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

Low-metallicity massive single stars with rotation. Evolutionary models applicable to I Zwicky 18

Massive rotating single stars with an initial metal composition appropriate for the dwarf galaxy I Zw 18 ([Fe/H]=$-$1.7) are modelled during hydrogen burning for initial masses of 9-300 M$_{\odot}$ and rotational velocities of 0-900 km s$^{-1}$. Internal mixing processes in these models were calibrated based on an observed sample of OB-type stars in the Magellanic Clouds. Even moderately fast rotators, which may be abundant at this metallicity, are found to undergo efficient mixing induced by rotation resulting in quasi chemically-homogeneous evolution. These homogeneously-evolving models reach effective temperatures of up to 90 kK during core hydrogen burning. This, together with their moderate mass-loss rates, make them Transparent Wind Ultraviolet INtense stars (TWUIN star), and their expected numbers might explain the observed HeII ionizing photon flux in I Zw 18 and other low-metallicity HeII galaxies. Our slowly rotating stars above $\sim$80 M$_{\odot}$ evolve into late B- to M-type supergiants during core hydrogen burning, with visual magnitudes up to 19$^{\mathrm{m}}$ at the distance of I Zw 18. Both types of stars, TWUIN stars and luminous late-type supergiants, are only predicted at low metallicity. Massive star evolution at low metallicity is shown to differ qualitatively from that in metal-rich environments. Our grid can be used to interpret observations of local star-forming dwarf galaxies and high-redshift galaxies, as well as the metal-poor components of our Milky Way and its globular clusters.Comment: accepted for publication in A\&

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

Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars

Context. Metal-poor massive stars are supposed to be progenitors of certain supernovae, gamma-ray bursts and compact object mergers, potentially contributing to the early epochs of the Universe with their strong ionizing radiation. However, they remain mainly theoretical as individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the SMC. Aims. This work aims at exploring what our state-of-the-art theories of stellar evolution combined with those of stellar atmospheres predict about a certain type of metal-poor (0.02 Z$_{\odot}$) hot massive stars, the chemically homogeneously evolving ones, called TWUIN stars. Methods. Synthetic spectra corresponding to a broad range in masses (20-130 M$_{\odot}$) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage were computed. Results. We find that TWUIN stars show almost no emission lines during most of their {core hydrogen-burning} lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission and even some metal lines (oxygen and carbon, but still almost no nitrogen) show up. Mass loss and clumping play a significant role in line-formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early O type giant or supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early O type star observed in a low-metallicity galaxy could be the outcome of chemically homogeneous evolution $-$ and therefore the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe due to their being hot without showing prominent emission lines during the majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres

The role of stellar expansion on the formation of gravitational wave sources

Massive stars are the progenitors of black holes and neutron stars, the mergers of which can be detected with gravitational waves (GW). The expansion of massive stars is one of the key factors affecting their evolution in close binary systems, but it remains subject to large uncertainties in stellar astrophysics. For population studies and predictions of GW sources, the stellar expansion is often simulated with the analytic formulae from Hurley et al. (2000). These formulae need to be extrapolated for stars beyond 50 solar masses and are often considered outdated. In this work we present five different prescriptions developed from 1D stellar models to constrain the maximum expansion of massive stars. We adopt these prescriptions to investigate how stellar expansion affects mass transfer interactions and in turn the formation of GW sources. We show that limiting radial expansion with updated 1D stellar models, when compared to the use of Hurley et al. (2000) radial expansion formulae, does not significantly affect GW source properties (rates and masses). This is because most mass transfer events leading to GW sources are initialised before the donor star reaches its maximum expansion. The only significant difference was found for the mass distribution of massive binary black hole mergers (total mass > 50 solar masses) formed from stars that may evolve beyond the Humphreys-Davidson limit, whose radial expansion is the most uncertain. We conclude that understanding the expansion of massive stars and the origin of the Humphrey-Davidson limit is a key factor for the study of GW sources.Comment: Accepted for publication in MNRA

The slow component of the delayed rectifier potassium current in undiseased human ventricular myocytes

Objective: The purpose of this study was to investigate the properties of the slow component of the delayed rectifier potassium current (I-Ks) in myocytes isolated from undiseased human left ventricles. Methods: The whole-cell configuration of the patch-clamp technique was applied in 58 left ventricular myocytes from 15 hearts at 37 degreesC. Nisoldipine (1 muM) was used to block inward calcium current (I-Ca) and E-4031 (1-5 muM) was applied to inhibit the rapid component of the delayed rectifier potassium current (I-Ks). Results: In 31 myocytes an E-4031 insensitive, but L-735,821 and chromanol 293B sensitive, tail current was identified which was attributed to the slow component of I-K (I-Ks). Activation of I-Ks was slow (tau = 903 +/- 101 ms at 50 mV, n = 24), but deactivation of the current was relatively rapid ( tau =122.4 +/- 11.7 ms at -40 mV, n = 19). The activation of I-Ks was voltage independent but its deactivation showed clear voltage dependence. The deactivation was faster at negative voltages (about 100 ms at -50 mV) and slower at depolarized potentials (about 300 ms at 0 mV). In six cells, the reversal potential was -81.6 +/- 2.8 mV on an average which is close to the K+ equilibrium potential suggesting K+ as the main charge carrier. Conclusion: In undiseased human ventricular myocytes, I-Ks exhibits slow activation and fast deactivation kinetics. Therefore, in humans I-Ks differs from that reported in guinea pig, and it best resembles I-Ks described in dog and rabbit ventricular myocytes

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