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1.
  • Kasliwal, M. M., et al. (författare)
  • Illuminating gravitational waves : A concordant picture of photons from a neutron star merger
  • 2017
  • Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 358:6370, s. 1559-
  • Tidskriftsartikel (refereegranskat)abstract
    • Merging neutron stars offer an excellent laboratory for simultaneously studying strong-field gravity and matter in extreme environments. We establish the physical association of an electromagnetic counterpart (EM170817) with gravitational waves (GW170817) detected from merging neutron stars. By synthesizing a panchromatic data set, we demonstrate that merging neutron stars are a long-sought production site forging heavy elements by r-process nucleosynthesis. The weak gamma rays seen in EM170817 are dissimilar to classical short gamma-ray bursts with ultrarelativistic jets. Instead, we suggest that breakout of a wide-angle, mildly relativistic cocoon engulfing the jet explains the low-luminosity gamma rays, the high-luminosity ultraviolet-optical-infrared, and the delayed radio and x-ray emission. We posit that all neutron star mergers may lead to a wide-angle cocoon breakout, sometimes accompanied by a successful jet and sometimes by a choked jet.
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2.
  • Leloudas, G., et al. (författare)
  • The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole
  • 2016
  • Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 1:1
  • Tidskriftsartikel (refereegranskat)abstract
    • When a star passes within the tidal radius of a supermassive black hole, it will be torn apart1. For a star with the mass of the Sun (M-circle dot) and a non-spinning black hole with a mass <10(8)M(circle dot), the tidal radius lies outside the black hole event horizon2 and the disruption results in a luminous flare(3-6). Here we report observations over a period of ten months of a transient, hitherto interpreted(7) as a superluminous supernova(8). Our data show that the transient rebrightened substantially in the ultraviolet and that the spectrum went through three different spectroscopic phases without ever becoming nebular. Our observations are more consistent with a tidal disruption event than a superluminous supernova because of the temperature evolution(6), the presence of highly ionized CNO gas in the line of sight(9) and our improved localization of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely(10,11). While the supermassive black hole has a mass >10(8)M(circle dot)(12,13), a star with the same mass as the Sun could be disrupted outside the event horizon if the black hole were spinning rapidly(14). The rapid spin and high black hole mass can explain the high luminosity of this event.
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3.
  • Gal-Yam, A., et al. (författare)
  • A WC/WO star exploding within an expanding carbon-oxygen-neon nebula
  • 2022
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 601:7892, s. 201-204
  • Tidskriftsartikel (refereegranskat)abstract
    • The final fate of massive stars, and the nature of the compact remnants they leave behind (black holes and neutron stars), are open questions in astrophysics. Many massive stars are stripped of their outer hydrogen envelopes as they evolve. Such Wolf-Rayet stars(1) emit strong and rapidly expanding winds with speeds greater than 1,000 kilometres per second. A fraction of this population is also helium-depleted, with spectra dominated by highly ionized emission lines of carbon and oxygen (types WC/WO). Evidence indicates that the most commonly observed supernova explosions that lack hydrogen and helium (types Ib/Ic) cannot result from massive WC/WO stars(2,3), leading some to suggest that most such stars collapse directly into black holes without a visible supernova explosion(4). Here we report observations of SN 2019hgp, beginning about a day after the explosion. Its short rise time and rapid decline place it among an emerging population of rapidly evolving transients(5-8). Spectroscopy reveals a rich set of emission lines indicating that the explosion occurred within a nebula composed of carbon, oxygen and neon. Narrow absorption features show that this material is expanding at high velocities (greater than 1,500 kilometres per second), requiring a compact progenitor. Our observations are consistent with an explosion of a massive WC/WO star, and suggest that massive Wolf-Rayet stars may be the progenitors of some rapidly evolving transients.
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4.
  • Sollerman, Jesper, et al. (författare)
  • The Type II supernova SN 2020jfo in M 61, implications for progenitor system, and explosion dynamics
  • 2021
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 655
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the discovery and extensive follow-up observations of SN 2020jfo, a Type IIP supernova (SN) in the nearby (14.5 Mpc) galaxy M 61. Optical light curves (LCs) and spectra from the Zwicky Transient Facility (ZTF), complemented with data from Swift/UVOT and near-infrared photometry is presented. These were used to model the 350-day duration bolometric light curve, which exhibits a relatively short (∼65 days) plateau. This implies a moderate ejecta mass (∼5 M⊙) at the time of explosion, whereas the deduced amount of ejected radioactive nickel is ∼0.025 M⊙. An extensive series of spectroscopy is presented, including spectropolarimetric observations. The nebular spectra are dominated by Hα, but also reveal emission lines from oxygen and calcium. Comparisons to synthetic nebular spectra indicate an initial progenitor mass of ∼12 M⊙. We also note the presence of stable nickel in the nebular spectrum, and SN 2020jfo joins a small group of SNe that have inferred super-solar Ni/Fe ratios. Several years of prediscovery data were examined, but no signs of precursor activity were found. Pre-explosion Hubble Space Telescope imaging reveals a probable progenitor star, detected only in the reddest band (MF814W ≈ −5.8) and it is fainter than expected for stars in the 10−15 M⊙ range. There is thus some tension between the LC analysis, the nebular spectral modeling, and the pre-explosion imaging. To compare and contrast, we present two additional core-collapse SNe monitored by the ZTF, which also have nebular Hα-dominated spectra. This illustrates how the absence or presence of an interaction with circumstellar material (CSM) affect both the LCs and in particular the nebular spectra. Type II SN 2020amv has a LC powered by CSM interaction, in particular after ∼40 days when the LC is bumpy and slowly evolving. The late-time spectra show strong Hα emission with a structure suggesting emission from a thin, dense shell. The evolution of the complex three-horn line profile is reminiscent of that observed for SN 1998S. Finally, SN 2020jfv has a poorly constrained early-time LC, but it is of interest because of the transition from a hydrogen-poor Type IIb to a Type IIn, where the nebular spectrum after the light-curve rebrightening is dominated by Hα, although with an intermediate line width.
