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News Article | December 12, 2016
Site: www.eurekalert.org

In 2015, the All Sky Automated Survey for SuperNovae (ASAS-SN) detected an event, named ASASSN-15lh, that was recorded as the brightest supernova ever -- and categorised as a superluminous supernova, the explosion of an extremely massive star at the end of its life. It was twice as bright as the previous record holder, and at its peak was 20 times brighter than the total light output of the entire Milky Way. An international team, led by Giorgos Leloudas at the Weizmann Institute of Science, Israel, and the Dark Cosmology Centre, Denmark, has now made additional observations of the distant galaxy, about 4 billion light-years from Earth, where the explosion took place and they have proposed a new explanation for this extraordinary event. "We observed the source for 10 months following the event and have concluded that the explanation is unlikely to lie with an extraordinarily bright supernova. Our results indicate that the event was probably caused by a rapidly spinning supermassive black hole as it destroyed a low-mass star," explains Leloudas. In this scenario, the extreme gravitational forces of a supermassive black hole, located in the centre of the host galaxy, ripped apart a Sun-like star that wandered too close -- a so-called tidal disruption event, something so far only observed about 10 times. In the process, the star was "spaghettified" and shocks in the colliding debris as well as heat generated in accretion led to a burst of light. This gave the event the appearance of a very bright supernova explosion, even though the star would not have become a supernova on its own as it did not have enough mass. The team based their new conclusions on observations from a selection of telescopes, both on the ground and in space. Among them was the Very Large Telescope at ESO's Paranal Observatory, the New Technology Telescope at ESO's La Silla Observatory and the NASA/ESA Hubble Space Telescope [1]. The observations with the NTT were made as part of the Public ESO Spectroscopic Survey of Transient Objects (PESSTO). "There are several independent aspects to the observations that suggest that this event was indeed a tidal disruption and not a superluminous supernova," explains coauthor Morgan Fraser from the University of Cambridge, UK (now at University College Dublin, Ireland). In particular, the data revealed that the event went through three distinct phases over the 10 months of follow-up observations. These data overall more closely resemble what is expected for a tidal disruption than a superluminous supernova. An observed re-brightening in ultraviolet light as well as a temperature increase further reduce the likelihood of a supernova event. Furthermore, the location of the event -- a red, massive and passive galaxy -- is not the usual home for a superluminous supernova explosion, which normally occur in blue, star-forming dwarf galaxies. Although the team say a supernova source is therefore very unlikely, they accept that a classical tidal disruption event would not be an adequate explanation for the event either. Team member Nicholas Stone from Columbia University, USA, elaborates: "The tidal disruption event we propose cannot be explained with a non-spinning supermassive black hole. We argue that ASASSN-15lh was a tidal disruption event arising from a very particular kind of black hole." The mass of the host galaxy implies that the supermassive black hole at its centre has a mass of at least 100 million times that of the Sun. A black hole of this mass would normally be unable to disrupt stars outside of its event horizon -- the boundary within which nothing is able to escape its gravitational pull. However, if the black hole is a particular kind that happens to be rapidly spinning -- a so-called Kerr black hole -- the situation changes and this limit no longer applies. "Even with all the collected data we cannot say with 100% certainty that the ASASSN-15lh event was a tidal disruption event," concludes Leloudas. "But it is by far the most likely explanation." [1] As well as the data from ESO's Very Large Telescope, the New Technology Telescope and the NASA/ESA Hubble Space Telescope the team used observations from NASA's Swift telescope, the Las Cumbres Observatory Global Telescope (LCOGT), the Australia Telescope Compact Array, ESA's XMM-Newton, the Wide-Field Spectrograph (WiFeS