Time filter

Source Type

Evans D.F.,Keele University | Southworth J.,Keele University | Maxted P.F.L.,Keele University | Skottfelt J.,Open University Milton Keynes | And 38 more authors.
Astronomy and Astrophysics | Year: 2016

Context. Wide binaries are a potential pathway for the formation of hot Jupiters. The binary fraction among host stars is an important discriminator between competing formation theories, but has not been well characterised. Additionally, contaminating light from unresolved stars can significantly affect the accuracy of photometric and spectroscopic measurements in studies of transiting exoplanets. Aims. We observed 101 transiting exoplanet host systems in the Southern hemisphere in order to create a homogeneous catalogue of both bound companion stars and contaminating background stars, in an area of the sky where transiting exoplanetary systems have not been systematically searched for stellar companions. We investigate the binary fraction among the host stars in order to test theories for the formation of hot Jupiters. Methods. Lucky imaging observations from the Two Colour Instrument on the Danish 1.54 m telescope at La Silla were used to search for previously unresolved stars at small angular separations. The separations and relative magnitudes of all detected stars were measured. For 12 candidate companions to 10 host stars, previous astrometric measurements were used to evaluate how likely the companions are to be physically associated. Results. We provide measurements of 499 candidate companions within 20 arcsec of our sample of 101 planet host stars. 51 candidates are located within 5 arcsec of a host star, and we provide the first published measurements for 27 of these. Calibrations for the plate scale and colour performance of the Two Colour Instrument are presented. Conclusions. We find that the overall multiplicity rate of the host stars is 38-13 +17%, consistent with the rate among solar-type stars in our sensitivity range, suggesting that planet formation does not preferentially occur in long period binaries compared to a random sample of field stars. Long period stellar companions (P > 10 yr) appear to occur independently of short period companions, and so the population of close-in stellar companions is unconstrained by our study. © ESO, 2016. Source

Savolainen P.,Metsahovi Radio Observatory | Paizis A.,National institute for astrophysics | Farinelli R.,University of Ferrara | Hannikainen D.C.,Metsahovi Radio Observatory | And 3 more authors.
AIP Conference Proceedings | Year: 2011

We have investigated the spectral and temporal evolution of Neutron Star Low-Mass X-ray Binaries (NS LMXBs) using a sample of eight bright sources. INTEGRAL spectra from 2003 to 2009 and some near-simultaneous RXTE and Swift spectra have been fitted with the recently developed thermal and bulk Comptonization model compTB. In this proposed scenario, the transient hard X-ray tails exhibited by the Z sources and GX 13+1 are connected to efficient bulk motion Comptonization in the inner Transition Layer (TL), while the lack of a tail could be due to too low or too high local accretion rates. Another difference between the source types - the long-term modulation of the Atoll source light curves (with the notable exception of GX 13+1) - is seen to be mostly independent of spectral evolution, while presumably driven by changes in the overall accretion rate from the companion. © 2011 American Institute of Physics. Source

Juvela M.,University of Helsinki | Harju J.,University of Helsinki | Harju J.,Finnish Center for Astronomy with | Ysard N.,University of Helsinki | Lunttila T.,University of Helsinki
Astronomy and Astrophysics | Year: 2012

