass RSGs result in rare short-plateau
SNe II, then these events might ease some of the apparent under-representation
of higher-luminosity RSGs in observed SN II progenitor samples.Comment: 26 pages, 16 figures, submitted to Ap
SN 2021zny: an early flux excess combined with late-time oxygen emission suggests a double white dwarf merger event
We present a photometric and spectroscopic analysis of the ultra-luminous and
slowly evolving 03fg-like Type Ia SN 2021zny. Our observational campaign starts
from hours after explosion (making SN 2021zny one of the earliest
observed members of its class), with dense multi-wavelength coverage from a
variety of ground- and space-based telescopes, and is concluded with a nebular
spectrum months after peak brightness. SN 2021zny displayed several
characteristics of its class, such as the peak brightness ( mag),
the slow decline ( mag), the blue early-time colours,
the low ejecta velocities and the presence of significant unburned material
above the photosphere. However, a flux excess for the first days
after explosion is observed in four photometric bands, making SN 2021zny the
third 03fg-like event with this distinct behavior, while its d spectrum
shows prominent [O I] lines, a very unusual characteristic of thermonuclear
SNe. The early flux excess can be explained as the outcome of the interaction
of the ejecta with of H/He-poor circumstellar
material at a distance of cm, while the low ionization state of
the late-time spectrum reveals low abundances of stable iron-peak elements. All
our observations are in accordance with a progenitor system of two
carbon/oxygen white dwarfs that undergo a merger event, with the disrupted
white dwarf ejecting carbon-rich circumstellar material prior to the primary
white dwarf detonation.Comment: 19 pages, 16 figures, accepted for publication in MNRA
Revealing the progenitor of SN 2021zby through analysis of the shock-cooling light curve
We present early observations and analysis of the double-peaked Type IIb
supernova (SN IIb) 2021zby. captured the prominent early shock cooling
peak of SN 2021zby within the first 10 days after explosion with a
30-minute cadence. We present optical and near-infrared spectral series of SN
2021zby, including three spectra during the shock cooling phase. Using a
multi-band model fit, we find that the inferred properties of its progenitor
are consistent with a red supergiant or yellow supergiant, with an envelope
mass of 0.3-3.0 M and an envelope radius of 50-350. These inferred progenitor properties are similar to those of other
SNe IIb with double-peak feature, such as SNe 1993J, 2011dh, 2016gkg and
2017jgh. This study further validates the importance of the high cadence and
early coverage in resolving the shape of the shock cooling light curve, while
the multi-band observations, especially UV, is also necessary to fully
constrain the progenitor properties.Comment: 12 pages, 5 figures, 2 tables, submitted to ApJ
SN 2019hcc: a Type II supernova displaying early O II lines
We present optical spectroscopy together with ultraviolet, optical, and near-infrared photometry of SN 2019hcc, which resides in a host galaxy at redshift 0.044, displaying a sub-solar metallicity. The supernova spectrum near peak epoch shows a 'w' shape at around 4000 Å which is usually associated with O II lines and is typical of Type I superluminous supernovae. SN 2019hcc post-peak spectra show a well-developed H α P-Cygni profile from 19 d past maximum and its light curve, in terms of its absolute peak luminosity and evolution, resembles that of a fast-declining Hydrogen-rich supernova (SN IIL). The object does not show any unambiguous sign of interaction as there is no evidence of narrow lines in the spectra or undulations in the light curve. Our TARDIS spectral modelling of the first spectrum shows that carbon, nitrogen, and oxygen (CNO) at 19 000 K reproduce the 'w' shape and suggests that a combination of non-thermally excited CNO and metal lines at 8000 K could reproduce the feature seen at 4000 Å. The Bolometric light-curve modelling reveals that SN 2019hcc could be fit with a magnetar model, showing a relatively strong magnetic field (B > 3 × 1014 G), which matches the peak luminosity and rise time without powering up the light curve to superluminous luminosities. The high-energy photons produced by the magnetar would then be responsible for the detected O II lines. As a consequence, SN 2019hcc shows that a 'w' shape profile at around 4000 Å, usually attributed to O II, is not only shown in superluminous supernovae and hence it should not be treated as the sole evidence of the belonging to such a supernova type...