and the Magellan Telescope. This research was presented in a paper entitled "The Superluminous Transient ASASSN-15lh as a Tidal Disruption Event from a Kerr Black Hole", by G. Leloudas et al. to appear in the new Nature Astronomy magazine. The team is composed of G. Leloudas (Weizmann Institute of Science, Rehovot, Israel; Niels Bohr Institute, Copenhagen, Denmark), M. Fraser (University of Cambridge, Cambridge, UK), N. C. Stone (Columbia University, New York, USA), S. van Velzen (The Johns Hopkins University, Baltimore, USA), P. G. Jonker (Netherlands Institute for Space Research, Utrecht, the Netherlands; Radboud University Nijmegen, Nijmegen, the Netherlands), I. Arcavi (Las Cumbres Observatory Global Telescope Network, Goleta, USA; University of California, Santa Barbara, USA), C. Fremling (Stockholm University, Stockholm, Sweden), J. R. Maund (University of Sheffield, Sheffield, UK), S. J. Smartt (Queen's University Belfast, Belfast, UK), T. Krühler (Max-Planck-Institut für extraterrestrische Physik, Garching b. München, Germany), J. C. A. Miller-Jones (ICRAR - Curtin University, Perth, Australia), P. M. Vreeswijk (Weizmann Institute of Science, Rehovot, Israel), A. Gal-Yam (Weizmann Institute of Science, Rehovot, Israel), P. A. Mazzali (Liverpool John Moores University, Liverpool, UK; Max-Planck-Institut für Astrophysik, Garching b. München, Germany), A. De Cia (European Southern Observatory, Garching b. München, Germany), D. A. Howell (Las Cumbres Observatory Global Telescope Network, Goleta, USA; University of California Santa Barbara, Santa Barbara, USA), C. Inserra (Queen's University Belfast, Belfast, UK), F. Patat (European Southern Observatory, Garching b. München, Germany), A. de Ugarte Postigo (Instituto de Astrofisica de Andalucia, Granada, Spain; Niels Bohr Institute, Copenhagen, Denmark), O. Yaron (Weizmann Institute of Science, Rehovot, Israel), C. Ashall (Liverpool John Moores University, Liverpool, UK), I. Bar (Weizmann Institute of Science, Rehovot, Israel), H. Campbell (University of Cambridge, Cambridge, UK; University of Surrey, Guildford, UK), T.-W. Chen (Max-Planck-Institut für extraterrestrische Physik, Garching b. München, Germany), M. Childress (University of Southampton, Southampton, UK), N. Elias-Rosa (Osservatoria Astronomico di Padova, Padova, Italy), J. Harmanen (University of Turku, Piikkiö, Finland), G. Hosseinzadeh (Las Cumbres Observatory Global Telescope Network, Goleta, USA; University of California Santa Barbara, Santa Barbara, USA), J. Johansson (Weizmann Institute of Science, Rehovot, Israel), T. Kangas (University of Turku, Piikkiö, Finland), E. Kankare (Queen's University Belfast, Belfast, UK), S. Kim (Pontificia Universidad Católica de Chile, Santiago, Chile), H. Kuncarayakti (Millennium Institute of Astrophysics, Santiago, Chile; Universidad de Chile, Santiago, Chile), J. Lyman (University of Warwick, Coventry, UK), M. R. Magee (Queen's University Belfast, Belfast, UK), K. Maguire (Queen's University Belfast, Belfast, UK), D. Malesani (University of Copenhagen, Copenhagen, Denmark; DTU Space, Denmark), S. Mattila (University of Turku, Piikkiö, Finland; Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Piikkiö, Finland; University of Cambridge, Cambridge, UK), C. V. McCully (Las Cumbres Observatory Global Telescope Network, Goleta, USA; University of California Santa Barbara, Santa Barbara, USA), M. Nicholl (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA), S. Prentice (Liverpool John Moores University, Liverpool, UK), C. Romero-Ca[ñ] - https:/ izales (Pontificia Universidad Católica de Chile, Santiago, Chile; Millennium Institute of Astrophysics, Santiago, Chile), S. Schulze (Pontificia Universidad Católica de Chile, Santiago, Chile; Millennium Institute of Astrophysics, Santiago, Chile), K. W. Smith (Queen's University Belfast, Belfast, UK), J. Sollerman (Stockholm University, Stockholm, Sweden), M. Sullivan (University of Southampton, Southampton, UK), B. E. Tucker (Australian National University, Canberra, Australia; ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Australia), S. Valenti (University of California, Davis, USA), J. C. Wheeler (University of Texas at Austin, Austin, USA), and D. R. Young (Queen's University Belfast, Belfast, UK). ESO is the foremost intergovernmental astronomy organisation in Europe and the world's most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become "the world's biggest eye on the sky".

