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 galaxies 1 . Previous candidate flares 2-6 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 jet 7-10 . Here we report the discovery of 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 decline of its light curve follows 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 2 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.When the pericenter of a star's orbit (R p ) passes within the tidal disruption radius of a massive black hole, R T ≈ R ⋆ (M BH /M ⋆ ) 1/3 , tidal forces overcome the binding energy of the 1 star, which breaks up with roughly half of the stellar debris remaining bound to the black hole and the rest being ejected at high velocity 1 . For black holes above a critical mass,, the star becomes trapped within the event horizon of the black hole before being disrupted. The mass accretion rate (Ṁ ) in a tidal disruption event (TDE) can be calculated directly from the orbital return-times of the bound debris 1,11,12 . For the simplest case of a star of uniform density this yields,Ṁ = 2 3 ( f M⋆ t min )( t t min ) −5/3 , where f is the fraction of the star accreted and t min is the orbital period of the most tightly bound debris and, therefore, the time delay between the time of disruption and the start of the flare, which scales asThe radiative output of the accreted debris is less certain, and depends on the ratio of the accretion rate to the Eddington rate 13 . Table 2). No source is detected in a deep coadd of all the TDS epochs in 2009, with a 3σ upper limit of > 25.6 mag implying a peak amplitude of variability in the NUV of > 6.4 mag. See the Supplementary Information for details on the PS1 and GALEX photometry. PS1-10jh is coincident with the centre of a galaxy within the 3σ positional uncertainty (0.036 arcsec; Supplementary Information) with rest-frame u, g, r, i, and z photometry from SDSS 16 and K photometry from UKIDSS 17 fitted with a galaxy template 18 with M stars = (3.6 ± 0.2) × 10 9 M ⊙ and M r = −18.7 mag, where M stars is the galaxy stellar mass and M r is the absolute r-band...

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ∼30 million midinfrared sources toward the inner Galaxy, 10Њ ≤ FlF ≤ 65Њ and FbF ≤ 1Њ, was used to determine the distribution of stars in Galactic longitude, l, latitude, b, and apparent magnitude, m. The counts versus longitude can be approximated by the modified Bessel function N p N 0 (l/l 0 )K 1 (l/l 0 ), where l 0 is insensitive to limiting magnitude, band choice, and side of Galactic center: l 0 p 17Њ-30Њ with a best-fit value in the 4.5 mm band of l 0 p 24Њ ‫ע‬ 4Њ. Modeling the source distribution as an exponential disk yields a radial scale length of H * p 3.9 ‫ע‬ 0.6 kpc. There is a pronounced north-south asymmetry in source counts for FlF Շ 30Њ, with ∼25% more stars in the north. For l p 10Њ-30Њ, there is a strong enhancement of stars of m p 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length R bar p 4.4 ‫ע‬ 0.5 kpc, tilted by f p 44Њ ‫01ע‬Њ to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l p 26Њ-28Њ, 31Њ .5-34Њ, and 306Њ-309Њ are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (m K Ϫm [8.0] 1 3) peak. In these areas, the counts are reduced by extinction due to molecular gas, high diffuse backgrounds associated with star formation, or both.

The star formation rate (SFR) of the Milky Way remains poorly known, with often-quoted values ranging from 1 to 10 M yr −1 . This situation persists despite the potential for the Milky Way to serve as the ultimate SFR calibrator for external galaxies. We show that various estimates for the Galactic SFR are consistent with one another once they have been normalized to the same initial mass function (IMF) and massive star models, converging to 1.9 ± 0.4 M yr −1 . However, standard SFR diagnostics are vulnerable to systematics founded in the use of indirect observational tracers sensitive only to high-mass stars. We find that absolute SFRs measured using resolved low/intermediate-mass stellar populations in Galactic H ii regions are systematically higher by factors of ∼2-3 compared with calibrations for SFRs measured from mid-IR and radio emission. We discuss some potential explanations for this discrepancy and conclude that it could be allayed if (1) the power-law slope of the IMF for intermediate-mass (1.5 M < m < 5M ) stars were steeper than the Salpeter slope or (2) a correction factor was applied to the extragalactic 24 μm SFR calibrations to account for the duration of star formation in individual mid-IR-bright H ii regions relative to the lifetimes of O stars. Finally, we present some approaches for testing whether a Galactic SFR of ∼2 M yr −1 is consistent with what we would measure if we could view the Milky Way as external observers. Using luminous radio supernova remnants and X-ray point sources, we find that the Milky Way deviates from expectations at the 1σ -3σ level, hinting that perhaps the Galactic SFR is overestimated or extragalactic SFRs need to be revised upward.

We present the discovery and analysis of two ultra-luminous supernovae (SNe) at z ≈ 0.9 with the Pan-STARRS1 Medium-Deep Survey. These SNe, PS1-10ky and PS1-10awh, are amongst the most luminous SNe ever discovered, comparable to the unusual transient SCP 06F6. Like SCP 06F6, they show characteristic high luminosities (M bol ≈ −22.5 mag), blue spectra with a few broad absorption lines, and no evidence for H or He. We have constructed a full multi-color light curve sensitive to the peak of the spectral energy distribution in the rest-frame ultraviolet, and we have obtained time-series spectroscopy for these SNe. Given the similarities between the SNe, we combine their light curves to estimate a total radiated energy over the course of explosion of (0.9 − 1.4) × 10 51 erg. We find expansion velocities of 12, 000 − 18, 000 km s −1 with no evidence for deceleration measured ∼3 restframe weeks either side of light-curve peak, consistent with the expansion of an optically-thick massive shell of material. We show that radioactive decay is not sufficient to power PS1-10ky, and discuss two plausible origins for these events: the initial spin-down of a newborn magnetar in a core-collapse SN, or SN shock breakout from the dense circumstellar wind surrounding a Wolf-Rayet star.