Abstract
We present deep X-ray and radio observations of the fast blue optical transient (FBOT) AT 2020xnd/ZTF 20acigmel at
z
= 0.2433 from 13 days to 269 days after explosion. AT 2020xnd belongs to ...the category of optically luminous FBOTs with similarities to the archetypal event AT 2018cow. AT 2020xnd shows luminous radio emission reaching
L
ν
≈ 8 × 10
29
erg s
−1
Hz
−1
at 20 GHz and 75 days post-explosion, accompanied by luminous and rapidly fading soft X-ray emission peaking at
L
X
≈ 6 × 10
42
erg s
−1
. Interpreting the radio emission in the context of synchrotron radiation from the explosion’s shock interaction with the environment, we find that AT 2020xnd launched a high-velocity outflow (
v
∼ 0.1
c
–0.2
c
) propagating into a dense circumstellar medium (effective
M
̇
≈
10
−
3
M
⊙
yr
−1
for an assumed wind velocity of
v
w
= 1000 km s
−1
). Similar to AT 2018cow, the detected X-ray emission is in excess compared to the extrapolated synchrotron spectrum and constitutes a different emission component, possibly powered by accretion onto a newly formed black hole or neutron star. These properties make AT 2020xnd a high-redshift analog to AT 2018cow, and establish AT 2020xnd as the fourth member of the class of optically luminous FBOTs with luminous multiwavelength counterparts.
We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising (1.4 0.1 mag hr−1) and luminous ( mag) transient. ...It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The high peak luminosity ( ), the short rise time ( in g band), and the blue colors at peak ( ) all resemble the high-redshift Ic-BL iPTF16asu, as well as several other unclassified fast transients. The early discovery of SN2018gep (within an hour of shock breakout) enabled an intensive spectroscopic campaign, including the highest-temperature ( ) spectra of a stripped-envelope SN. A retrospective search revealed luminous ( mag) emission in the days to weeks before explosion, the first definitive detection of precursor emission for a Ic-BL. We find a limit on the isotropic gamma-ray energy release , a limit on X-ray emission , and a limit on radio emission . Taken together, we find that the early ( ) data are best explained by shock breakout in a massive shell of dense circumstellar material (0.02 ) at large radii ( ) that was ejected in eruptive pre-explosion mass-loss episodes. The late-time ( ) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.
Abstract
We report the discovery and panchromatic follow-up observations of the young Type Ic supernova (SN Ic) SN 2020oi in M100, a grand-design spiral galaxy at a mere distance of 14 Mpc. We ...followed up with observations at radio, X-ray, and optical wavelengths from only a few days to several months after explosion. The optical behavior of the supernova is similar to those of other normal SNe Ic. The event was not detected in the X-ray band but our radio observations revealed a bright mJy source (
L
ν
≈
1.2
×
10
27
erg
s
−
1
Hz
−
1
). Given the relatively small number of stripped envelope SNe for which radio emission is detectable, we used this opportunity to perform a detailed analysis of the comprehensive radio data set we obtained. The radio-emitting electrons initially experience a phase of inverse Compton cooling, which leads to steepening of the spectral index of the radio emission. Our analysis of the cooling frequency points to a large deviation from equipartition at the level of
ϵ
e
/
ϵ
B
≳ 200, similar to a few other cases of stripped envelope SNe. Our modeling of the radio data suggests that the shock wave driven by the SN ejecta into the circumstellar matter (CSM) is moving at
∼
3
×
10
4
km
s
−
1
. Assuming a constant mass loss from the stellar progenitor, we find that the mass-loss rate is
M
̇
≈
1.4
×
10
−
4
M
⊙
yr
−
1
for an assumed wind velocity of
1000
km
s
−
1
. The temporal evolution of the radio emission suggests a radial CSM density structure steeper than the standard
r
−2
.
Abstract We present observations of a peculiar hydrogen- and helium-poor stripped-envelope (SE) supernova (SN) 2020wnt, primarily in the optical and near-infrared (near-IR). Its peak absolute ...bolometric magnitude of −20.9 mag ( L bol, peak = (6.8 ± 0.3) × 10 43 erg s −1 ) and a rise time of 69 days are reminiscent of hydrogen-poor superluminous SNe (SLSNe I), luminous transients potentially powered by spinning-down magnetars. Before the main peak, there is a brief peak lasting <10 days post explosion, likely caused by interaction with circumstellar medium (CSM) ejected ∼years before the SN explosion. The optical spectra near peak lack a hot continuum and O ii absorptions, which are signs of heating from a central engine; they quantitatively resemble those of radioactivity-powered hydrogen/helium-poor Type Ic SESNe. At ∼1 yr after peak, nebular spectra reveal a blue pseudo-continuum and narrow O i recombination lines associated with magnetar heating. Radio observations rule out strong CSM interactions as the dominant energy source at +266 days post peak. Near-IR observations at +200–300 days reveal carbon monoxide and dust formation, which causes a dramatic optical light-curve dip. Pair-instability explosion models predict slow light curve and spectral features incompatible with observations. SN 2020wnt is best explained as a magnetar-powered core-collapse explosion of a 28 M ⊙ pre-SN star. The explosion kinetic energy is significantly larger than the magnetar energy at peak, effectively concealing the magnetar-heated inner ejecta until well after peak. SN 2020wnt falls into a continuum between normal SNe Ic and SLSNe I, and demonstrates that optical spectra at peak alone cannot rule out the presence of a central engine.