Harrison C.M.,Durham University | Thomson A.P.,Durham University | Alexander D.M.,Durham University | Bauer F.E.,University of Santiago de Chile | And 6 more authors.
Astrophysical Journal | Year: 2015

We present multi-frequency (1-8 GHz) Very Large Array data, combined with VIsible MultiObject Spectrograph integral field unit data and Hubble Space Telescope imaging, of a z = 0.085 radio-quiet type 2 quasar (with L 1.4 GHz 5 × 1023 W Hz-1 and L AGN 2 × 1045 erg s-1). Due to the morphology of its emission-line region, the target (J1430+1339) has been referred to as the "Teacup" active galactic nucleus (AGN) in the literature. We identify "bubbles" of radio emission that are extended 10-12 kpc to both the east and west of the nucleus. The edge of the brighter eastern bubble is co-spatial with an arc of luminous ionized gas. We also show that the "Teacup" AGN hosts a compact radio structure, located 0.8 kpc from the core position, at the base of the eastern bubble. This radio structure is co-spatial with an ionized outflow with an observed velocity of v = -740 km s-1. This is likely to correspond to a jet, or possibly a quasar wind, interacting with the interstellar medium at this position. The large-scale radio bubbles appear to be inflated by the central AGN, which indicates that the AGN can also interact with the gas on ≳ 10 kpc scales. Our study highlights that even when a quasar is formally "radio-quiet" the radio emission can be extremely effective for observing the effects of AGN feedback. © 2015. The American Astronomical Society. All rights reserved..

Richert A.J.W.,Pennsylvania State University | Feigelson E.D.,Pennsylvania State University | Feigelson E.D.,University Park | Getman K.V.,Pennsylvania State University | And 2 more authors.
Astrophysical Journal | Year: 2015

Hubble Space Telescope images of proplyds in the Orion Nebula, as well as submillimeter/radio measurements, show that the dominant O7 star Ori C photoevaporates nearby disks around pre-main-sequence stars. Theory predicts that massive stars photoevaporate disks within distances of the order of 0.1 pc. These findings suggest that young, OB-dominated massive H ii regions are inhospitable to the survival of protoplanetary disks and, subsequently, to the formation and evolution of planets. In the current work, we test this hypothesis using large samples of pre-main-sequence stars in 20 massive star-forming regions selected with X-ray and infrared photometry in the MYStIX survey. Complete disk destruction would lead to a deficit of cluster members with an excess in JHKS and Spitzer/IRAC bands in the vicinity of O stars. In four MYStIX regions containing O stars and a sufficient surface density of disk-bearing sources to reliably test for spatial avoidance, we find no evidence for the depletion of inner disks around pre-main-sequence stars in the vicinity of O-type stars, even very luminous O2-O5 stars. These results suggest that massive star-forming regions are not very hostile to the survival of protoplanetary disks and, presumably, to the formation of planets. © 2015. The American Astronomical Society. All rights reserved..

Thuan T.X.,University of Virginia | Bauer F.E.,University of Santiago de Chile | Bauer F.E.,Millennium Institute of Astrophysics | Bauer F.E.,Space Science Institute | Izotov Y.I.,Ukrainian Academy of Sciences
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present XMM-Newton and Chandra observations of two low-metallicity cometary blue compact dwarf (BCD) galaxies, Mrk 59 and Mrk 71. The first BCD, Mrk 59, contains two ultraluminous X-ray sources (ULXs), IXO 72 and IXO 73, both associated with bright massivestars and HII complexes, as well as one fainter extended source associated with a massive HII complex at the head of the cometary structure. The low metallicity of Mrk 59 appears to be responsible for the presence of the two ULXs. IXO 72 has varied little over thelast 10 yr, while IXO 73 has demonstrated a variability factor of ~4 over the same period. The second BCD, Mrk 71, contains two faint X-ray point sources and two faint extendedsources. One point source is likely a background AGN, while the other appears to be coincident with a very luminous star and a compact HII region at the 'head' of the cometary structure. The two faint extended sources are also associated with massive HII complexes. Although both BCDs have the same metallicity, the three sources in Mrk 71 have X-ray luminosities ~1-2 orders of magnitude fainter than those in Mrk 59. The age of the starburst may play a role. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Huijse P.,Millennium Institute of Astrophysics | Estevez P.A.,University of Chile | Protopapas P.,Harvard University | Principe J.C.,University of Florida | Zegers P.,University of Los Andes, Chile
IEEE Computational Intelligence Magazine | Year: 2014