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5.
  • Ahumada, T., et al. (författare)
  • Discovery and confirmation of the shortest gamma-ray burst from a collapsar
  • 2021
  • Ingår i: Nature Astronomy. - : Springer Nature. - 2397-3366. ; 5:9, s. 917-927
  • Tidskriftsartikel (refereegranskat)abstract
    • Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the Universe. The duration and hardness distribution of GRBs has two clusters1, now understood to reflect (at least) two different progenitors2. Short-hard GRBs (SGRBs; T90 < 2 s) arise from compact binary mergers, and long-soft GRBs (LGRBs; T90 > 2 s) have been attributed to the collapse of peculiar massive stars (collapsars)3. The discovery of SN 1998bw/GRB 980425 (ref. 4) marked the first association of an LGRB with a collapsar, and AT 2017gfo (ref. 5)/GRB 170817A/GW170817 (ref. 6) marked the first association of an SGRB with a binary neutron star merger, which also produced a gravitational wave. Here, we present the discovery of ZTF20abwysqy (AT2020scz), a fast-fading optical transient in the Fermi satellite and the Interplanetary Network localization regions of GRB 200826A; X-ray and radio emission further confirm that this is the afterglow. Follow-up imaging (at rest-frame 16.5 days) reveals excess emission above the afterglow that cannot be explained as an underlying kilonova, but which is consistent with being the supernova. Although the GRB duration is short (rest-frame T90 of 0.65 s), our panchromatic follow-up data confirm a collapsar origin. GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets.
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6.
  • Ahumada, Tomas, et al. (författare)
  • In Search of Short Gamma-Ray Burst Optical Counterparts with the Zwicky Transient Facility
  • 2022
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 932:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The Fermi Gamma-ray Burst Monitor (GBM) triggers on-board in response to ∼40 short gamma-ray bursts (SGRBs) per year; however, their large localization regions have made the search for optical counterparts a challenging endeavour. We have developed and executed an extensive program with the wide field of view of the Zwicky Transient Facility (ZTF) camera, mounted on the Palomar 48 inch Oschin telescope (P48), to perform target-of-opportunity (ToO) observations on 10 Fermi-GBM SGRBs during 2018 and 2020–2021. Bridging the large sky areas with small field-of-view optical telescopes in order to track the evolution of potential candidates, we look for the elusive SGRB afterglows and kilonovae (KNe) associated with these high-energy events. No counterpart has yet been found, even though more than 10 ground-based telescopes, part of the Global Relay of Observatories Watching Transients Happen (GROWTH) network, have taken part in these efforts. The candidate selection procedure and the follow-up strategy have shown that ZTF is an efficient instrument for searching for poorly localized SGRBs, retrieving a reasonable number of candidates to follow up and showing promising capabilities as the community approaches the multi-messenger era. Based on the median limiting magnitude of ZTF, our searches would have been able to retrieve a GW170817-like event up to ∼200 Mpc and SGRB afterglows to z = 0.16 or 0.4, depending on the assumed underlying energy model. Future ToOs will expand the horizon to z = 0.2 and 0.7, respectively.
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7.
  • Anand, S., et al. (författare)
  • Optical follow-up of the neutron star–black hole mergers S200105ae and S200115j
  • 2020
  • Ingår i: Nature Astronomy. - : Nature Research. - 2397-3366.
  • Tidskriftsartikel (refereegranskat)abstract
    • LIGO and Virgo’s third observing run revealed the first neutron star–black hole (NSBH) merger candidates in gravitational waves. These events are predicted to synthesize r-process elements1,2 creating optical/near-infrared ‘kilonova’ emission. The joint gravitational wave and electromagnetic detection of an NSBH merger could be used to constrain the equation of state of dense nuclear matter3, and independently measure the local expansion rate of the Universe4. Here, we present the optical follow-up and analysis of two of the only three high-significance NSBH merger candidates detected to date, S200105ae and S200115j, with the Zwicky Transient Facility5. The Zwicky Transient Facility observed ~48% of S200105ae and ~22% of S200115j’s localization probabilities, with observations sensitive to kilonovae brighter than −17.5 mag fading at 0.5 mag d−1 in the g- and r-bands; extensive searches and systematic follow-up of candidates did not yield a viable counterpart. We present state-of-the-art kilonova models tailored to NSBH systems that place constraints on the ejecta properties of these NSBH mergers. We show that with observed depths of apparent magnitude ~22 mag, attainable in metre-class, wide-field-of-view survey instruments, strong constraints on ejecta mass are possible, with the potential to rule out low mass ratios, high black hole spins and large neutron star radii.