Context. The temperature is a central parameter affecting the chemical and physical properties of dense cores of interstellar clouds and their potential evolution towards star formation. The chemistry and the dust properties are temperature dependent and, therefore, interpretation of any observation requires the knowledge of the temperature and its variations. Direct measurement of the gas kinetic temperature is possible with molecular line spectroscopy, the ammonia molecule, NH 3, being the most commonly used tracer. Aims. We want to determine the accuracy of the temperature estimates derived from ammonia spectra. The normal interpretation of NH 3 observations assumes that all the hyperfine line components are tracing the same volume of gas. However, in the case of strong temperature gradients they may be sensitive to different layers and this could cause errors in the optical depth and gas temperature estimates. Methods. We examine a series of spherically symmetric cloud models, 1.0 and 0.5 M ⊙ Bonnor-Ebert spheres, with different radial temperature profiles. We calculate synthetic NH 3 spectra and compare the derived column densities and temperatures to the actual values in the models. Results. For high signal-to-noise observations, the estimated gas kinetic temperatures are within ∼0.3 K of the real mass averaged temperature and the column densities are correct to within ∼10%. When the S/N ratio of the (2, 2) spectrum decreases below 10, the temperature errors are of the order of 1 K but without a significant bias. Only when the density of the models is increased by a factor of a few, the results begin to show significant bias because of the saturation of the (1, 1) main group. Conclusions. The ammonia spectra are found to be a reliable tracer of the real mass averaged gas temperature. Because the radial temperature profiles of the cores are not well constrained, the central temperature could still be different from this value. If the cores are optically very thick, there are no longer guarantees of the accuracy of the estimates. © 2012 ESO. Source

Aleksic J.,IFAE | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | Backes M.,TU Dortmund | And 163 more authors.
Astrophysical Journal Letters | Year: 2010

We report on the detection with the MAGIC telescopes of very high energy (VHE) γ -rays from IC 310, a head-tail radio galaxy in the Perseus galaxy cluster, observed during the interval 2008 November to 2010 February. The Fermi satellite has also detected this galaxy. The source is detected by MAGIC at a high statistical significance of 7.6σ in 20.6 hr of stereo data. The observed spectral energy distribution is flat with a differential spectral index of -2.00 ± 0.14. The mean flux above 300 GeV, between 2009 October and 2010 February, (3.1 ± 0.5)×10-12 cm-2 s -1, corresponds to (2.5 ± 0.4)% of Crab Nebula units. Only an upper limit, of 1.9% ofCrab Nebula units above 300 GeV,was obtained with the 2008 data. This, together with strong hints (>3σ) of flares in themiddle of 2009 October and November, implies that the emission is variable. TheMAGIC results favor a scenario with the VHE emission originating from the inner jet close to the central engine. More complicated models than a simple one-zone synchrotron self-Compton (SSC) scenario, e.g., multi-zone SSC, external Compton, or hadronic, may be required to explain the very flat spectrum and its extension overmore than three orders ofmagnitude in energy. © 2010. The American Astronomical Society. All rights reserved. Source

Aleksic J.,IFAE | Alvarez E.A.,Complutense University of Madrid | Antonelli L.A.,National institute for astrophysics | Antoranz P.,University of Siena | And 165 more authors.
Journal of Cosmology and Astroparticle Physics | Year: 2011

We report the results of the observation of the nearby satellite galaxy Segue 1 performed by the MAGIC-I ground-based gamma-ray telescope between November 2008 and March 2009 for a total of 43.2 hours. No significant gamma-ray emission was found above the background. Differential upper limits on the gamma-ray flux are derived assuming various power-law slopes for the possible emission spectrum. Integral upper limits are also calculated for several power-law spectra and for different energy thresholds. The values are of the order of 10-11 ph cm-2 s-1 above 100 GeV and 10-12 ph cm-2 s-1 above 200 GeV. Segue 1 is currently considered one of the most interesting targets for indirect dark matter searches. In these terms, the upper limits have been also interpreted in the context of annihilating dark matter particles. For such purpose, we performed a grid scan over a reasonable portion of the parameter space for the minimal SuperGravity model and computed the flux upper limit for each point separately, taking fully into account the peculiar spectral features of each model. We found that in order to match the experimental upper limits with the model predictions, a minimum flux boost of 103 is required, and that the upper limits are quite dependent on the shape of the gamma-ray energy spectrum predicted by each specific model. Finally we compared the upper limits with the predictions of some dark matter models able to explain the PAMELA rise in the positron ratio, finding that Segue 1 data are in tension with the dark matter explanation of the PAMELA spectrum in the case of a dark matter candidate annihilating into τ+τ-. A complete exclusion however is not possible due to the uncertainties in the Segue 1 astrophysical factor. © 2011 IOP Publishing Ltd and SISSA. Source

Discover hidden collaborations