53 research outputs found
The metallicity dependence of envelope inflation in massive stars
Recently it has been found that models of massive stars reach the Eddington
limit in their interior, which leads to dilute extended envelopes. We perform a
comparative study of the envelope properties of massive stars at different
metallicities, with the aim to establish the impact of the stellar metallicity
on the effect of envelope inflation. We analyse published grids of
core-hydrogen burning massive star models computed with metallicities
appropriate for massive stars in the Milky Way, the LMC and the SMC, the very
metal poor dwarf galaxy I Zwicky 18, and for metal-free chemical composition.
Stellar models of all the investigated metallicities reach and exceed the
Eddington limit in their interior, aided by the opacity peaks of iron, helium
and hydrogen, and consequently develop inflated envelopes. Envelope inflation
leads to a redward bending of the zero-age main sequence and a broadening of
the main sequence band in the upper part of the Hertzsprung-Russell diagram. We
derive the limiting L/M-values as function of the stellar surface temperature
above which inflation occurs, and find them to be larger for lower metallicity.
While Galactic models show inflation above ~29 Msun, the corresponding mass
limit for Population III stars is ~150 Msun. While the masses of the inflated
envelopes are generally small, we find that they can reach 1-100 Msun in models
with effective temperatures below ~8000 K, with higher masses reached by models
of lower metallicity. Envelope inflation is expected to occur in sufficiently
massive stars at all metallicities, and is expected to lead to rapidly growing
pulsations, high macroturbulent velocities, and might well be related to the
unexplained variability observed in Luminous Blue Variables like S Doradus and
Eta Carina.Comment: 16 pages (with Appendix), accepted in A&
Dependence of the optical brightness on the gamma and X-ray properties of GRBs
The Swift satellite made a real break through with measuring simultaneously
the gamma X-ray and optical data of GRBs, effectively. Although, the satellite
measures the gamma, X-ray and optical properties almost in the same time a
significant fractions of GRBs remain undetected in the optical domain. In a
large number of cases only an upper bound is obtained. Survival analysis is a
tool for studying samples where a part of the cases has only an upper (lower)
limit. The obtained survival function may depend on some other variables. The
Cox regression is a way to study these dependencies. We studied the dependence
of the optical brightness (obtained by the UVOT) on the gamma and X-ray
properties, measured by the BAT and XRT on board of the Swift satellite. We
showed that the gamma peak flux has the greatest impact on the afterglow's
optical brightness while the gamma photon index and the X-ray flux do not. This
effect probably originates in the energetics of the jet launched from the
central engine of the GRB which triggers the afterglow.Comment: 2012 Fermi Symposium proceedings - eConf C12102
Searching for electromagnetic counterpart of LIGO gravitational waves in the Fermi GBM data with ADWO
The Fermi collaboration identified a possible electromagnetic counterpart of
the gravitational wave event of September 14, 2015. Our goal is to provide an
unsupervised data analysis algorithm to identify similar events in Fermi's
Gamma-ray Burst Monitor CTTE data stream. We are looking for signals that are
typically weak. Therefore, they can only be found by a careful analysis of
count rates of all detectors and energy channels simultaneously. Our
Automatized Detector Weight Optimization (ADWO) method consists of a search for
the signal, and a test of its significance. We developed ADWO, a virtual
detector analysis tool for multi-channel multi-detector signals, and performed
successful searches for short transients in the data-streams. We have
identified GRB150522B, as well as possible electromagnetic candidates of the
transients GW150914 and LVT151012. ADWO is an independently developed,
unsupervised data analysis tool that only relies on the raw data of the Fermi
satellite. It can therefore provide a strong, independent test to any
electromagnetic signal accompanying future gravitational wave observations.Comment: 4 pages and 4 figures, A&A Letters accepte
The clustering of gamma-ray bursts in the Hercules-Corona Borealis Great Wall: the largest structure in the Universe?