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8.
  • Andreoni, Igor, et al. (författare)
  • Fast-transient Searches in Real Time with ZTFReST : Identification of Three Optically Discovered Gamma-Ray Burst Afterglows and New Constraints on the Kilonova Rate
  • 2021
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 918:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The most common way to discover extragalactic fast transients, which fade within a few nights in the optical, is via follow-up of gamma-ray burst and gravitational-wave triggers. However, wide-field surveys have the potential to identify rapidly fading transients independently of such external triggers. The volumetric survey speed of the Zwicky Transient Facility (ZTF) makes it sensitive to objects as faint and fast fading as kilonovae, the optical counterparts to binary neutron star mergers, out to almost 200 Mpc. We introduce an open-source software infrastructure, the ZTF REaltime Search and Triggering, ZTFReST, designed to identify kilonovae and fast transients in ZTF data. Using the ZTF alert stream combined with forced point-spread-function photometry, we have implemented automated candidate ranking based on their photometric evolution and fitting to kilonova models. Automated triggering, with a human in the loop for monitoring, of follow-up systems has also been implemented. In 13 months of science validation, we found several extragalactic fast transients independently of any external trigger, including two supernovae with post-shock cooling emission, two known afterglows with an associated gamma-ray burst (ZTF20abbiixp, ZTF20abwysqy), two known afterglows without any known gamma-ray counterpart (ZTF20aajnksq, ZTF21aaeyldq), and three new fast-declining sources (ZTF20abtxwfx, ZTF20acozryr, ZTF21aagwbjr) that are likely associated with GRB200817A, GRB201103B, and GRB210204A. However, we have not found any objects that appear to be kilonovae. We constrain the rate of GW170817-like kilonovae to R < 900 Gpc(-3) yr(-1) (95% confidence). A framework such as ZTFReST could become a prime tool for kilonova and fast-transient discovery with the Vera Rubin Observatory.
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9.
  • Fremling, C., et al. (författare)
  • The Zwicky Transient Facility Bright Transient Survey. I. Spectroscopic Classification and the Redshift Completeness of Local Galaxy Catalogs
  • 2020
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 895:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The Zwicky Transient Facility (ZTF) is performing a three-day cadence survey of the visible northern sky (similar to 3 pi) with newly found transient candidates announced via public alerts. The ZTF Bright Transient Survey (BTS) is a large spectroscopic campaign to complement the photometric survey. BTS endeavors to spectroscopically classify all extragalactic transients with m(peak) <= 18.5 mag in either the g(ZTF) or r(ZTF) filters, and publicly announce said classifications. BTS discoveries are predominantly supernovae (SNe), making this the largest flux-limited SN survey to date. Here we present a catalog of 761 SNe, classified during the first nine months of ZTF (2018 April 1-2018 December 31). We report BTS SN redshifts from SN template matching and spectroscopic host-galaxy redshifts when available. We analyze the redshift completeness of local galaxy catalogs, the redshift completeness fraction (RCF; the ratio of SN host galaxies with known spectroscopic redshift prior to SN discovery to the total number of SN hosts). Of the 512 host galaxies with SNe Ia, 227 had previously known spectroscopic redshifts, yielding an RCF estimate of 44% 4%. The RCF decreases with increasing distance and decreasing galaxy luminosity (for z < 0.05, or similar to 200 Mpc, RCF 0.6). Prospects for dramatically increasing the RCF are limited to new multifiber spectroscopic instruments or wide-field narrowband surveys. Existing galaxy redshift catalogs are only similar to 50% complete at r 16.9 mag. Pushing this limit several magnitudes deeper will pay huge dividends when searching for electromagnetic counterparts to gravitational wave events or sources of ultra-high-energy cosmic rays or neutrinos.
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10.
  • Gezari, S., et al. (författare)
  • iPTF Discovery of the Rapid Turn-on of a Luminous Quasar
  • 2017
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 835:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a radio-quiet quasar at z = 0.237 discovered turning on by the intermediate Palomar Transient Factory (iPTF). The transient, iPTF 16bco, was detected by iPTF in the nucleus of a galaxy with an archival Sloan Digital Sky Survey spectrum with weak narrow-line emission characteristic of a low-ionization nuclear emission-line region (LINER). Our follow-up spectra show the dramatic appearance of broad Balmer lines and a power-law continuum characteristic of a luminous (L-bol approximate to 10(45) erg s(-1)) type 1 quasar 12 yr later. Our photometric monitoring with PTF from 2009-2012 and serendipitous X-ray observations from the XMM-Newton Slew Survey in 2011 and 2015 constrain the change of state to have occurred less than 500 days before the iPTF detection. An enhanced broad H alpha/[O III] lambda 5007 line ratio in the type 1 state relative to other changing-look quasars also is suggestive of the most rapid change of state yet observed in a quasar. We argue that the > 10 increase in Eddington ratio inferred from the brightening in UV and X-ray continuum flux is more likely due to an intrinsic change in the accretion rate of a preexisting accretion disk than an external mechanism such as variable obscuration, microlensing, or the tidal disruption of a star. However, further monitoring will be helpful in better constraining the mechanism driving this change of state. The rapid turn-on of the quasar is much shorter than the viscous infall timescale of an accretion disk and requires a disk instability that can develop around a similar to 10(8) M circle dot black hole on timescales less than 1 yr.
