The origin of cosmic high-energy neutrinos remains largely unexplained. For high-energy neutrino alerts from IceCube, a coincidence with time-variable emission has been seen for three different types ...of accreting black holes: (1) a gamma-ray flare from a blazar (TXS 0506+056), (2) an optical transient following a stellar tidal disruption event (TDE; AT2019dsg), and (3) an optical outburst from an active galactic nucleus (AGN; AT2019fdr). For the latter two sources, infrared follow-up observations revealed a powerful reverberation signal due to dust heated by the flare. This discovery motivates a systematic study of neutrino emission from all supermassive black hole with similar dust echoes. Because dust reprocessing is agnostic to the origin of the outburst, our work unifies TDEs and high-amplitude flares from AGN into a population that we dub accretion flares. Besides the two known events, we uncover a third flare that is coincident with a PeV-scale neutrino (AT2019aalc). Based solely on the optical and infrared properties, we estimate a significance of 3.6\(\sigma\) for this association of high-energy neutrinos with three accretion flares. Our results imply that at least ~10% of the IceCube high-energy neutrino alerts could be due to accretion flares. This is surprising because the sum of the fluence of these flares is at least three orders of magnitude lower compared to the total fluence of normal AGN. It thus appears that the efficiency of high-energy neutrino production in accretion flares is increased compared to non-flaring AGN. We speculate that this can be explained by the high Eddington ratio of the flares.
The fate of stars in the zero-age main-sequence (ZAMS) range \(\approx 8-12\) Msun is unclear. They could evolve to form white dwarfs or explode as electron-capture supernovae (SNe) or iron ...core-collapse SNe (CCSNe). Even though the initial mass function indicates that this mass range should account for over 40% of all CCSNe progenitors, few have been observationally confirmed, likely owing to the faintness and rapid evolution of these transients. In this paper, we present a sample of nine Ca-rich/O-poor Type IIb SNe detected by the Zwicky Transient Facility with progenitors likely in this mass range. We perform a holistic analysis of the spectroscopic and photometric properties of the sample. These sources have a flux ratio of Ca II \(\lambda \lambda\)7291, 7324 to O I \(\lambda \lambda\)6300, 6364 of \(\gtrsim\) 2 in their nebular spectra. Comparing the measured O I luminosity (\(\lesssim 10^{39} \mathrm{erg\ s^{-1}}\)) and derived oxygen mass (\(\lesssim 0.1\) Msun) with theoretical models, we infer that the progenitor ZAMS mass for these explosions is less than 12 Msun. These correspond to He-stars with core masses less than around 3 Msun. We find that the ejecta properties (Mej \(\lesssim 1\) Msun) are also consistent with those expected for such low mass He-stars. The low ejecta mass of these sources indicates a class of strongly-stripped SNe that is a transition between the regular stripped-envelope SNe and ultra-stripped SNe. The progenitor could be stripped by a main sequence companion and result in the formation of a neutron star \(-\) main sequence binary. Such binaries have been suggested to be progenitors of neutron star \(-\) white dwarf systems that could merge within a Hubble time, and be detectable with LISA.
We present the tidal disruption event (TDE) AT2022lri, hosted in a nearby (\(\approx\!144\) Mpc) quiescent galaxy with a low-mass massive black hole (\(10^4\,M_\odot < M_{\rm BH} < 10^6\,M_\odot\)). ...AT2022lri belongs to the TDE-H+He subtype. More than 1 Ms of X-ray data were collected with NICER, Swift, and XMM-Newton from 187 d to 672 d after peak. The X-ray luminosity gradually declined from \(1.5\times 10^{44}\,{\rm erg\,s^{-1}}\) to \(1.5\times 10^{43}\,{\rm erg\,s^{-1}}\) and remains much above the UV and optical luminosity, consistent with a super-Eddington accretion flow viewed face-on. Sporadic strong X-ray dips atop a long-term decline are observed, with variability timescale of \(\approx\!0.5\) hr--1 d and amplitude of \(\approx\!2\)--8. When fitted with simple continuum models, the X-ray spectrum is dominated by a thermal disk component with inner temperature going from \(\sim\! 146\) eV to \(\sim\! 86\) eV. However, there are residual features that peak around 1 keV, which, in some cases, cannot be reproduced by a single broad emission line. We analyzed a subset of time-resolved spectra with two physically motivated models describing either a scenario where ionized absorbers contribute extra absorption and emission lines or where disk reflection plays an important role. Both models provide good and statistically comparable fits, show that the X-ray dips are correlated with drops in the inner disk temperature, and require the existence of sub-relativistic (0.1--0.3\(c\)) ionized outflows. We propose that the disk temperature fluctuation stems from episodic drops of the mass accretion rate triggered by magnetic instabilities or/and wobbling of the inner accretion disk along the black hole's spin axis.
