Person:

Zauderer, B

Active galactic nuclei, which are powered by long-term accretion onto central supermassive black holes, produce1 relativistic jets with lifetimes of at least one million years, and the observation of the birth of such a jet is therefore unlikely. Transient accretion onto a supermassive black hole, for example through the tidal disruption2, 3 of a stray star, thus offers a rare opportunity to study the birth of a relativistic jet. On 25 March 2011, an unusual transient source (Swift J164449.3+573451) was found4, potentially representing5, 6 such an accretion event. Here we report observations spanning centimetre to millimetre wavelengths and covering the first month of evolution of a luminous radio transient associated with Swift J164449.3+573451. The radio transient coincides7 with the nucleus of an inactive galaxy. We conclude that we are seeing a newly formed relativistic outflow, launched by transient accretion onto a million-solar-mass black hole. A relativistic outflow is not predicted in this situation, but we show that the tidal disruption of a star naturally explains the observed high-energy properties and radio luminosity and the inferred rate of such events. The weaker beaming in the radio-frequency spectrum relative to γ-rays or X-rays suggests that radio searches may uncover similar events out to redshifts of z ≈ 6.

We present continued multi-frequency radio observations of the relativistic tidal disruption event Swift J164449.3+573451 (Sw 1644+57) extending to δt ≈ 600 d. The data were obtained with the JVLA and AMI Large Array as part of our on-going study of the jet energetics and the density structure of the parsecscale environment around the disrupting supermassive black hole (SMBH). We combine these data with public Swift/XRT and Chandra X-ray observations over the same time-frame to show that the jet has undergone a dramatic transition starting at ≈ 500 d, with a sharp decline in the X-ray flux by about a factor of 170 on a timescale of δt/t . 0.2 (and by a factor of 15 in δt/t ≈ 0.05). The rapid decline rules out a forward shock origin (direct or reprocessing) for the X-ray emission at . 500 d, and instead points to internal dissipation in the inner jet. On the other hand, our radio data uniquely demonstrate that the low X-ray flux measured by Chandra at ≈ 610 d is consistent with emission from the forward shock. Furthermore, the Chandra data are inconsistent with thermal emission from the accretion disk itself since the expected temperature of ∼ 30 − 60 eV and inner radius of ∼ 2 − 10Rs cannot accommodate the observed flux level or the detected emission at & 1 keV. We associate the rapid decline with a turn off of the relativistic jet when the mass accretion rate dropped below ∼ M˙ Edd ≈ 0.006 M⊙ yr−1 (for a 3 × 106 M⊙ black hole and order unity efficiency) indicating that the peak accretion rate was about 330M˙ Edd, and the total accreted mass by δt ≈ 500 d is about 0.15 M⊙. From the radio data we further find significant flattening in the integrated energy of the forward shock at δt & 250 d with Ej,iso ≈ 2 × 1054 erg (Ej ≈ 1052 erg for a jet opening angle, θj = 0.1) following a rise by about a factor of 15 at ≈ 30 − 250 d. Projecting forward, we predict that the emission in the radio and X-ray bands will evolve in tandem with similar decline rates.

We present the Pan-STARRS1 discovery of the long-lived and blue transient PS1-11af, which was also detected by Galaxy Evolution Explorer with coordinated observations in the near-ultraviolet (NUV) band. PS1-11af is associated with the nucleus of an early type galaxy at redshift z = 0.4046 that exhibits no evidence for star formation or active galactic nucleus activity. Four epochs of spectroscopy reveal a pair of transient broad absorption features in the UV on otherwise featureless spectra. Despite the superficial similarity of these features to P-Cygni absorptions of supernovae (SNe), we conclude that PS1-11af is not consistent with the properties of known types of SNe. Blackbody fits to the spectral energy distribution are inconsistent with the cooling, expanding ejecta of a SN, and the velocities of the absorption features are too high to represent material in homologous expansion near a SN photosphere. However, the constant blue colors and slow evolution of the luminosity are similar to previous optically selected tidal disruption events (TDEs). The shape of the optical light curve is consistent with models for TDEs, but the minimum accreted mass necessary to power the observed luminosity is only ∼0.002 M , which points to a partial disruption model. A full disruption model predicts higher bolometric luminosities, which would require most of the radiation to be emitted in a separate component at high energies where we lack observations. In addition, the observed temperature is lower than that predicted by pure accretion disk models for TDEs and requires reprocessing to a constant, lower temperature. Three deep non-detections in the radio with the Very Large Array over the first two years after the event set strict limits on the production of any relativistic outflow comparable to Swift J1644+57, even if off-axis.