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11.
  • Kasliwal, Mansi M., et al. (författare)
  • Kilonova Luminosity Function Constraints Based on Zwicky Transient Facility Searches for 13 Neutron Star Merger Triggers during O3
  • 2020
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 905:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run (O3). We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization area of 4480 deg(2), a median distance of 267 Mpc, and false-alarm rates ranging from 1.5 to 10(-25) yr(-1). The ZTF coverage in the g and r bands had a median enclosed probability of 39%, median depth of 20.8 mag, and median time lag between merger and the start of observations of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UltraViolet/Optical/InfraRed (UVOIR) photometric points, 64 OIR spectra, and three radio images using 17 different telescopes. We find no promising kilonovae (radioactivity-powered counterparts), and we show how to convert the upper limits to constrain the underlying kilonova luminosity function. Initially, we assume that all GW triggers are bona fide astrophysical events regardless of false-alarm rate and that kilonovae accompanying BNS and NSBH mergers are drawn from a common population; later, we relax these assumptions. Assuming that all kilonovae are at least as luminous as the discovery magnitude of GW170817 (-16.1 mag), we calculate that our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than -16.6 mag (the extrapolated peak magnitude of GW170817) and fade at a rate of 1 mag day(-1) (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations based on the online classifications, the joint probability of zero detections, assuming all kilonovae are brighter than -16.6 mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, no more than <57% (<89%) of putative kilonovae could be brighter than -16.6 mag assuming flat evolution (fading by 1 mag day(-1)) at the 90% confidence level. If we further take into account the online terrestrial probability for each GW trigger, we find that no more than <68% of putative kilonovae could be brighter than -16.6 mag. Comparing to model grids, we find that some kilonovae must have M-ej M, X-lan > 10(-4), or > 30 degrees to be consistent with our limits. We look forward to searches in the fourth GW observing run; even 17 neutron star mergers with only 50% coverage to a depth of -16 mag would constrain the maximum fraction of bright kilonovae to <25%.
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12.
  • Kool, Erik C., et al. (författare)
  • SN 2020bqj : A Type Ibn supernova with a long-lasting peak plateau
  • 2021
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 652
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Type Ibn supernovae (SNe Ibn) are a rare class of stripped envelope supernovae interacting with a helium-rich circumstellar medium (CSM). The majority of the SNe Ibn reported in the literature display a surprising homogeneity in their fast-evolving lightcurves and are typically found in actively starforming spiral galaxies.Aims. We present the discovery and the study of SN 2020bqj (ZTF20aalrqbu), a SN Ibn with a long-duration peak plateau lasting 40 days and hosted by a faint low-mass galaxy. We aim to explain its peculiar properties using an extensive photometric and spectroscopic data set.Methods. We compare the photometric and spectral evolution of SN 2020bqj with regular SNe Ibn from the literature, as well as with other outliers in the SN Ibn subclass. We fit the bolometric and multi-band lightcurves with powering mechanism models such as radioactive decay and CSM interaction. We also model the host galaxy of SN 2020bqj.Results. The risetime, peak magnitude and spectral features of SN 2020bqj are consistent with those of most SNe Ibn, but the SN is a clear outlier in the subclass based on its bright, long-lasting peak plateau and the low mass of its faint host galaxy. We show through modeling that the lightcurve of SN 2020bqj can be powered predominantly by shock heating from the interaction of the SN ejecta and a dense CSM, combined with radioactive decay. The peculiar Type Ibn SN 2011hw is a close analog to SN 2020bqj in terms of lightcurve and spectral evolution, suggesting a similar progenitor and CSM scenario. In this scenario a very massive progenitor star in the transitional phase between a luminous blue variable and a compact Wolf-Rayet star undergoes core-collapse, embedded in a dense helium-rich CSM with an elevated opacity compared to normal SNe Ibn, due to the presence of residual hydrogen. This scenario is consistent with the observed properties of SN 2020bqj and the modeling results.Conclusions. SN 2020bqj is a compelling example of a transitional SN Ibn/IIn based on not only its spectral features, but also its lightcurve, host galaxy properties and the inferred progenitor properties. The strong similarity with SN 2011hw suggests this subclass may be the result of a progenitor in a stellar evolution phase that is distinct from those of progenitors of regular SNe Ibn.
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13.