Early observations of transient explosions can provide vital clues to their progenitor origins. In this paper we present the nearby Type Iax (02cx-like) supernova (SN), SN 2020udy that was discovered ...within hours (\(\sim\)7 hr) of estimated first light. An extensive dataset of ultra-violet, optical, and near-infrared observations was obtained, covering out to \(\sim\)150 d after explosion. SN 2020udy peaked at -17.86\(\pm\)0.43 mag in the r band and evolved similarly to other 'luminous' SNe Iax, such as SNe 2005hk and 2012Z. Its well-sampled early light curve allows strict limits on companion interaction to be placed. Main-sequence companion stars with masses of 2 and 6 M\(_\odot\) are ruled out at all viewing angles, while a helium-star companion is allowed from a narrow range of angles (140-180\(^\circ\) away from the companion). The spectra and light curves of SN2020udy are in good agreement with those of the 'N5def' deflagration model of a near Chandrasekhar-mass carbon-oxygen white dwarf. However, as has been seen in previous studies of similar luminosity events, SN 2020udy evolves slower than the model. Broad-band linear polarisation measurements taken at and after peak are consistent with no polarisation, in agreement with the predictions of the companion-star configuration from the early light curve measurements. The host galaxy environment is low metallicity and is consistent with a young stellar population. Overall, we find the most plausible explosion scenario to be the incomplete disruption of a CO white dwarf near the Chandrasekhar-mass limit, with a helium-star companion.
The Zwicky Transient Facility (ZTF) performs a systematic neutrino follow-up program, searching for optical counterparts to high-energy neutrinos with dedicated Target-of-Opportunity (ToO) ...observations. Since first light in March 2018, ZTF has taken prompt observations for 24 high-quality neutrino alerts from the IceCube Neutrino Observatory, with a median latency of 12.2 hours from initial neutrino detection. From two of these campaigns, we have already reported tidal disruption event (TDE) AT 2019dsg and likely TDE AT 2019fdr as probable counterparts, suggesting that TDEs contribute >7.8% of the astrophysical neutrino flux. We here present the full results of our program through to December 2021. No additional candidate neutrino sources were identified by our program, allowing us to place the first constraints on the underlying optical luminosity function of astrophysical neutrino sources. Transients with optical absolutes magnitudes brighter that \(-21\) can contribute no more than 87% of the total, while transients brighter than \(-22\) can contribute no more than 58% of the total, neglecting the effect of extinction and assuming they follow the star formation rate. These are the first observational constraints on the neutrino emission of bright populations such as superluminous supernovae. None of the neutrinos were coincident with bright optical AGN flares comparable to that observed for TXS 0506+056/IC170922A, with such optical blazar flares producing no more than 26% of the total neutrino flux. We highlight the outlook for electromagnetic neutrino follow-up programs, including the expected potential for the Rubin Observatory.
