Jordi Isern

Gamma-ray bursts as collimated jets from neutron star/black hole mergers

THE distribution of more than 150 γ-ray bursts detected by the BATSE experiment is isotropic on the sky but radially non-uniform1,2. This raises the possibility that bursts are cosmological (at z≲l) and therefore very energetic events, releasing ∼1050 erg sr−1 on a timescale of seconds. The coalescence of two neutron stars3–7 or the accretion-induced collapse of a white dwarf8 can release up to 1053 erg in the form of neutrino-antineutrino pairs, so that the conversion of <1% into γ-rays by annihilation9 could generate γ-ray bursts, but in all such models an optically thick wind tends to form10,11, preventing the γ-rays from escaping and converting their energy into kinetic energy of the ejected material. We present here a possible solution to this difficulty. When a stellar-mass neutron star is disrupted by a black hole, it forms a thick disk which emits νν̄ pairs. These neutrinos expel a wind from the disk, but angular momentum conservation means that a clear funnel forms along the rotation axis. Neutrino annihilation within the matter-free funnel can then create γ-rays which escape to the distant observer.

A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

NGC 6791 is a well studied open cluster that it is so close to us that can be imaged down to very faint luminosities. The main-sequence turn-off age (∼8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (∼6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf (12C and 16O) is expected to occur. This sequence of events is expected to introduce long delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791.

MAX: a gamma-ray lens for nuclear astrophysics

The mission concept MAX is a space borne crystal diffraction telescope, featuring a broad-band Laue lens optimized for the observation of compact sources in two wide energy bands of high astrophysical relevance. For the first time in this domain, gamma-rays will be focused from the large collecting area of a crystal diffraction lens onto a very small detector volume. As a consequence, the background noise is extremely low, making possible unprecedented sensitivities. The primary scientific objective of MAX is the study of type Ia supernovae by measuring intensities, shifts and shapes of their nuclear gamma-ray lines. When finally understood and calibrated, these profoundly radioactive events will be crucial in measuring the size, shape, and age of the Universe. Observing the radioactivities from a substantial sample of supernovae and novae will significantly improve our understanding of explosive nucleosynthesis. Moreover, the sensitive gamma-ray line spectroscopy performed with MAX is expected to clarify the nature of galactic microquasars (e+e- annihilation radiation from the jets), neutrons stars and pulsars, X-ray Binaries, AGN, solar flares and, last but not least, gamma-ray afterglow from gamma-burst counterparts.

Pulsations of massive ZZ Ceti stars with carbon/oxygen and oxygen/neon cores

We explore the adiabatic pulsational properties of massive white dwarf stars with hydrogen-rich envelopes and oxygen/neon and carbon/oxygen cores. To this end, we compute the cooling of massive white dwarf models for both core compositions taking into account the evolutionary history of the progenitor stars and the chemical evolution caused by time-dependent element diffusion. In particular, for the oxygen/neon models we adopt the chemical profile resulting from repeated carbon-burning shell flashes expected in very massive white dwarf progenitors. For carbon/oxygen white dwarfs we consider the chemical profiles resulting from phase separation upon crystallization. For both compositions we also take into account the effects of crystallization on the oscillation eigenmodes. We find that the pulsational properties of oxygen/neon white dwarfs are noticeably different from those made of carbon/oxygen, thus making asteroseismological techniques a promising way to distinguish between the two types of stars and, hence, to obtain valuable information about their progenitors.

New evolutionary models for massive ZZ Ceti stars II. the effects of crystallization on their pulsational properties

In view of recent claims that asteroseismology could supply invaluable insight into the crystallization process oc- curring in the interiors of massive white dwarf stars, we present in this work new pulsational calculations for improved carbon- oxygen DA white dwarf models suitable for the study of massive ZZ Ceti stars. The background models employed in this study, presented in detail in a recent paper by Althaus et al. (2003, A&A, 404, 593), are the result of the complete evolution of massive white dwarf progenitors from the zero-age main sequence through the Asymptotic Giant Branch (AGB) and mass loss phases to the white dwarf regime. Abundance changes are accounted for by means of a full coupling between nuclear evolution and time- dependent mixing due to convection, salt fingers, and diffusive overshoot. In addition, time-dependent element diffusion for multicomponent gases has been considered during the white dwarf evolution. Crystallization and chemical rehomogenization due to phase separation upon crystallization in the core of our models have been fully considered. The effects of crystallization on the period spectrum of these massive white dwarf models are assessed by means of a detailed pulsational analysis of linear, nonradial, adiabatic gravity modes. To properly account for the effects of the presence of a solid phase in the models we impose special conditions on the oscillation eigenfunctions at the solid-liquid interface. We find that the theoretical pulsation spectrum is strongly modified when crystallization is considered, in particular concerning the mode trapping properties of the equilibrium models. We show that the strong mode trapping seen in the models with overshooting can be reproduced by means of a simple analytical model. We also discuss at some length the implications of our study for BPM 37093, the most massive ZZ Ceti star presently known. In particular, we attempt to place constraints on the physical processes occurring prior to the formation of this white dwarf. We find that if BPM 37093 has a stellar mass of ≈1.00 Mits observed spectrum could bear the signature of overshoot episodes during the helium core burning.