  • Malyali, A., et al. (författare)
  • AT 2019avd : a novel addition to the diverse population of nuclear transients
  • 2021
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 647
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on SRG/eROSITA, ZTF, ASAS-SN, Las Cumbres, NEOWISE-R, and Swift XRT/UVOT observations of the unique ongoing event AT 2019avd, located in the nucleus of a previously inactive galaxy at z = 0.029. eROSITA first observed AT 2019avd on 2020-04-28 during its first all sky survey, when it was detected as an ultra-soft X-ray source (kT ~ 85 eV) that was ≳90 times brighter in the 0.2−2 keV band than a previous 3σ upper flux detection limit (with no archival X-ray detection at this position). The ZTF optical light curve in the ~450 days preceding the eROSITA detection is double peaked, and the eROSITA detection coincides with the rise of the second peak. Follow-up optical spectroscopy shows the emergence of a Bowen fluorescence feature and high-ionisation coronal lines ([Fe X] 6375 Å, [Fe XIV] 5303 Å), along with persistent broad Balmer emission lines (FWHM ~ 1400 km s−1). Whilst the X-ray properties make AT 2019avd a promising tidal disruption event (TDE) candidate, the optical properties are atypical for optically selected TDEs. We discuss potential alternative origins that could explain the observed properties of AT 2019avd, such as a stellar binary TDE candidate, or a TDE involving a super massive black hole binary.
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14.
  • Miller, A. A., et al. (författare)
  • ZTF Early Observations of Type Ia Supernovae. II. First Light, the Initial Rise, and Time to Reach Maximum Brightness
  • 2020
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 902:1
  • Tidskriftsartikel (refereegranskat)abstract
    • While it is clear that Type Ia supernovae (SNe) are the result of thermonuclear explosions in C/O white dwarfs (WDs), a great deal remains uncertain about the binary companion that facilitates the explosive disruption of the WD. Here, we present a comprehensive analysis of a large, unique data set of 127 SNe Ia with exquisite coverage by the Zwicky Transient Facility (ZTF). High-cadence (six observations per night) ZTF observations allow us to measure the SN rise time and examine its initial evolution. We develop a Bayesian framework to model the early rise as a power law in time, which enables the inclusion of priors in our model. For a volume-limited subset of normal SNe Ia, we find that the mean power-law index is consistent with 2 in the r(ZTF)-band (alpha(r) = 2.01 +/- 0.02), as expected in the expanding fireball model. There are, however, individual SNe that are clearly inconsistent with alpha(r) = 2. We estimate a mean rise time of 18.9 days (with a range extending from similar to 15 to 22 days), though this is subject to the adopted prior. We identify an important, previously unknown, bias whereby the rise times for higherredshift SNe within a flux-limited survey are systematically underestimated. This effect can be partially alleviated if the power-law index is fixed to alpha = 2, in which case we estimate a mean rise time of 21.7 days (with a range from similar to 18 to 23 days). The sample includes a handful of rare and peculiar SNe Ia. Finally, we conclude with a discussion of lessons learned from the ZTF sample that can eventually be applied to observations from the Vera C..Rubin Observatory.
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15.
  • Perley, Daniel A., et al. (författare)
  • Real-time discovery of AT2020xnd : a fast, luminous ultraviolet transient with minimal radioactive ejecta
  • 2021
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 508:4, s. 5138-5147
  • Tidskriftsartikel (refereegranskat)abstract
    • The many unusual properties of the enigmatic AT2018cow suggested that at least some subset of the empirical class of fast blue optical transients (FBOTs) represents a genuinely new astrophysical phenomenon. Unfortunately, the intrinsic rarity and fleeting nature of these events have made it difficult to identify additional examples early enough to acquire the observations necessary to constrain theoretical models. We present here the Zwicky Transient Facility discovery of AT2020xnd (ZTF20acigmel, the 'Camel') at z = 0.243, the first unambiguous AT2018cow analogue to be found and confirmed in real time. AT2018cow and AT2020xnd share all key observational properties: a fast optical rise, sustained high photospheric temperature, absence of a second peak attributable to ejection of a radioactively heated stellar envelope, extremely luminous radio, millimetre, and X-ray emission, and a dwarf-galaxy host. This supports the argument that AT2018cow-like events represent a distinct phenomenon from slower-evolving radio-quiet supernovae, likely requiring a different progenitor or a different central engine. The sample properties of the four known members of this class to date disfavour tidal disruption models but are consistent with the alternative model of an accretion powered jet following the direct collapse of a massive star to a black hole. Contextual filtering of alert streams combined with rapid photometric verification using multiband imaging provides an efficient way to identify future members of this class, even at high redshift.
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16.
  • Sollerman, Jesper, et al. (författare)
  • Two stripped envelope supernovae with circumstellar interaction : But only one really shows it
  • 2020
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 643
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. We present observations of SN 2019tsf (ZTF19ackjszs) and SN 2019oys (ZTF19abucwzt). These two stripped envelope (SE) Type Ib supernovae (SNe) suddenly showed a (re-)brightening in their late light curves. We investigate this in the context of circumstellar material (CSM) interaction with previously ejected material, a phenomenon that is unusual among SE SNe.Aims. We use our follow-up photometry and spectroscopy for these supernovae to demonstrate the presence of CSM interaction, estimate the properties of the CSM, and discuss why the signals are so different for the two objects.Methods. We present and analyze observational data, consisting of optical light curves and spectra. For SN 2019oys, we also have detections in radio as well as limits from UV and X-rays.Results. Both light curves show spectacular re-brightening after about 100 days. In the case of SN 2019tsf, the re-brightening is followed by a new period of decline, and the spectra never show signs of narrow emission lines that would indicate CSM interaction. On the contrary, SN 2019oys made a spectral makeover from a Type Ib to a spectrum clearly dominated by CSM interaction at the light curve brightening phase. Deep Keck spectra reveal a plethora of narrow high-ionization lines, including coronal lines, and the radio observations show strong emission.Conclusions. The rather similar light curve behavior – with a late linear re-brightening – of these two Type Ib SE SNe indicate CSM interaction as the powering source. For SN 2019oys the evidence for a phase where the ejecta hit H-rich material, likely ejected from the progenitor star, is conspicuous. We observe strong narrow lines of H and He, but also a plethora of high-ionization lines, including coronal lines, revealing shock interaction. Spectral simulations of SN 2019oys show two distinct density components, one with density ≳109 cm−3, dominated by somewhat broader, low-ionization lines of H I, He I, Na I, and Ca II, and one with narrow, high-ionization lines at a density ∼106 cm−3. The former is strongly affected by electron scattering, while the latter is unaffected. The evidence for CSM interaction in SN 2019oys is corroborated by detections in radio. On the contrary, for SN 2019tsf, we find little evidence in the spectra for any CSM interaction. 