The Hercules-Corona Borealis Great Wall is a statistically significant
clustering of gamma-ray bursts around redshift 2. Motivated by recent
theoretical results indicating that a maximal Universal structure size may
indeed coincide with its estimated size (2-3Gpc), we reexamine the question of
this Great Wall's existence from both observational and theoretical
perspectives. Our statistical analyses confirm the clustering's presence in the
most reliable data set currently available, and we present a video showing what
this data set looks like in~3D. Cosmological explanations (i.e. having to do
with the distribution of gravitating matter) and astrophysical explanations
(i.e. having to do with the rate of star formation over cosmic time and space)
regarding the origin of such a structure are presented and briefly discussed
and the role of observational bias is also discussed at length. This, together
with the scientific importance of using gamma-ray bursts as unique cosmological
probes, emphasises the need for future missions such as the THESEUS satellite
which will provide us with unprecedentedly homogeneous data of gamma-ray bursts
with measured redshifts. We conclude from all this that the Hercules-Corona
Borealis Great Wall may indeed be the largest structure in the Universe - but
to be able to decide conclusively whether it actually exists, we need THESEUS.Comment: Accepted for publication in MNRAS. 12 pages, 3 figures, 3 table
Exploration of the high-redshift universe enabled by THESEUS
At peak, long-duration gamma-ray bursts are the most luminous sources of electromagnetic radiation known. Since their progenitors are massive stars, they provide a tracer of star formation and star-forming galaxies over the whole of cosmic history. Their bright power-law afterglows provide ideal backlights for absorption studies of the interstellar and intergalactic medium back to the reionization era. The proposed THESEUS mission is designed to detect large samples of GRBs at z > 6 in the 2030s, at a time when supporting observations with major next generation facilities will be possible, thus enabling a range of transformative science. THESEUS will allow us to explore the faint end of the luminosity function of galaxies and the star formation rate density to high redshifts; constrain the progress of re-ionisation beyond z greater than or similar to 6; study in detail early chemical enrichment from stellar explosions, including signatures of Population III stars; and potentially characterize the dark energy equation of state at the highest redshifts
THESEUS::A key space mission concept for Multi-Messenger Astrophysics
The recent discovery of the electromagnetic counterpart of the gravitational
wave source GW170817, has demonstrated the huge informative power of
multi-messenger observations. During the next decade the nascent field of
multi-messenger astronomy will mature significantly. Around 2030, third
generation gravitational wave detectors will be roughly ten times more
sensitive than the current ones. At the same time, neutrino detectors currently
upgrading to multi km^3 telescopes, will include a 10 km^3 facility in the
Southern hemisphere that is expected to be operational around 2030. In this
review, we describe the most promising high frequency gravitational wave and
neutrino sources that will be detected in the next two decades. In this
context, we show the important role of the Transient High Energy Sky and Early
Universe Surveyor (THESEUS), a mission concept proposed to ESA by a large
international collaboration in response to the call for the Cosmic Vision
Programme M5 missions. THESEUS aims at providing a substantial advancement in
early Universe science as well as playing a fundamental role in multi-messenger
and time-domain astrophysics, operating in strong synergy with future
gravitational wave and neutrino detectors as well as major ground- and
space-based telescopes. This review is an extension of the THESEUS white paper
(Amati et al. 2017), also in light of the discovery of GW170817/GRB170817A that
was announced on October 16th, 2017.Comment: 25 pages, 13 figures, accepted to Advances in Space Research with
minor revisions. Details on the THESEUS instrumentation, science case and
expected performances can also be found in Amati et al. 2017
(arXiv:1710.04638) and in the presentations of the THESEUS Workshop 2017
(http://www.isdc.unige.ch/theseus/workshop2017-programme.html). v2 few typos
correcte
The THESEUS space mission concept: science case, design and expected performances
THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5Âż1 arcmin localization, an energy band extending from several MeV down to 0.3ÂżkeV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) follow-up with a 0.7Âżm class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift
10, signatures of Pop III stars, sources and physics of re-ionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late ¿20s/early ¿30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA).© 2018 COSPARS.E. acknowledges the financial support from contracts ASI-INAF 1/009/10/0, NARO15 ASI-INAF 1/037/12/0 and ASI 2015-046-R.0. R.H. acknowledges GACR grant 13-33324S. S.V. research leading to these results has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 606176. D.S. was supported by the Czech grant 1601116S GA CR. Maria Giovanna Dainotti acknowledges funding from the European Union through the Marie Curie Action FP7-PEOPLE-2013-IOF, under grant agreement No. 626267 (>Cosmological Candles>)
- âŠ