2 research outputs found
The unpolarized macronova associated with the gravitational wave event GW 170817
The merger of two dense stellar remnants including at least one neutron star
(NS) is predicted to produce gravitational waves (GWs) and short duration gamma
ray bursts (GRBs). In the process, neutron-rich material is ejected from the
system and heavy elements are synthesized by r-process nucleosynthesis. The
radioactive decay of these heavy elements produces additional transient
radiation termed "kilonova" or "macronova". We report the detection of linear
optical polarization P = (0.50 +/- 0.07)% at 1.46 days after detection of the
GWs from GW170817, a double neutron star merger associated with an optical
macronova counterpart and a short GRB. The optical emission from a macronova is
expected to be characterized by a blue, rapidly decaying, component and a red,
more slowly evolving, component due to material rich of heavy elements, the
lanthanides. The polarization measurement was made when the macronova was still
in its blue phase, during which there is an important contribution from a
lanthanide-free outflow. The low degree of polarization is consistent with
intrinsically unpolarized emission scattered by Galactic dust, suggesting a
symmetric geometry of the emitting region and low inclination of the merger
system. Stringent upper limits to the polarization degree from 2.45 - 9.48 days
post-burst are consistent with the lanthanides-rich macronova interpretation.Comment: 18 pages, 1 figure, 2 tables, Nature Astronomy, in pres
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PACS photometry of the herschel reference survey - far-infrared/ submillimetre colours as tracers of dust properties in nearby galaxies
We present Herschel/PACS 100 and 160 μm integrated photometry for the 323 galaxies in the Herschel Reference Survey (HRS), a K-band, volume-limited sample of galaxies in the local Universe. Once combined with the Herschel/SPIRE observations already available, these data make the HRS the largest representative sample of nearby galaxies with homogeneous coverage across the 100-500 μm wavelength range. In this paper, we take advantage of this unique data set to investigate the properties and shape of the far-infrared/submillimetre spectral energy distribution in nearby galaxies. We show that, in the stellar mass range covered by the HRS (8 ≤ log (M*/M⊙) ≤ 12), the far-infrared/submillimetre colours are inconsistent with a single modified blackbody having the same dust emissivity index β for all galaxies. In particular, either β decreases or multiple temperature components are needed, when moving from metal-rich/gas-poor to metal-poor/gas-rich galaxies. We thus investigate how the dust temperature and mass obtained from a single modified blackbody depend on the assumptions made on β. We show that, while the correlations between dust temperature, galaxy structure and star formation rate are strongly model dependent, the dust mass scaling relations are much more reliable, and variations of β only change the strength of the observed trends. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society