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17.
  • Yang, Sheng, et al. (författare)
  • A low-energy explosion yields the underluminous Type IIP SN 2020cxd
  • 2021
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 655
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. We present our observations and analysis of SN 2020cxd, a low-luminosity (LL), long-lived Type IIP supernova (SN). This object is a clear outlier in the magnitude-limited SN sample recently presented by the Zwicky Transient Facility’s (ZTF) Bright Transient Survey.Aims. We demonstrate that SN 2020cxd is an additional member of the group of LL SNe and we discuss the rarity of LL SNe in the context of the ZTF survey. We consider how further studies of these faintest members of the core-collapse (CC) SN family might help improve the general understanding of the underlying initial mass function for stars that explode.Methods. We used optical light curves (LCs) from the ZTF in the gri bands and several epochs of ultraviolet data from the Neil Gehrels Swift observatory as well as a sequence of optical spectra. We constructed the colour curves and a bolometric LC. Then we compared the evolution of the ejecta velocity and black-body temperature for LL SNe as well as for typical Type II SNe. Furthermore, we adopted a Monte Carlo code that fits semi-analytic models to the LC of SN 2020cxd, which allows for the estimation of the physical parameters. Using our late-time nebular spectra, we also make a comparison against SN II spectral synthesis models from the literature to constrain the progenitor properties of SN 2020cxd.Results. The LCs of SN 2020cxd show a great similarity with those of LL SNe IIP in terms of luminosity, timescale, and colours. Also, the spectral evolution of SN 2020cxd is that of a Type IIP SN. The spectra show prominent and narrow P-Cygni lines, indicating low expansion velocities. This is one of the faintest LL SNe observed, with an absolute plateau magnitude of Mr = −14.5 mag and also one with the longest plateau lengths, with a duration of 118 days. Finally, the velocities measured from the nebular emission lines are among the lowest ever seen in a SN, with an intrinsic full width at half maximum value of 478 km s−1. The underluminous late-time exponential LC tail indicates that the mass of 56Ni ejected during the explosion is much smaller than the average of normal SNe IIP, we estimate M56Ni = 0.003 M⊙. The Monte Carlo fitting of the bolometric LC suggests that the progenitor of SN 2020cxd had a radius of R0 = 1.3 × 1013 cm, kinetic energy of Ekin = 4.3 × 1050 erg, and ejecta mass of Mej = 9.5 M⊙. From the bolometric LC, we estimated the total radiated energy Erad = 1.52 × 1048 erg. Using our late-time nebular spectra, we compared these results against SN II spectral synthesis models to constrain the progenitor zero-age main sequence mass and found that it is likely to be ≲15 M⊙.Conclusions. SN 2020cxd is a LL Type IIP SN. The inferred progenitor parameters and the features observed in the nebular spectrum favour a low-energy, Ni-poor, iron CC SN from a low-mass (∼12 M⊙) red supergiant.
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18.
  • Andreoni, Igor, et al. (författare)
  • Constraining the Kilonova Rate with Zwicky Transient Facility Searches Independent of Gravitational Wave and Short Gamma-Ray Burst Triggers
  • 2020
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 904:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The first binary neutron star merger, GW170817, was accompanied by a radioactivity-powered optical/infrared transient called a kilonova. To date, no compelling kilonova has been found in all-sky optical surveys, independently of short gamma-ray burst and gravitational-wave triggers. In this work, we searched the first 23 months of the Zwicky Transient Facility (ZTF) data stream for candidate kilonovae in the form of rapidly evolving transients. We combined ZTF alert queries with forced point-spread-function photometry and nightly flux stacking to increase our sensitivity to faint and fast transients. Automatic queries yielded >11,200 candidates, 24 of which passed quality checks and selection criteria based on a grid of kilonova models tailored for both binary neutron star and neutron star-black hole mergers. None of the candidates in our sample was deemed a possible kilonova after thorough vetting. The sources that passed our selection criteria are dominated by Galactic cataclysmic variables. We identified two fast transients at high Galactic latitude, one of which is the confirmed afterglow of long-duration GRB.190106A, the other is a possible cosmological afterglow. Using a survey simulation code, we constrained the kilonova rate for a range of models including top-hat, linearly decaying light curves, and synthetic light curves obtained with radiative transfer simulations. For prototypical GW170817-like kilonovae, we constrain the rate to be R < 1775 Gpc(-3) yr(-1) (95% confidence). By assuming a population of kilonovae with the same geometry and composition of GW170817 observed under a uniform viewing angle distribution, we obtained a constraint on the rate of R.<.4029 Gpc(-3) yr(-1).