The detonation of a thin ($\lesssim$$0.03\,\mathrm{M_\odot}\() helium shell (He-shell) atop a \)\sim$$1\,\mathrm{M_\odot}\( white dwarf (WD) is a promising mechanism to explain normal Type Ia ...supernovae (SNe Ia), while thicker He-shells and less massive WDs may explain some recently observed peculiar SNe Ia. We present observations of SN 2020jgb, a peculiar SN Ia discovered by the Zwicky Transient Facility (ZTF). Near maximum light, SN 2020jgb is slightly subluminous (ZTF \)g\(-band absolute magnitude \)M_g\( between \)-18.2\( and \)-18.7\( mag depending on the amount of host galaxy extinction) and shows an unusually red color (\)g_\mathrm{ZTF}-r_\mathrm{ZTF}\( between 0.4 and 0.2 mag) due to strong line-blanketing blueward of \)\sim\(5000 \)Å\(. These properties resemble those of SN 2018byg, a peculiar SN Ia consistent with a thick He-shell double detonation (DDet) SN. Using detailed radiative transfer models, we show that the optical spectroscopic and photometric evolution of SN 2020jgb are broadly consistent with a \)\sim$$0.95\,\mathrm{M_\odot}\( (C/O core + He-shell; up to \)\sim$$1.00\,\mathrm{M_\odot}\( depending on the total host extinction) progenitor ignited by a thick (\)\sim$$0.13\,\mathrm{M_\odot}\() He-shell. We detect a prominent absorption feature at \)\sim\(1 \)\mu\mathrm{m}\( in the near-infrared (NIR) spectrum of SN 2020jgb, which could originate from unburnt helium in the outermost ejecta. While the sample size is limited, similar 1 \)\mu\mathrm{m}$ features have been detected in all the thick He-shell DDet candidates with NIR spectra obtained to date. SN 2020jgb is also the first subluminous, thick He-shell DDet SN discovered in a star-forming galaxy, indisputably showing that He-shell DDet objects occur in both star-forming and passive galaxies, consistent with the normal SN Ia population.
We present the discovery and analysis of SN\,2022oqm, a Type Ic supernova (SN) detected \(<1\)\,day after explosion. The SN rises to a blue and short-lived (2\,days) initial peak. Early-time spectral ...observations of SN\,2022oqm show a hot (40,000\,K) continuum with high-ionization C and O absorption features at velocities of 4000\,km\,s\(^{-1}\), while its photospheric radius expands at 20,000\,\kms, indicating a pre-existing distribution of expanding C/O material. After \(\sim2.5\)\,days, both the spectrum and light curves evolve into those of a typical SN Ic, with line velocities of \(\sim10,000\)\,km\,s\(^{-1}\), in agreement with the photospheric radius evolution. The optical light curves reach a second peak at \(t\approx15\)\,days. By \(t=60\)\,days, the spectrum of \oqm\ becomes nearly nebular, displaying strong \ion{Ca}{2} and \ion{Ca}{2} emission with no detectable \ion{O}{1}, marking this event as Ca-rich. The early behavior can be explained by \(10^{-3}\)\,\msun\ of optically thin circumstellar material (CSM) surrounding either (1) a massive compact progenitor such as a Wolf-Rayet star, (2) a massive stripped progenitor with an extended envelope, or (3) a binary system with a white dwarf. We propose that the early-time light curve is powered by both interaction of the ejecta with the optically thin CSM and shock cooling (in the massive-star scenario). The observations can be explained by CSM that is optically thick to X-ray photons, is optically thick in the lines as seen in the spectra, and is optically thin to visible-light continuum photons that come either from downscattered X-rays or from the shock-heated ejecta. Calculations show that this scenario is self-consistent.
