Cambridge, MA, United States
Cambridge, MA, United States

Time filter

Source Type

Sanchez-Ramirez R.,Institute Astrofisica Of Andalucia Iaa Csic | Sanchez-Ramirez R.,University of the Basque Country | Sanchez-Ramirez R.,Ikerbasque | Hancock P.J.,University of Sydney | And 50 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2017

We present the extensive follow-up campaign on the afterglowof GRB 110715A at 17 different wavelengths, from X-ray to radio bands, starting 81 s after the burst and extending up to 74 d later.We performed for the first time aGRBafterglowobservationwith theALMAobservatory. We find that the afterglow of GRB 110715A is very bright at optical and radio wavelengths. We use the optical and near-infrared spectroscopy to provide further information about the progenitor's environment and its host galaxy. The spectrum shows weak absorption features at a redshift z = 0.8225, which reveal a host-galaxy environment with low ionization, column density, and dynamical activity. Late deep imaging shows a very faint galaxy, consistent with the spectroscopic results. The broad-band afterglow emission is modelled with synchrotron radiation using a numerical algorithm and we determine the best-fitting parameters using Bayesian inference in order to constrain the physical parameters of the jet and the medium in which the relativistic shock propagates. We fitted our data with a variety of models, including different density profiles and energy injections. Although the general behaviour can be roughly described by thesemodels, none of them are able to fully explain all data points simultaneously. GRB 110715A shows the complexity of reproducing extensive multiwavelength broad-band afterglow observations, and the need of good sampling in wavelength and time and more complex models to accurately constrain the physics of GRB afterglows. © 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Chen C.,Harvard Smithonian Center for Astrophysics | Schneps M.H.,University of Massachusetts Boston | Schneps M.H.,Massachusetts Institute of Technology | Sonnert G.,Harvard Smithonian Center for Astrophysics
Journal of Science Education and Technology | Year: 2016

Teachers choosing between different models to facilitate students’ understanding of an abstract system must decide whether to adopt a model that is simplified and striking or one that is realistic and complex. Only recently have instructional technologies enabled teachers and learners to change presentations swiftly and to provide for learning based on multiple models, thus giving rise to questions about the order of presentation. Using disjoint individual growth modeling to examine the learning of astronomical concepts using a simulation of the solar system on tablets for 152 high school students (age 15), the authors detect both a model effect and an order effect in the use of the Orrery, a simplified model that exaggerates the scale relationships, and the True-to-scale, a proportional model that more accurately represents the realistic scale relationships. Specifically, earlier exposure to the simplified model resulted in diminution of the conceptual gain from the subsequent realistic model, but the realistic model did not impede learning from the following simplified model. © 2016 Springer Science+Business Media New York

De Plaa J.,SRON Netherlands Institute for Space Research | Zhuravleva I.,Max Planck Institute for Astrophysics | Werner N.,Kavli Institute for Particle Astrophysics and Cosmology | Kaastra J.S.,SRON Netherlands Institute for Space Research | And 3 more authors.
AIP Conference Proceedings | Year: 2012

We aim to estimate the level of turbulence in the hot gas of elliptical galaxies using resonant scattering in Fe XVII lines. Using the RGS grating spectrometer aboard XMM-Newton, we measure the ratio between the Fe XVII lines at 15 Å and 17 Å in two deep observations of NGC 5044 and NGC 5813. The 15 Å line has a large oscillator strength and is therefore sensitive to resonant scattering. We find a significant difference in the (I 17.05+I 17.10)/I 15.01 line ratio between the two objects. The level of turbulence in NGC 5044 appears to be significantly higher than in NGC 5813, which could be explained by their spatial structures seen in Chandra images. © 2012 American Institute of Physics.

De Plaa J.,SRON Netherlands Institute for Space Research | Zhuravleva I.,Max Planck Institute for Astrophysics | Werner N.,Kavli Institute for Particle Astrophysics and Cosmology | Kaastra J.S.,SRON Netherlands Institute for Space Research | And 6 more authors.
Astronomy and Astrophysics | Year: 2012