Time-domain astronomy (TDA) is facing a paradigm shift caused by the exponential growth of the sample size, data complexity and data generation rates of new astronomical sky surveys. For example, the Large Synoptic Survey Telescope (LSST), which will begin operations in northern Chile in 2022, will generate a nearly 150 Petabyte imaging dataset of the southern hemisphere sky. The LSST will stream data at rates of 2 Terabytes per hour, effectively capturing an unprecedented movie of the sky. The LSST is expected not only to improve our understanding of time-varying astrophysical objects, but also to reveal a plethora of yet unknown faint and fast-varying phenomena. To cope with a change of paradigm to data-driven astronomy, the fields of astroinformatics and astrostatistics have been created recently. The new data-oriented paradigms for astronomy combine statistics, data mining, knowledge discovery, machine learning and computational intelligence, in order to provide the automated and robust methods needed for the rapid detection and classification of known astrophysical objects as well as the unsupervised characterization of novel phenomena. In this article we present an overview of machine learning and computational intelligence applications to TDA. Future big data challenges and new lines of research in TDA, focusing on the LSST, are identified and discussed from the viewpoint of computational intelligence/machine learning. Interdisciplinary collaboration will be required to cope with the challenges posed by the deluge of astronomical data coming from the LSST. © 2014 IEEE.

Anderson J.P.,European Southern Observatory | James P.A.,Liverpool John Moores University | Habergham S.M.,Liverpool John Moores University | Galbany L.,Millennium Institute of Astrophysics | And 3 more authors.
Publications of the Astronomical Society of Australia | Year: 2015

Mapping the diversity of SNe to progenitor properties is key to our understanding of stellar evolution and explosive stellar death. Investigations of the immediate environments of SNe allow statistical constraints to be made on progenitor properties such as mass and metallicity. Here, we review the progress that has been made in this field. Pixel statistics using tracers of e.g. star formation within galaxies show intriguing differences in the explosion sites of, in particular SNe types II and Ibc (SNe II and SNe Ibc respectively), suggesting statistical differences in population ages. Of particular interest is that SNe Ic are significantly more associated with host galaxy Hα emission than SNe Ib, implying shorter lifetimes for the former. In addition, such studies have shown (unexpectedly) that the interacting SNe IIn do not explode in regions containing the most massive stars, which suggests that at least a significant fraction of their progenitors arise from the lower end of the core-collapse SN mass range. Host H ii region spectroscopy has been obtained for a significant number of core-collapse events, however definitive conclusions on differences between distinct SN types have to-date been elusive. Single stellar evolution models predict that the relative fraction of SNe Ibc to SNe II should increase with increasing metallicity, due to the dependence of mass-loss rates on progenitor metallicity. We present a meta-analysis of all current host H ii region oxygen abundances for CC SNe. It is concluded that the SN II to SN Ibc ratio shows little variation with oxygen abundance, with only a suggestion that the ratio increases in the lowest bin. Radial distributions of different SNe are discussed, where a central excess of SNe Ibc has been observed within disturbed galaxy systems, which is difficult to ascribe to metallicity or selection effects. Environment studies are also being undertaken for SNe Ia, where constraints can be made on the shortest delay times of progenitor systems. It is shown that 'redder' SNe Ia are more often found within star-forming regions. Environment studies are evolving to enable studies at higher spatial resolutions than previously possible, while in addition the advent of wide-field integral field unit instruments allows galaxy-wide spectral analyses which will provide fruitful results to this field. Some example contemporary results are shown in that direction. © 2015 Astronomical Society of Australia.