Fast and Not-so-Furious: Case Study of the Fast and Faint Type IIb SN 2021bxu
We present photometric and spectroscopic observations and analysis of
SN~2021bxu (ATLAS21dov), a low-luminosity, fast-evolving Type IIb supernova
(SN). SN~2021bxu is unique, showing a large initial decline in brightness
followed by a short plateau phase. With
during the plateau, it is at the lower end of the luminosity distribution of
stripped-envelope supernovae (SE-SNe) and shows a distinct 10 day plateau
not caused by H- or He-recombination. SN~2021bxu shows line velocities which
are at least slower than typical SE-SNe. It is
photometrically and spectroscopically similar to Type IIb SNe during the
photospheric phases of evolution, with similarities to Ca-rich IIb SNe. We find
that the bolometric light curve is best described by a composite model of shock
interaction between the ejecta and an envelope of extended material, combined
with a typical SN~IIb powered by the radioactive decay of Ni. The
best-fit parameters for SN~2021bxu include a Ni mass of , an ejecta mass of
, and an ejecta
kinetic energy of . From the fits to the properties of the extended material of
Ca-rich IIb SNe we find a trend of decreasing envelope radius with increasing
envelope mass. SN~2021bxu has on the low end compared to
SE-SNe and Ca-rich SNe in the literature, demonstrating that SN~2021bxu-like
events are rare explosions in extreme areas of parameter space. The progenitor
of SN~2021bxu is likely a low mass He star with an extended envelope.Comment: 18 pages, 15 figures, submitted to MNRA
Long-term follow-up observations of extreme coronal line emitting galaxies
We present new spectroscopic and photometric follow-up observations of the
known sample of extreme coronal line emitting galaxies (ECLEs) identified in
the Sloan Digital Sky Survey (SDSS). With these new data, observations of the
ECLE sample now span a period of two decades following their initial SDSS
detections. We confirm the nonrecurrence of the iron coronal line signatures in
five of the seven objects, further supporting their identification as the
transient light echoes of tidal disruption events (TDEs). Photometric
observations of these objects in optical bands show little overall evolution.
In contrast, mid-infrared (MIR) observations show ongoing long-term declines.
The remaining two objects had been classified as active galactic nuclei (AGN)
with unusually strong coronal lines rather than being TDE related, given the
persistence of the coronal lines in earlier follow-up spectra. We confirm this
classification, with our spectra continuing to show the presence of strong,
unchanged coronal-line features and AGN-like MIR colours and behaviour. We have
constructed spectral templates of both subtypes of ECLE to aid in
distinguishing the likely origin of newly discovered ECLEs. We highlight the
need for higher cadence, and more rapid, follow-up observations of such objects
to better constrain their properties and evolution. We also discuss the
relationships between ECLEs, TDEs, and other identified transients having
significant MIR variability.Comment: Submitted to MNRAS. 33 pages, 15 figure
SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features
We present panchromatic observations and modeling of the Calcium-rich supernova (SN) 2019ehk in the star-forming galaxy M100 (d ≈ 16.2 Mpc) starting 10 hr after explosion and continuing for ~300 days. SN 2019ehk shows a double-peaked optical light curve peaking at t = 3 and 15 days. The first peak is coincident with luminous, rapidly decaying Swift-XRT–discovered X-ray emission (L_x ≈ 10⁴¹ erg s⁻¹ at 3 days; L_x ∝ t⁻³), and a Shane/Kast spectral detection of narrow Hα and He II emission lines (v ≈ 500 km s⁻¹) originating from pre-existent circumstellar material (CSM). We attribute this phenomenology to radiation from shock interaction with extended, dense material surrounding the progenitor star at r (0.1–1) × 10¹⁷ cm. The photometric and spectroscopic properties during the second light-curve peak are consistent with those of Ca-rich transients (rise-time of t_r = 13.4 ± 0.210 days and a peak B-band magnitude of M_B = −15.1 ± 0.200 mag). We find that SN 2019ehk synthesized (3.1 ± 0.11) × 10⁻² M_⊙ of ⁵⁶Ni and ejected M_(ej) = (0.72 ± 0.040) M⊙ total with a kinetic energy E_k = (1.8 ± 0.10) × 10⁵⁰ erg. Finally, deep HST pre-explosion imaging at the SN site constrains the parameter space of viable stellar progenitors to massive stars in the lowest mass bin (~10 M_⊙) in binaries that lost most of their He envelope or white dwarfs (WDs). The explosion and environment properties of SN 2019ehk further restrict the potential WD progenitor systems to low-mass hybrid HeCO WD+CO WD binaries