Context. The interstellar and intra-cluster medium (ICM) in giant elliptical galaxies and clusters of galaxies is often assumed to be in hydrostatic equilibrium. Numerical simulations, however, show that about 5-30% of the pressure in a cluster is provided by turbulence induced by, for example, the central active galactic nucleus (AGN) and merger activity. Aims. We aim to put constraints on the turbulent velocities and the turbulent pressure in the ICM of the giant elliptical galaxies NGC 5044 and NGC 5813 using XMM-Newton reflection grating spectrometer (RGS) observations. Methods. The magnitude of the turbulence is estimated using the Fe XVII lines at 15.01 Å, 17.05 Å, and 17.10 Å in the RGS spectra. At low turbulent velocities, the gas becomes optically thick in the 15.01 Å line due to resonant scattering, while the 17 Å lines remain optically thin. By comparing the (I 17.05 + I 17.10)/I 15.01 line ratio from RGS with simulated line ratios for different Mach numbers, the level of turbulence is constrained. The measurement is, however, limited by the systematic uncertainty in the line ratio for an optically thin plasma, which is about 20-30%. Results. We find that the (I 17.05 + I 17.10)/I 15.01 line ratio in NGC 5813 is significantly higher than in NGC 5044. This difference can be explained by a higher level of turbulence in NGC 5044. The best estimates for the turbulent velocities using resonant scattering and upper limits from the line widths, are 320 < V turb < 720 km s -1 for NGC 5044 and 140 < V turb < 540 km s -1 for NGC 5813 at the 90% confidence limit. Conclusions. The high turbulent velocities and the fraction of the turbulent pressure support of >40% in NGC 5044, assuming isotropic turbulence, confirm that it is a highly disturbed system, probably due to an off-axis merger. The turbulent pressure support in NGC 5813 is more modest at 15-45%. The (I 17.05 + I 17.10)/I 15.01 line ratio in an optically thin plasma, calculated using AtomDB v2.0.1, is 2σ above the ratio measured in NGC 5044, which cannot be explained by resonant scattering. This shows that the discrepancies between theoretical, laboratory, and astrophysical data on Fe XVII lines need to be reduced to improve the accuracy of the determination of turbulent velocities using resonant scattering. © 2012 ESO.

Rahoui F.,Harvard University | Rahoui F.,Harvard Smithonian Center for Astrophysics | Coriat M.,University of Southampton | Corbel S.,University Paris Diderot | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

The microquasar GX 339-4, known to exhibit powerful compact jets that dominate its radio to near-infrared emission, entered an outburst in 2010 for the fifth time in about 15yr. An extensive radio to X-ray multiwavelength campaign was immediately triggered, and we report here on European Southern Observatory/FORS2+ISAAC optical and near-infrared spectroscopic observations, supported by Australia Telescope Compact Array radio and Rossi X-ray Timing Experiment/Swift X-ray quasi-simultaneous data. GX 339-4 was observed at three different epochs, once in the soft state and twice in the hard state. In the soft state, the optical and near-infrared continuum is largely consistent with the Raleigh-Jeans tail of a thermal process. As an explanation, we favour irradiation of the outer accretion disc by its inner regions, enhanced by disc warping. An excess is also present at low frequencies, likely due to an M subgiant companion star. During the first hard state, the optical/near-infrared continuum is well described by the optically thin synchrotron emission of the compact jet combined with disc irradiation and perhaps another component peaking in the ultraviolet. The spectral break where the jet transits from the optically thick to thin regimes, located below 1.20 × 10 14Hz, is not detected and the extension of the optically thin synchrotron is consistent with the 3-50keV spectrum. In contrast, the emission during the second hard state is more difficult to understand and points towards a more complex jet continuum. In both cases, the near-infrared continuum is found to be variable at time-scales at least as short as 20s, although these variabilities are smoothed out beyond a few hundred seconds. This implies rapid variations - in flux and frequency - of the location of the spectral break, i.e. dramatic short time-scale changes of the physical conditions at the base of the jet, such as the magnetic field and/or the base radius. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Greiner J.,Max Planck Institute for Extraterrestrial Physics | Michalowski M.J.,University of Edinburgh | Klose S.,Thuringer Landessternwarte Tautenburg | Hunt L.K.,National institute for astrophysics | And 16 more authors.
Astronomy and Astrophysics | Year: 2016

Context. As a result of their relation to massive stars, long-duration gamma-ray bursts (GRBs) allow the pinpointing of star formation in galaxies independent of redshift, dust obscuration, or galaxy mass/size, thus providing a unique tool to investigate star formation history over cosmic time. Aims. About half of the optical afterglows of long-duration GRBs are missed owing to dust extinction and are primarily located in the most massive GRB hosts. It is important to investigate the amount of obscured star formation in these GRB host galaxies to understand this bias. Methods. Radio emission of galaxies correlates with star formation, but does not suffer extinction as do the optical star formation estimators. We selected 11 GRB host galaxies with either large stellar mass or large UV-based and optical-based star formation rates (SFRs) and obtained radio observations of these with the Australia Telescope Compact Array and the Karl Jansky Very Large Array. Results. Despite intentionally selecting GRB hosts with expected high SFRs, we do not find any radio emission related to star formation in any of our targets. Our upper limit for GRB 100621A implies that the earlier reported radio detection was due to afterglow emission. We detect radio emission from the position of GRB 020819B, but argue that it is in large part, if not completely, due to afterglow contamination. Conclusions. Half of our sample has radio-derived SFR limits, which are only a factor 2-3 above the optically measured SFRs. This supports other recent studies that the majority of star formation in GRB hosts is not obscured by dust. © 2016 ESO.