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19.
  • Bellm, Eric C., et al. (författare)
  • The Zwicky Transient Facility : System Overview, Performance, and First Results
  • 2019
  • Ingår i: Publications of the Astronomical Society of the Pacific. - : IOP Publishing. - 0004-6280 .- 1538-3873. ; 131:995
  • Tidskriftsartikel (refereegranskat)abstract
    • The Zwicky Transient Facility (ZTF) is a new optical time-domain survey that uses the Palomar 48 inch Schmidt telescope. A custom-built wide-field camera provides a 47 deg(2) field of view and 8 s readout time, yielding more than an order of magnitude improvement in survey speed relative to its predecessor survey, the Palomar Transient Factory. We describe the design and implementation of the camera and observing system. The ZTF data system at the Infrared Processing and Analysis Center provides near-real-time reduction to identify moving and varying objects. We outline the analysis pipelines, data products, and associated archive. Finally, we present on-sky performance analysis and first scientific results from commissioning and the early survey. ZTF's public alert stream will serve as a useful precursor for that of the Large Synoptic Survey Telescope.
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20.
  • Biswas, Rahul, et al. (författare)
  • Two c's in a pod : cosmology-independent measurement of the Type Ia supernova colour-luminosity relation with a sibling pair
  • 2022
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 509:4, s. 5340-5356
  • Tidskriftsartikel (refereegranskat)abstract
    • Using Zwicky Transient Facility (ZTF) observations, we identify a pair of ‘sibling’ Type Ia supernovae (SNe Ia), i.e. hosted by the same galaxy at z = 0.0541. They exploded within 200 d from each other at a separation of 0.6arcsec0.6arcsec corresponding to a projected distance of only 0.6 kpc. Performing SALT2 light-curve fits to the gri ZTF photometry, we show that for these equally distant ‘standardizable candles’, there is a difference of 2 mag in their rest-frame B-band peaks, and the fainter supernova (SN) has a significantly red SALT2 colour c = 0.57 ± 0.04, while the stretch values x1 of the two SNe are similar, suggesting that the fainter SN is attenuated by dust in the interstellar medium of the host galaxy. We use these measurements to infer the SALT2 colour standardization parameter, β = 3.5 ± 0.3, independent of the underlying cosmology and Malmquist bias. Assuming the colour excess is entirely due to dust, the result differs by 2σ from the average Milky Way total-to-selective extinction ratio, but is in good agreement with the colour–brightness corrections empirically derived from the most recent SN Ia Hubble–Lemaitre diagram fits. Thus we suggest that SN ‘siblings’, which will increasingly be discovered in the coming years, can be used to probe the validity of the colour and light-curve shape corrections using in SN Ia cosmology while avoiding important systematic effects in their inference from global multiparameter fits to inhomogeneous data sets, and also help constrain the role of interstellar dust in SN Ia cosmology.
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21.
  • Bolin, Bryce T., et al. (författare)
  • The discovery and characterization of (594913) 'Ayló'chaxnim, a kilometre sized asteroid inside the orbit of Venus
  • 2022
  • Ingår i: Monthly notices of the Royal Astronomical Society. Letters. - : Oxford University Press. - 1745-3925 .- 1745-3933. ; 517:1, s. L49-L54
  • Tidskriftsartikel (refereegranskat)abstract
    • Near-Earth asteroid population models predict the existence of bodies located inside the orbit of Venus. Despite searches up to the end of 2019, none had been found. We report discovery and follow-up observations of (594913) 'Ayló'chaxnim, an asteroid with an orbit entirely interior to Venus. (594913) 'Ayló'chaxnim has an aphelion distance of ∼0.65 au, is ∼2 km in diameter and is red in colour. The detection of such a large asteroid inside the orbit of Venus is surprising given their rarity according to near-Earth asteroid population models. As the first officially numbered and named asteroid located entirely within the orbit of Venus, we propose that the class of interior to Venus asteroids be referred to as 'Ayló'chaxnim asteroids.
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22.
  • Goobar, Ariel, et al. (författare)
  • THE RISE OF SN 2014J IN THE NEARBY GALAXY M82
  • 2014
  • Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 784:1
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the discovery of SN 2014J in the nearby galaxy M82. Given its proximity, it offers the best opportunity to date to study a thermonuclear supernova (SN) over a wide range of the electromagnetic spectrum. Optical, near-IR, and mid-IR observations on the rising light curve, orchestrated by the intermediate Palomar Transient Factory, show that SN 2014J is a spectroscopically normal Type Ia supernova (SN Ia), albeit exhibiting high-velocity features in its spectrum and heavily reddened by dust in the host galaxy. Our earliest detections start just hours after the fitted time of explosion. We use high-resolution optical spectroscopy to analyze the dense intervening material and do not detect any evolution in the resolved absorption features during the light curve rise. Similar to other highly reddened SNe Ia, a low value of total-to-selective extinction, R-V less than or similar to 2, provides the best match to our observations. We also study pre-explosion optical and near-IR images from Hubble Space Telescope with special emphasis on the sources nearest to the SN location.