Pejcha O.,Princeton University | Prieto J.L.,Diego Portales University | Prieto J.L.,Millennium Institute of Astrophysics
Astrophysical Journal | Year: 2015

We present a new self-consistent and versatile method that derives photospheric radius and temperature variations of Type II-Plateau supernovae based on their expansion velocities and photometric measurements. We apply the method to a sample of 26 well-observed, nearby supernovae with published light curves and velocities. We simultaneously fit ∼230 velocity and ∼6800 mag measurements distributed over 21 photometric passbands spanning wavelengths from 0.19 to 2.2 μm. The light-curve differences among the Type II-Plateau supernovae are well modeled by assuming different rates of photospheric radius expansion, which we explain as different density profiles of the ejecta, and we argue that steeper density profiles result in flatter plateaus, if everything else remains unchanged. The steep luminosity decline of Type II-Linear supernovae is due to fast evolution of the photospheric temperature, which we verify with a successful fit of SN 1980K. Eliminating the need for theoretical supernova atmosphere models, we obtain self-consistent relative distances, reddenings, and nickel masses fully accounting for all internal model uncertainties and covariances.We use our global fit to estimate the time evolution of any missing band tailored specifically for each supernova, and we construct spectral energy distributions and bolometric light curves. We produce bolometric corrections for all filter combinations in our sample. We compare our model to the theoretical dilution factors and find good agreement for the B and V filters. Our results differ from the theory when the I, J, H, or K bands are included. We investigate the reddening law toward our supernovae and find reasonable agreement with standard script RV ∼ 3.1 reddening law in UBVRI bands. Results for other bands are inconclusive. We make our fitting code publicly available. © 2015. The American Astronomical Society. All rights reserved.

Pejcha O.,Princeton University | Prieto J.L.,Diego Portales University | Prieto J.L.,Millennium Institute of Astrophysics
Astrophysical Journal | Year: 2015

Hydrogen-rich Type II-Plateau supernovae (SNe) exhibit correlations between the plateau luminosity Lpl, the nickel mass MNi, the explosion energy Eexp, and the ejecta mass Mej. Using our global, self-consistent, multi-band model of nearby well-observed SNe, we find that the covariances of these quantities are strong and that the confidence ellipsoids are oriented in the direction of the correlations, which reduces their significance. By proper treatment of the covariance matrix of the model, we discover a significant intrinsic width to the correlations between Lpl, Eexp and MNi, where the uncertainties due to the distance and the extinction dominate. For fixed Eexp, the spread in MNi is about 0.25 dex, which we attribute to the differences in the progenitor internal structure. We argue that the effects of incomplete γ-ray trapping are not important in our sample. Similarly, the physics of the Type II-Plateau SN light curves leads to inherently degenerate estimates of Eexp and Mej, which makes their observed correlation weak. Ignoring the covariances of SN parameters or the intrinsic width of the correlations causes significant biases in the slopes of the fitted relations. Our results imply that Type II-Plateau SN explosions are not described by a single physical parameter or a simple one-dimensional trajectory through the parameter space, but instead reflect the diversity of the core and surface properties of their progenitors. We discuss the implications for the physics of the explosion mechanism and possible future observational constraints. © 2015. The American Astronomical Society. All rights reserved.

Zoccali M.,University of Santiago de Chile | Zoccali M.,Millennium Institute of Astrophysics | Valenti E.,European Southern Observatory
Publications of the Astronomical Society of Australia | Year: 2016

We review the observational evidences concerning the three-dimensional structure of the Galactic bulge. Although the inner few kpc of our Galaxy are normally referred to as the bulge, all the observations demonstrate that this region is dominated by a bar, i.e., the bulge is a bar. The bar has a boxy/peanut (X-shaped) structure in its outer regions, while it seems to become less and less elongated in its innermost region. A thinner and longer structure departing from the main bar has also been found, although the observational evidences that support the scenario of two separate structures has been recently challenged. Metal-poor stars ([Fe/H] ≲ −0.5 dex) trace a different structure, and also have different kinematics. Copyright © Astronomical Society of Australia 2016

Agliozzo C.,Millennium Institute of Astrophysics
EAS Publications Series | Year: 2015

The physical mechanism responsible for high mass-loss in luminous blue variable stars (LBVs) is still poorly understood. The possibility that it is independent of metallicity is of paramount importance to LBVs in the metal-poor Universe. Our approach to investigate this possibility is to study LBVs in different environments and to use multiwavelength observations to resolve their nebulae. These contain the fingerprints of their mass-loss, as shown here by the galactic example G79.29+0.46. I also present our pilot study of objects in the Magellanic Clouds (MCs), where a paucity of data existed so far. © 2015 EAS, EDP Sciences.

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