We present the discovery and early evolution of ASASSN-19bt, a tidal disruption event (TDE) discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of \(d\simeq115\) Mpc and ...the first TDE to be detected by TESS. As the TDE is located in the TESS Continuous Viewing Zone, our dataset includes 30-minute cadence observations starting on 2018 July 25, and we precisely measure that the TDE begins to brighten \(\sim8.3\) days before its discovery. Our dataset also includes 18 epochs of Swift UVOT and XRT observations, 2 epochs of XMM-Newton observations, 13 spectroscopic observations, and ground data from the Las Cumbres Observatory telescope network, spanning from 32 days before peak through 37 days after peak. ASASSN-19bt thus has the most detailed pre-peak dataset for any TDE. The TESS light curve indicates that the transient began to brighten on 2019 January 21.6 and that for the first 15 days its rise was consistent with a flux \(\propto t^2\) power-law model. The optical/UV emission is well-fit by a blackbody SED, and ASASSN-19bt exhibits an early spike in its luminosity and temperature roughly 32 rest-frame days before peak and spanning up to 14 days that has not been seen in other TDEs, possibly because UV observations were not triggered early enough to detect it. It peaked on 2019 March 04.9 at a luminosity of \(L\simeq1.3\times10^{44}\) ergs s\(^{-1}\) and radiated \(E\simeq3.2\times10^{50}\) ergs during the 41-day rise to peak. X-ray observations after peak indicate a softening of the hard X-ray emission prior to peak, reminiscent of the hard/soft states in X-ray binaries.
We present nearly 500 days of observations of the tidal disruption event ASASSN-18pg, spanning from 54 days before peak light to 441 days after peak light. Our dataset includes X-ray, UV, and optical ...photometry, optical spectroscopy, radio observations, and the first published spectropolarimetric observations of a TDE. ASASSN-18pg was discovered on 2018 July 11 by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of \(d=78.6\) Mpc, and with a peak UV magnitude of \(m\simeq14\) it is both one of the nearest and brightest TDEs discovered to-date. The photometric data allow us to track both the rise to peak and the long-term evolution of the TDE. ASASSN-18pg peaked at a luminosity of \(L\simeq2.2\times10^{44}\) erg s\(^{-1}\), and its late-time evolution is shallower than a flux \(\propto t^{-5/3}\) power-law model, similar to what has been seen in other TDEs. ASASSN-18pg exhibited Balmer lines and spectroscopic features consistent with Bowen fluorescence prior to peak which remained detectable for roughly 225 days after peak. Analysis of the two-component H\(\alpha\) profile indicates that, if they are the result of reprocessing of emission from the accretion disk, the different spectroscopic lines may be coming from regions between \(\sim10\) and \(\sim60\) light-days from the black hole. No X-ray emission is detected from the TDE and there is no evidence of a jet or strong outflow detected in the radio. Our spectropolarimetric observations give no strong evidence for significant asphericity in the emission region, with the emission region having an axis ratio of at least \(\sim0.65\).
We report the discovery of the first likely black hole in a non-interacting binary system with a field red giant. By combining radial velocity measurements from the Apache Point Observatory Galactic ...Evolution Experiment (APOGEE) with photometric variability data from the All-Sky Automated Survey for Supernovae (ASAS-SN), we identified the bright rapidly-rotating giant 2MASS J05215658+4359220 as a binary system with a massive unseen companion. Subsequent radial velocity measurements reveal a system with an orbital period of 83 days and near-zero eccentricity. The photometric variability period of the giant is consistent with the orbital period, indicative of star spots and tidal synchronization. Constraints on the giant's mass and radius from its luminosity, surface gravity, and temperature imply an unseen companion with mass of \(3.3^{+2.8}_{-0.7}\) M\(_\odot\), indicating a low-mass black hole or an exceedingly massive neutron star. Measurement of the astrometric binary motion by {\it Gaia} will further characterize the system. This discovery demonstrates the potential of massive spectroscopic surveys like APOGEE and all-sky, high-cadence photometric surveys like ASAS-SN to revolutionize our understanding of the compact object mass function, and to test theories of binary star evolution and the supernova mechanism.
We report the discovery of a stripped giant + lower giant binary, 2M04123153+6738486 (2M0412), identified during a search for non-interacting compact object-star binaries. 2M0412 is an evolved ...(\(T_{\rm eff, giant}\simeq4000\) K), luminous (\(L_{\rm giant}\simeq150~L_\odot\)) red giant in a circular \(P=81.2\) day binary. 2M0412 is a known variable star previously classified as a semi-regular variable. The cross-correlation functions of follow-up Keck/HIRES and LBT/PEPSI spectra show an RV-variable second component with implied mass ratio \(q=M_{\rm giant}/M_{\rm comp}\simeq0.20\pm0.01\). The ASAS-SN, ATLAS, TESS and ZTF light curves show that the giant is a Roche lobe filling ellipsoidal variable with an inclination of \(49.4^\circ{}\pm{0.3^{\circ}}\), and a giant mass of \(M_{\rm giant}=0.38\pm0.01~ M_\odot\) for a distance of \(\simeq3.7\) kpc. The mass of the giant indicates that its envelope has been stripped. The giant companion on the lower red giant branch has a mass of \(M_{\rm comp}=1.91\pm0.03~M_\odot\) with \(T_{\rm eff, comp}\simeq5000\) K, \(L_{\rm comp}\simeq60~L_\odot\) and \(R_{\rm comp}\simeq11~R_\odot\). We also identify an orbital phase dependent, broad \(\rm H\alpha\) emission line which could indicate ongoing accretion from the stripped red giant onto the companion.