The Luminosity Function of Massive White Dwarfs

A method to extract information about the star formation rate history of our galaxy is presented. It is based on the fact that massive stars spend a very short time on the main sequence. Thus, their remnants are white dwarfs that are born almost at the same time that their progenitors. This allows to traceback the star formation rate from the white dwarf luminosity function of massive white dwarfs.

Gamma-rays from SNIa

Type Ia supernovae are thought to be the outcome of the thermonuclear explosion of a carbon/oxygen white dwarf in a close binary system. Their optical light curve is powered by thermalized gamma-rays produced by the radioactive decay of 56Ni, the most abundant isotope present in the debris. The maximum and the shape of the light curve strongly depends on the total amount and distribution of this freshly synthesized isotope, as well as on the velocity and density distribution of the ejecta. Gamma-rays escaping the ejecta have the advantage of their lower interaction with the ejecta, the possibility to distinguish among isotopes and the relative simplicity of their transport modelling, and can be used as a diagnostic tool for studying the structure of the exploding star and the characteristics of the explosion, as it has been proved in the case of SN2014J.

Observations of SN2011fe with INTEGRAL

Context. SN2011fe was detected by the Palomar Transient Factory in M101 on August 24, 2011, a few hours after the explosion. From the early optical spectra it was immediately realized that it was a Type Ia supernova, thus making this event the brightest one discovered in the past twenty years. Aims. The distance of the event offered the rare opportunity of performing a detailed observation with the instruments onboard INTEGRAL to detect the γ-ray emission expected from the decay chains of 56Ni. The observations were performed in two runs, one before and around the optical maximum, aimed to detect the early emission from the decay of 56Ni, and another after this maximum aimed to detect the emission of 56Co. Methods. The observations performed with the instruments onboard INTEGRAL (SPI, IBIS/ISGRI, JEMX, and OMC) were analyzed and compared with the existing models of γ-ray emission from this kind of supernova. In this paper, the analysis of the γ-ray emission has been restricted to the first epoch. Results. SPI and IBIS/ISGRI only provide upper limits to the expected emission due to the decay of 56Ni. These upper limits on the gamma-ray flux are 7.1 × 10−5 ph/s/cm2 for the 158 keV line and 2.3 × 10−4 ph/s/cm2 for the 812 keV line. These bounds allow rejecting at the 2σ level explosions involving a massive white dwarf, ∼1 M in the sub-Chandrasekhar scenario and specifically all models that would have substantial amounts of radioactive 56Ni in the outer layers of the exploding star responsible for the SN2011fe event. The optical light curve obtained with the OMC camera also suggests that SN2011fe was the outcome of the explosion of a CO white dwarf, possibly through the delayed detonation mode, although other ones are possible, of a CO that synthesized ∼0.55 M of 56Ni. For this specific model, INTEGRAL would have only been able to detect this early γ-ray emission if the supernova had occurred at a distance <∼2 Mpc. Conclusions. The detection of the early γ-ray emission of 56Ni is difficult, and it can only be achieved with INTEGRAL if the distance of the event is close enough. The exact distance depends on the specific SNIa subtype. The broadness and rapid rise of the lines are probably at the origin of this difficulty.

Gamma‐ray bursts from relativistic beams in neutron star mergers

The coalescence of two neutron stars produces a merged configuration consisting of a dense core surrounded by a thick rotating disk. Two opposite relativistic beams can form along the system axis, feeded in energy by the annihilation of neutrinos and antineutrinos emitted from the hot disk. We show that the relativistic factor of the beams can reach 102–103 because the core polar cap remains cooler than the disk and a large fraction of the energy is released beyond the sonic point. The interaction of the ejected shell with the interstellar medium may eventually lead to a gamma‐ray burst if the kinetic energy of the shell can be efficiently radiated through nonthermal processes in shocks.

NUMBER COUNTS OF WHITE DWARFS: THE IMPACT OF GAIA

In the next years, several space missions will be devoted to measure with very high accuracy the motions of a sizeable fraction of the stars of our Galaxy. The most promising one is the ESA astrometric satellite GAIA, which will provide very precise astrometry (< 10 μas in parallax and < 10 μas yr−1 in proper motion at V ∼ 15, increasing to 0.2 mas yr−1 at V ∼ 20) and multicolor photometry, for all 1.3 billion objects to V ∼ 20, and radial velocities with accuracies of a few km s−1 for most stars brighter than V ∼ 17. Consequently, full homogeneous six-dimensional phase-space information for a huge number of white dwarfs will become available. Our Monte Carlo simulator has been used to estimate the number of white dwarfs potentially observable by GAIA. Our results show which could be the impact of a mission like GAIA in the current understanding of our Galaxy. Scientific attainable goals include, among others, a reliable determination of the age of our galactic disk, a better knowledge of the structure of the halo of the Milky Way or the reconstruction of the past history of the Star Formation Rate of the galactic disk.