The double explosion of SN 2009ip in 2012 raises questions about our understanding of the late stages of massive star evolution. Here we present a comprehensive study of SN 2009ip during its remarkable rebrightenings. High-cadence photometric and spectroscopic observations from the GeV to the radio band obtained from a variety of ground-based and space facilities (including the Very Large Array, Swift, Fermi, Hubble Space Telescope, and XMM) constrain SN 2009ip to be a low energy (E ~ 1050 erg for an ejecta mass ~0.5 M ☉) and asymmetric explosion in a complex medium shaped by multiple eruptions of the restless progenitor star. Most of the energy is radiated as a result of the shock breaking out through a dense shell of material located at ~5 × 1014 cm with M ~ 0.1 M ☉, ejected by the precursor outburst ~40 days before the major explosion. We interpret the NIR excess of emission as signature of material located further out, the origin of which has to be connected with documented mass-loss episodes in the previous years. Our modeling predicts bright neutrino emission associated with the shock break-out if the cosmic-ray energy is comparable to the radiated energy. We connect this phenomenology with the explosive ejection of the outer layers of the massive progenitor star, which later interacted with material deposited in the surroundings by previous eruptions. Future observations will reveal if the massive luminous progenitor star survived. Irrespective of whether the explosion was terminal, SN 2009ip brought to light the existence of new channels for sustained episodic mass loss, the physical origin of which has yet to be identified.

We present multi-wavelength observations of the afterglow of the short GRB 111117A, and follow-up observations of its host galaxy. From rapid optical and radio observations we place limits of r & 25.5 mag at δt ≈ 0.55 d and Fν(5.8GHz) . 18 µJy at δt ≈ 0.50 d, respectively. However, using a Chandra observation at δt ≈ 3.0 d we locate the absolute position of the X-ray afterglow to an accuracy of 0.22′′ (1σ), a factor of about 6 times better than the Swift/XRT position. This allows us to robustly identify the host galaxy and to locate the burst at a projected offset of 1.25 ± 0.20′′ from the host centroid. Using optical and near-IR observations of the host galaxy we determine a photometric redshift of z = 1.3+0.3−0.2, one of the highest for any short GRB, and leading to a projected physical offset for the burst of 10.5 ± 1.7 kpc, typical of previous short GRBs. At this redshift, the isotropic γ-ray energy is Eγ,iso ≈ 3.0×1051 erg (rest-frame 23 − 2300 keV) with a peak energy of Epk ≈ 850 − 2300 keV (rest-frame). In conjunction with the isotropic X-ray energy, GRB 111117A appears to follow our recently-reported Ex,iso-Eγ,iso-Epk universal scaling. Using the X-ray data along with the optical and radio non-detections we find that for a blastwave kinetic energy of EK,iso ≈ Eγ,iso erg, the circumburst density is n0 ≈ 3×10−4 − 1 cm−3 (for a range of ǫB = 0.001 − 0.1). Similarly, from the non-detection of a break in the X-ray light curve at δt . 3 d, we infer a minimum opening angle for the outflow of θj & 3 − 10◦ (depending on the circumburst density). We conclude that Chandra observations of short GRBs are effective at determining precise positions and robust host galaxy associations in the absence of optical and radio detections.