We report and characterize a white-light superflare on a previously
undiscovered M dwarf detected by the ASAS-SN survey. Employing various
color-magnitude and color-spectral type relationships, we ...estimate several
stellar parameters, including the quiescent V-band magnitude, from which we
derive a flare amplitude of $\Delta V \sim 10$. We determine an r-band absolute
magnitude of $M_{r} = 11.4$, consistent with a mid-M dwarf, and an approximate
distance to the source of $2.2$ kpc. Using classical-flare models, we infer a
flare energy of $E_{V} \simeq (4.1\pm 2.2)\times 10^{36}$ ergs, making this one
of the strongest flares documented on an M dwarf.
We report comprehensive multi-wavelength observations of a peculiar Type Ia-like supernova ("SN Ia-pec") ASASSN-15pz. ASASSN-15pz is a spectroscopic "twin" of SN 2009dc, a so-called ..."Super-Chandrasekhar-mass" SN, throughout its evolution, but it has a peak luminosity M_B,peak = -19.69 +/- 0.12 mag that is \approx 0.6 mag dimmer and comparable to the SN 1991T sub-class of SNe Ia at the luminous end of the normal width-luminosity relation. The synthesized Ni56 mass of M_Ni56 = 1.13 +/- 0.14 M_sun is also substantially less than that found for several 2009dc-like SNe. Previous well-studied 2009dc-like SNe have generally suffered from large and uncertain amounts of host-galaxy extinction, which is negligible for ASASSN-15pz. Based on the color of ASASSN-15pz, we estimate a host extinction for SN 2009dc of E(B-V)_host=0.12 mag and confirm its high luminosity (M_B, peak2009dc \approx -20.3 mag). The 2009dc-like SN population, which represents ~1% of SNe Ia, exhibits a range of peak luminosities, and do not fit onto the tight width-luminosity relation. Their optical light curves also show significant diversity of late-time (>~ 50 days) decline rates. The nebular-phase spectra provide powerful diagnostics to identify the 2009dc-like events as a distinct class of SNe Ia. We suggest referring to these sources using the phenomenology-based "2009dc-like SN Ia-pec" instead of "Super-Chandrasekhar SN Ia," which is based on an uncertain theoretical interpretation.
Radiation pressure on dust grains may be an important physical mechanism driving galaxy-wide superwinds in rapidly star-forming galaxies. We calculate the combined dust and gas Eddington ratio ...(Gamma) for the archetypal superwind of M82. By combining archival GALEX data, a standard dust model, Monte Carlo dust scattering calculations, and the Herschel map of the dust surface density distribution, the observed FUV/NUV surface brightness in the outflow constrains both the total UV luminosity escaping from the starburst along its minor axis (L_*, UV) and the flux-mean opacity, thus allowing a calculation of Gamma. We find that L_(*, UV) ~ 1-6*10^42 ergs s^-1, ~2-12 times greater than the UV luminosity observed from our line of sight. On a scale of 1-3 kpc above the plane of M82, we find that Gamma ~ 0.01 - 0.06. On smaller scales (~0.25-0.5 kpc), where the enclosed mass decreases, our calculation of L_(*, UV) implies that Gamma ~ 0.1 with factor of few uncertainties. Within the starburst itself, we estimate the single-scattering Eddington ratio to be of order unity. Thus, although radiation pressure is weak compared to gravity on kpc scales above the plane of M82, it may yet be important in launching the observed outflow. We discuss the primary uncertainties in our calculation, the sensitivity of Gamma to the dust grain size distribution, and the time evolution of the wind following M82's recent starburst episodes.
We report the discovery of 19 new R Coronae Borealis (RCB) star and DY Per candidates with light curves from the All-Sky Automated Survey for Supernovae (ASAS-SN). We examined both an existing set of ...1602 near/mid-IR selected candidates and an additional 2615 candidates selected to have near/mid-IR SEDs consistent with those of known R Coronae Borealis stars. We visually inspected the light curves for the characteristic variability of these systems.
We employ VLTI GRAVITY to resolve, for the first time, the two images generated by a gravitational microlens. The measurements of the image separation \theta_{-,+}=3.78 +/- 0.05 mas, and hence the ...Einstein radius \theta_E =1.87 +/- 0.03 mas, are precise. This demonstrates the robustness of the method, provided that the source is bright enough for GRAVITY (K <~ 10.5) and the image separation is of order or larger than the fringe spacing. When \theta_E is combined with a measurement of the "microlens parallax" \pi_E, the two will together yield the lens mass and lens-source relative parallax and proper motion. Because the source parallax and proper motion are well measured by Gaia, this means that the lens characteristics will be fully determined, whether or not it proves to be luminous. This method can be a powerful probe of dark, isolated objects, which are otherwise quite difficult to identify, much less characterize. Our measurement contradicts Einstein's (1936) prediction that "the luminous circle i.e., microlensed image cannot be distinguished" from a star.
We derive an upper limit to the energy of nuclei accelerated via the Fermi mechanism in any relativistic shockwave, driven by any astrophysical engine. This bound is accessible to current and ...upcoming ultra-high energy neutrino experiments. Detection of a single neutrino with energy above the upper limit would exclude all sites of shock acceleration, and imply physics beyond the Standard Model. We comment on the possibility that relativistic flows launched by supermassive black hole mergers are the source of the observed ultra-high energy cosmic rays.