We conduct a systematic tidal disruption event (TDE) demographics analysis using the largest sample of optically selected TDEs. A flux-limited, spectroscopically complete sample of 33 TDEs is ...constructed using the Zwicky Transient Facility over three years (from October 2018 to September 2021). We infer the black hole (BH) mass (\(M_{\rm BH}\)) with host galaxy scaling relations, showing that the sample \(M_{\rm BH}\) ranges from \(10^{5.1}\,M_\odot\) to \(10^{8.2}\,M_\odot\). We developed a survey efficiency corrected maximum volume method to infer the rates. The rest-frame \(g\)-band luminosity function (LF) can be well described by a broken power-law of \(\phi (L_g)\propto (L_g / L_{\rm bk})^{0.3} + (L_g / L_{\rm bk})^{2.6}^{-1}\), with \(L_{\rm bk}=10^{43.1}\,{\rm erg\,s^{-1}}\). In the BH mass regime of \(10^{5.3}\lesssim (M_{\rm BH}/M_\odot) \lesssim 10^{7.3}\), the TDE mass function follows \(\phi(M_{\rm BH})\propto M_{\rm BH}^{-0.25}\), which favors a flat local BH mass function (\(dn_{\rm BH}/d{\rm log}M_{\rm BH}\approx{\rm constant}\)). We confirm the significant rate suppression at the high-mass end (\(M_{\rm BH}\gtrsim 10^{7.5}\,M_\odot\)), which is consistent with theoretical predictions considering direct capture of hydrogen-burning stars by the event horizon. At a host galaxy mass of \(M_{\rm gal}\sim 10^{10}\,M_\odot\), the average optical TDE rate is \(\approx 3.2\times 10^{-5}\,{\rm galaxy^{-1}\,yr^{-1}}\). We constrain the optical TDE rate to be 3.7, 7.4, and 1.6\(\times 10^{-5}\,{\rm galaxy^{-1}\,yr^{-1}}\) in galaxies with red, green, and blue colors.
One of the open questions following the discovery of GW170817 is whether neutron star mergers are the only astrophysical sites capable of producing \(r\)-process elements. Simulations have shown that ...0.01-0.1M\(_\odot\) of \(r\)-process material could be generated in the outflows originating from the accretion disk surrounding the rapidly rotating black hole that forms as a remnant to both neutron star mergers and collapsing massive stars associated with long-duration gamma-ray bursts (collapsars). The hallmark signature of \(r\)-process nucleosynthesis in the binary neutron star merger GW170817 was its long-lasting near-infrared emission, thus motivating a systematic photometric study of the light curves of broadlined stripped-envelope (Ic-BL) supernovae (SNe) associated with collapsars. We present the first systematic study of 25 SNe Ic-BL -- including 18 observed with the Zwicky Transient Facility and 7 from the literature -- in the optical/near-infrared bands to determine what quantity of \(r\)-process material, if any, is synthesized in these explosions. Using semi-analytic models designed to account for \(r\)-process production in SNe Ic-BL, we perform light curve fitting to derive constraints on the \(r\)-process mass for these SNe. We also perform independent light curve fits to models without \(r\)-process. We find that the \(r\)-process-free models are a better fit to the light curves of the objects in our sample. Thus we find no compelling evidence of \(r\)-process enrichment in any of our objects. Further high-cadence infrared photometric studies and nebular spectroscopic analysis would be sensitive to smaller quantities of \(r\)-process ejecta mass or indicate whether all collapsars are completely devoid of \(r\)-process nucleosynthesis.
Multi-peaked supernovae with precursors, dramatic light-curve rebrightenings, and spectral transformation are rare, but are being discovered in increasing numbers by modern night-sky transient ...surveys like the Zwicky Transient Facility (ZTF). Here, we present the observations and analysis of SN 2023aew, which showed a dramatic increase in brightness following an initial luminous (-17.4 mag) and long (~100 days) unusual first peak (possibly precursor). SN 2023aew was classified as a Type IIb supernova during the first peak but changed its type to resemble a stripped-envelope supernova (SESN) after the marked rebrightening. We present comparisons of SN 2023aew's spectral evolution with SESN subtypes and argue that it is similar to SNe Ibc during its main peak. P-Cygni Balmer lines are present during the first peak, but vanish during the second peak's photospheric phase, before H\(\alpha\) resurfaces again during the nebular phase. The nebular lines (O I, Ca II, Mg I, H\(\alpha\)) exhibit a double-peaked structure which hints towards a clumpy or non-spherical ejecta. We analyze the second peak in the light curve of SN 2023aew and find it to be broader than normal SESNe as well as requiring a very high \(^{56}\)Ni mass to power the peak luminosity. We discuss the possible origins of SN 2023aew including an eruption scenario where a part of the envelope is ejected during the first peak which also powers the second peak of the light curve through SN-CSM interaction.