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23.
  • Hammerstein, Erica, et al. (författare)
  • The Final Season Reimagined : 30 Tidal Disruption Events from the ZTF-I Survey
  • 2023
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 942:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Tidal disruption events (TDEs) offer a unique way to study dormant black holes. While the number of observed TDEs has grown thanks to the emergence of wide-field surveys in the past few decades, questions regarding the nature of the observed optical, UV, and X-ray emission remain. We present a uniformly selected sample of 30 spectroscopically classified TDEs from the Zwicky Transient Facility Phase I survey operations with follow-up Swift UV and X-ray observations. Through our investigation into correlations between light-curve properties, we recover a shallow positive correlation between the peak bolometric luminosity and decay timescales. We introduce a new spectroscopic class of TDE, TDE-featureless, which are characterized by featureless optical spectra. The new TDE-featureless class shows larger peak bolometric luminosities, peak blackbody temperatures, and peak blackbody radii. We examine the differences between the X-ray bright and X-ray faint populations of TDEs in this sample, finding that X-ray bright TDEs show higher peak blackbody luminosities than the X-ray faint subsample. This sample of optically selected TDEs is the largest sample of TDEs from a single survey yet, and the systematic discovery, classification, and follow-up of this sample allows for robust characterization of TDE properties, an important stepping stone looking forward toward the Rubin era.
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24.
  • Ho, Anna Y. Q., et al. (författare)
  • A Search for Extragalactic Fast Blue Optical Transients in ZTF and the Rate of AT2018cow-like Transients
  • 2023
  • Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 949:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a search for extragalactic fast blue optical transients (FBOTs) during Phase I of the Zwicky Transient Facility (ZTF). We identify 38 candidates with durations above half-maximum light 1 day < t (1/2) < 12 days, of which 28 have blue (g - r less than or similar to -0.2 mag) colors at peak light. Of the 38 transients (28 FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae (SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting (Type IIn/Ibn) SNe, and 2 (1) H&He-poor (Type Ic/Ic-BL) SNe. Two FBOTs (published previously) had predominantly featureless spectra and luminous radio emission: AT2018lug (The Koala) and AT2020xnd (The Camel). Seven (five) did not have a definitive classification: AT 2020bdh showed tentative broad H alpha in emission, and AT 2020bot showed unidentified broad features and was 10 kpc offset from the center of an early-type galaxy. Ten (eight) have no spectroscopic observations or redshift measurements. We present multiwavelength (radio, millimeter, and/or X-ray) observations for five FBOTs (three Type Ibn, one Type IIn/Ibn, one Type IIb). Additionally, we search radio-survey (VLA and ASKAP) data to set limits on the presence of radio emission for 24 of the transients. All X-ray and radio observations resulted in nondetections; we rule out AT2018cow-like X-ray and radio behavior for five FBOTs and more luminous emission (such as that seen in the Camel) for four additional FBOTs. We conclude that exotic transients similar to AT2018cow, the Koala, and the Camel represent a rare subset of FBOTs and use ZTF's SN classification experiments to measure the rate to be at most 0.1% of the local core-collapse SN rate.
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25.
  • Karamehmetoglu, Emir, et al. (författare)
  • The luminous and rapidly evolving SN 2018bcc : Clues toward the origin of Type Ibn SNe from the Zwicky Transient Facility
  • 2021
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 649
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Supernovae (SNe) Type Ibn are rapidly evolving and bright ( MR;peak similar to 19) transients interacting with He-rich circumstellar material (CSM). SN 2018bcc, detected by the ZTF shortly after explosion, provides the best constraints on the shape of the rising light curve (LC) of a fast Type Ibn.Aims. We used the high-quality data set of SN 2018bcc to study observational signatures of the class. Additionally, the powering mechanism of SN 2018bcc o ffers insights into the debated progenitor connection of Type Ibn SNe.Methods. We compared well-constrained LC properties obtained from empirical models with the literature. We fit the pseudobolometric LC with semi-analytical models powered by radioactive decay and CSM interaction. Finally, we modeled the line profiles and emissivity of the prominent He i lines, in order to study the formation of P-Cygni profiles and to estimate CSM properties.Results. SN 2018bcc had a rise time to peak of the LC of 5:6+0:2 0:1 days in the restframe with a rising shape power-law index close to 2, and seems to be a typical rapidly evolving Type Ibn SN. The spectrum lacked signatures of SN-like ejecta and was dominated by over 15 He emission features at 20 days past peak, alongside Ca and Mg, all with VFWHM similar to 2000 km s 1. The luminous and rapidly evolving LC could be powered by CSM interaction but not by the decay of radioactive 56Ni. Modeling of the He i lines indicated a dense and optically thick CSM that can explain the P-Cygni profiles.Conclusions. Like other rapidly evolving Type Ibn SNe, SN 2018bcc is a luminous transient with a rapid rise to peak powered by shock interaction inside a dense and He-rich CSM. Its spectra do not support the existence of two Type Ibn spectral classes. We also note the remarkable observational match to pulsational pair instability SN models.
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