Cseh D.,Radboud University Nijmegen | Kaaret P.,University of Iowa | Corbel S.,University Paris Diderot | Grise F.,Institute of Astrophysics of Canarias | And 12 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2014

We report on the discovery of an apparent triple radio structure hidden inside the radio bubble of the ultraluminous X-ray source Holmberg II X-1. The morphology is consistent with a collimated jet structure, which is observed to emit optically thin synchrotron radiation. The central component has a steep radio spectrum and is brighter than the outer components indicating a renewed radio activity.We estimate a minimum time-averaged jet power of ~2 × 1039 erg s-1 that is associated with a time-averaged isotropic X-ray luminosity of at least 4 × 1039 erg s-1. Our results suggest that Holmberg II X-1 is powered by a black hole of MBH ≥ 25 M⊙, that is inferred to be accreting at a high Eddington rate with intermittent radio activity. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Fontani F.,ESO | Fontani F.,Institute Of Radio Astronomie Millimetrique | Palau A.,Institute Of Ciencies Of Lespai Csic Ieec | Caselli P.,University of Leeds | And 8 more authors.
Astronomy and Astrophysics | Year: 2011

Context. Theory predicts, and observations confirm, that the column density ratio of a molecule containing D to its counterpart containing H can be used as an evolutionary tracer in the low-mass star formation process. Aims. Since it remains unclear if the high-mass star formation process is a scaled-up version of the low-mass one, we investigated whether the relation between deuteration and evolution can be applied to the high-mass regime. Methods. With the IRAM-30 m telescope, we observed rotational transitions of N2D+ and N2H+ and derived the deuterated fraction in 27 cores within massive star-forming regions understood to represent different evolutionary stages of the massive-star formation process. Results. The abundance of N2D+ is higher at the pre-stellar/cluster stage, then drops during the formation of the protostellar object(s) as in the low-mass regime, remaining relatively constant during the ultra-compact HII region phase. The objects with the highest fractional abundance of N 2D+ are starless cores with properties very similar to typical pre-stellar cores of lower mass. The abundance of N2D + is lower in objects with higher gas temperatures as in the low-mass case but does not seem to depend on gas turbulence. Conclusions. Our results indicate that the N2D+-to-N2H+ column density ratio can be used as an evolutionary indicator in both low- and high-mass star formation, and that the physical conditions influencing the abundance of deuterated species likely evolve similarly during the processes that lead to the formation of both low- and high-mass stars. © 2011 ESO.

Mohler-Fischer M.,Max Planck Institute for Astronomy | Mancini L.,Max Planck Institute for Astronomy | Hartman J.D.,Princeton University | Hartman J.D.,Harvard Smithonian Center for Astrophysics | And 28 more authors.
Astronomy and Astrophysics | Year: 2013

We report the discovery of HATS-2b, the second transiting extrasolar planet detected by the HATSouth survey. HATS-2b is moving on a circular orbit around a V = 13.6 mag, K-type dwarf star (GSC 6665-00236), at a separation of 0.0230 ± 0.0003 AU and with a period of 1.3541 days. The planetary parameters have been robustly determined using a simultaneous fit of the HATSouth, MPG/ESO 2.2 m/GROND, Faulkes Telescope South/Spectral transit photometry, and MPG/ESO 2.2 m/FEROS, Euler 1.2 m/CORALIE, AAT 3.9 m/CYCLOPS radial-velocity measurements. HATS-2b has a mass of 1.37 ± 0.16 MJ, a radius of 1.14 ± 0.03 RJ, and an equilibrium temperature of 1567 ± 30 K. The host star has a mass of 0.88 ± 0.04 M ⊙ and a radius of 0.89 ± 0.02 R ⊙, and it shows starspot activity. We characterized the stellar activity by analyzing two photometric follow-up transit light curves taken with the GROND instrument, both obtained simultaneously in four optical bands (covering the wavelength range of 3860-9520 Å). The two light curves contain anomalies compatible with starspots on the photosphere of the host star along the same transit chord. © ESO, 2013.

Niederhofer F.,European Southern Observatory | Humphreys E.,European Southern Observatory | Goddi C.,European Southern Observatory | Greenhill L.J.,Harvard Smithonian Center for Astrophysics
Proceedings of the International Astronomical Union | Year: 2012

Radio Source I in the Orion BN/KL region provides the closest example of high mass star formation. It powers a rich ensemble of SiO and H2O masers, and is one of only three star-forming regions known to display SiO maser emission. Previous monitoring of different SiO masers with the VLBA and VLA has enabled the resolution of a compact disk and a protostellar wind at radii <100 AU from Source I, which collimates into a bipolar outflow at radii of 100-1000 AU (see contribution by Greenhill et al., this volume). Source I may provide the best case of disk-mediated accretion and outflow recollimation in massive star formation. Here, we report preliminary results of sub-arcsecond resolution 325 GHz H2O maser observations made with the SMA. We find that 325 GHz H2O masers trace a more collimated portion of the Source I outflow than masers at 22 GHz, but occur at similar radii suggesting similar excitation conditions. A velocity gradient perpendicular to the outflow axis, indicating rotation, supports magneto-centrifugal driving of the flow. Copyright © International Astronomical Union 2012.

Loading Harvard Smithonian Center for Astrophysics collaborators
Loading Harvard Smithonian Center for Astrophysics collaborators