Abstract
The optical-ultraviolet transient AT 2021loi is located at the center of its host galaxy. Its spectral features identify it as a member of the Bowen fluorescence flare (BFF) class. The first ...member of this class was considered to be related to a tidal disruption event, but enhanced accretion onto an already active supermassive black hole was suggested as an alternative explanation. Having occurred in a previously known unobscured active galactic nucleus, AT 2021loi strengthens the latter interpretation. Its light curve is similar to those of previous BFFs, showing a rebrightening approximately 1 yr after the main peak (which was not explicitly identified but might be the case in all previous BFFs). An emission feature around 4680 Å, seen in the preflare spectrum, strengthens by a factor of ∼2 around the optical peak of the flare and is clearly seen as a double-peaked feature then, suggesting a blend of N
iii
λ
4640 with He
ii
λ
4686 as its origin. The appearance of O
iii
λ
3133 and possible N
iii
λλ
4097, 4103 (blended with H
δ
) during the flare further support a Bowen fluorescence classification. Here we present ZTF, ATLAS, Keck, Las Cumbres Observatory, NEOWISE-R, Swift AMI, and Very Large Array observations of AT 2021loi, making it one of the best-observed BFFs to date. It thus provides some clarity on the nature of BFFs but also further demonstrates the diversity of nuclear transients.
Abstract
We report here radio follow-up observations of the optical tidal disruption event (TDE) AT 2019azh. Previously reported X-ray observations of this TDE showed variability at early times and a ...dramatic increase in luminosity, by a factor of ∼10, about 8 months after optical discovery. The X-ray emission is mainly dominated by intermediate hard-soft X-rays and is exceptionally soft around the X-ray peak, which is
L
X
∼ 10
43
erg s
−1
. The high cadence 15.5 GHz observations reported here show an early rise in radio emission followed by an approximately constant light curve, and a late-time flare. This flare starts roughly at the time of the observed X-ray peak luminosity and reaches its peak about 110 days after the peak in the X-ray, and a year after optical discovery. The radio flare peaks at
ν
L
ν
∼ 10
38
erg s
−1
, a factor of two higher than the emission preceding the flare. In light of the late-time radio and X-ray flares, and the X-ray spectral evolution, we speculate a possible transition in the accretion state of this TDE, similar to the observed behavior in black hole X-ray binaries. We compare the radio properties of AT 2019azh to other known TDEs, and focus on the similarities to the late-time radio flare of the TDE ASASSN-15oi.
ABSTRACT
The Tidal Disruption Event (TDE) AT 2018hyz exhibited a delayed radio flare almost three years after the stellar disruption. Here, we report new radio observations of the TDE AT 2018hyz with ...the AMI-LA and ATCA spanning from a month to more than four years after the optical discovery and 200 d since the last reported radio observation. We detected no radio emission from 30–220 d after the optical discovery in our observations at 15.5 GHz down to a 3σ level of <0.14 mJy. The fast-rising, delayed radio flare is observed in our radio data set and continues to rise almost ∼1580 d after the optical discovery. We find that the delayed radio emission, first detected 972 d after optical discovery, evolves as t4.2 ± 0.9, at 15.5 GHz. Here, we present an off-axis jet model that can explain the full set of radio observations. In the context of this model, we require a powerful narrow jet with an isotropic equivalent kinetic energy Ek, iso ∼ 1055 erg, an opening angle of ∼7°, and a relatively large viewing angle of ∼42°, launched at the time of the stellar disruption. Within our framework, we find that the minimal collimated energy possible for an off-axis jet from AT 2018hyz is Ek ≥ 3 × 1052 erg. Finally, we provide predictions based on our model for the light curve turnover time, and for the proper motion of the radio emitting source.
Abstract
Radio emission from tidal disruption events (TDEs) originates from an interaction of an outflow with the super-massive black hole (SMBH) circumnuclear material (CNM). In turn, this radio ...emission can be used to probe properties of both the outflow launched at the event and the CNM. Until recently, radio emission was detected only for a relatively small number of events. While the observed radio emission pointed to either relativistic or sub-relativistic outflows of different nature, it also indicated that the outflow has been launched shortly after stellar disruption. Recently, however, delayed radio flares, several months and years after stellar disruption, were reported in the case of the TDE ASASSN-15oi. These delayed flares suggest a delay in the launching of outflows and thus may provide new insights into SMBH accretion physics. Here, we present a new radio data set of another TDE, iPTF 16fnl, and discuss the possibility that a delayed radio flare also has been observed in this case, ∼5 months after optical discovery, suggesting that this phenomenon may be common in TDEs. Unlike ASASSN-15oi, the data for iPTF 16fnl is sparse and the delayed radio flare can be explained by several alternative models: among them are a complex varying CNM density structure and a delayed outflow ejection.
Binaries containing a compact object orbiting a supermassive black hole are thought to be precursors of gravitational wave events, but their identification has been extremely challenging. Here, we ...report quasi-periodic variability in x-ray absorption, which we interpret as quasi-periodic outflows (QPOuts) from a previously low-luminosity active galactic nucleus after an outburst, likely caused by a stellar tidal disruption. We rule out several models based on observed properties and instead show using general relativistic magnetohydrodynamic simulations that QPOuts, separated by roughly 8.3 days, can be explained with an intermediate-mass black hole secondary on a mildly eccentric orbit at a mean distance of about 100 gravitational radii from the primary. Our work suggests that QPOuts could be a new way to identify intermediate/extreme-mass ratio binary candidates.
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