Person:

Narayan, Gautham

The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies1. Previous candidate flares2, 3, 4, 5, 6 have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two ‘relativistic’ candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet7, 8, 9, 10. Here we report a luminous ultraviolet–optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decay of the light curve follow the predicted mass accretion rate and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about two million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.

The ESSENCE survey discovered 213 type Ia supernovae at redshifts 0.10 < z < 0.81 between 2002 and 2008. We present their R and I band light curve measurements, obtained using the MOSAIC II imager at the CTIO 4 m, along with rapid response spectroscopy for each object from a range of large aperture ground based telescopes. We detail our program to obtain quantitative classifications and precise redshifts from our spectroscopic follow-up of each object. We describe our efforts to improve the precision of the calibration of the CTIO 4 m natural photometric system. We use several empirical metrics to measure our internal photometric consistency and our absolute calibration of the survey. We assess the effect of various sources of systematic error on our measured fluxes, and estimate that the total systematic error budget from the photometric calibration is ~1%. We combine 108 ESSENCE SNIa that pass stringent quality cuts with a compilation of 441 SNIa from 3 year results presented by the Supernova Legacy Survey and Baryon Acoustic Oscillation measurements from the Sloan Digital Sky Survey to produce preliminary cosmological constraints employing the SNIa . This constitutes the largest sample of well-calibrated, spectroscopically confirmed SNIa to date. Assuming a flat Universe, we obtain a joint constraint of (\Omega_M = 0.266^{+0.026}{-0.016}(stat 1\sigma)), and (w = -1.112^{+0.069}{-0.072}(stat 1\sigma)). These measurements are consistent with a cosmological constant.