Rosalba Perna

Neutrino trapping and accretion models for gamma-ray bursts

Many models of gamma-ray bursts (GRBs) invoke a central engine consisting of a black hole of a few solar masses accreting matter from a disk at a rate of a fraction to a few solar masses per second. Popham et al. and Narayan et al. have shown that, for 0.1 M☉ s-1, accretion proceeds via neutrino cooling and neutrinos can carry away a significant amount of energy from the inner regions of the disks. We improve on these calculations by including a simple prescription for neutrino transfer and neutrino opacities in such regions. We find that the flows become optically thick to neutrinos inside a radius R ~ 6RS-40RS for in the range of 0.1-10 M☉ s-1, where RS is the black hole Schwarzchild radius. Most of the neutrino emission comes from outside this region, and the neutrino luminosity stays roughly constant at a value Lν ~ 1053 ergs s-1. We show that, for 1 M☉ s-1, neutrinos are sufficiently trapped that energy advection becomes the dominant cooling mechanism in the flow. These results imply that ν annihilation in hyperaccreting black holes is an inefficient mechanism for liberating large amounts of energy. Extraction of rotational energy by magnetic processes remains the most viable mechanism.

A Study of Compact Object Mergers as Short Gamma-Ray Burst Progenitors

We present a theoretical study of double compact objects as potential short/hard gamma-ray burst (GRB) progenitors. An updated population synthesis code, StarTrack, is used to calculate properties of double neutron stars and black hole-neutron star binaries. We obtain their formation rates, estimate merger times, and finally predict their most likely merger locations and afterglow properties for different types of host galaxies. Our results serve for a direct comparison with the recent HETE-2 and Swift observations of several short bursts, for which afterglows and host galaxies were detected. We also discuss the possible constraints these observations put on the evolutionary models of double compact object formation. We emphasize that our double compact object models can successfully reproduce at the same time short GRBs within both young, star-forming galaxies (e.g., GRB 050709 and GRB 051221A), as well as within old, elliptical hosts (e.g., GRB 050724 and probably GRB 050509B).

Unifying the observational diversity of isolated neutron stars via magneto-thermal evolution models

Observations of magnetars and some of the high magnetic field pulsars have shown that their thermal luminosity is systematically higher than that of classical radiopulsars, thus confirming the idea that magnetic fields are involved in their X-ray emission. Here we present the results of 2D simulations of the fully-coupled evolution of temperature and magnetic field in neutron stars, including the state-of-the-art kinetic coefficients and, for the first time, the important effect of the Hall term. After gathering and thoroughly re-analysing in a consistent way all the best available data on isolated, thermally emitting neutron stars, we compare our theoretical models to a data sample of 40 sources. We find that our evolutionary models can explain the phenomenological diversity of magnetars, high-B radio-pulsars, and isolated nearby neutron stars by only varying their initial magnetic field, mass and envelope composition. Nearly all sources appear to follow the expectations of the standard theoretical models. Finally, we discuss the expected outburst rates and the evolutionary links between different classes. Our results constitute a major step towards the grand unification of the isolated neutron star zoo.

Radio Foregrounds for the 21 Centimeter Tomography of the Neutral Intergalactic Medium at High Redshifts

Absorption or emission against the cosmic microwave background (CMB) radiation may be observed in the redshifted 21 cm line if the spin temperature of the neutral intergalactic medium (IGM) prior to reionization differs from the CMB temperature. This so-called 21 cm tomography should reveal important information on the physical state of the IGM at high redshifts. The fluctuations in the redshifted 21 cm line, due to gas density inhomogeneities at early times, should be observed at meter wavelengths by the next-generation radio telescopes such as the proposed Square Kilometer Array (SKA). Here we show that the extragalactic radio sources provide a serious contamination to the brightness temperature fluctuations expected in the redshifted 21 cm emission from the IGM at high redshifts. Unless the radio source population cuts off at flux levels above the planned sensitivity of SKA, its clustering noise component will dominate the angular fluctuations in the 21 cm signal. The integrated foreground signal is smooth in frequency space, and it should nonetheless be possible to identify the sharp spectral feature arising from the nonuniformities in the neutral hydrogen density during the epoch when the first UV sources reionize the IGM.

Flares in Long and Short Gamma-Ray Bursts: A Common Origin in a Hyperaccreting Accretion Disk

Early-time X-ray observations of gamma-ray bursts (GRBs) with the Swift satellite have revealed a more complicated phenomenology than was known before. In particular, the presence of flaring activity on a wide range of timescales probably requires late-time energy production within the GRB engine. Since the flaring activity is observed in both long and short GRBs, its origin must be within what is in common for the two likely progenitors of the two classes of bursts: a hyperaccreting accretion disk around a black hole of a few solar masses. Here we show that some of the observational properties of the flares, such as the duration-timescale correlation, and the duration-peak luminosity anticorrelation displayed by most flares within a given burst, are qualitatively consistent with viscous disk evolution, provided that the disk at large radii either fragments or otherwise suffers large-amplitude variability. We discuss the physical conditions in the outer parts of the disk and conclude that gravitational instability, possibly followed by fragmentation, is the most likely candidate for this variability.

Emission Spectra of Fallback Disks around Young Neutron Stars

The nature of the energy source powering anomalous X-ray pulsars (AXPs) is uncertain. Proposed scenarios involve either an ultramagnetized neutron star or accretion onto a neutron star. We consider the accretion model proposed recently by Chatterjee, Hernquist, & Narayan, in which a disk is fed by fallback material following a supernova. We compute the optical, infrared, and submillimeter emission expected from such a disk, including both viscous dissipation and reradiation of X-ray flux impinging on the disk from the pulsar. We find that it is possible with current instruments to put serious constraints on this and on other accretion models of AXPs. Fallback disks could also be found around isolated radio pulsars, and we compute the corresponding spectra. We show that the excess emission in the R and I bands observed for the pulsar PSR 0656+14 is broadly consistent with emission from a disk.

A New Low Magnetic Field Magnetar: The 2011 Outburst of Swift J1822.3-1606

We report on the long-term X-ray monitoring withSwift,RXTE,Suzaku,Chandra, andXMM-Newton of the outburst of the newly discovered magnetar Swift J1822.3−1606 (SGR 1822−1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time span from 2011 July until the end of 2012 April). We also report on archival ROSAT observations which detected the source during its likely quiescent state, and on upper limits on Swift J1822.3−1606’s radiopulsed and optical emission during outburst, with the Green Bank Telescope and the Gran Telescopio Canarias, respectively. Our X-ray timing analysis finds the source rotating with a period of P = 8.43772016(2) s and a period derivative ˙ P = 8.3(2) × 10 −14 ss −1 , which implies an inferred dipolar surface magnetic field of B � 2.7 × 10 13 G at the equator. This measurement makes Swift J1822.3−1606 the second lowest magnetic field magnetar (after SGR 0418+5729). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of Swift J1822.3−1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of Bp ∼ 1.5 × 10 14 G and Btor ∼ 7 × 10 14 G, respectively, and if its current age

Magnetic Drag on Hot Jupiter Atmospheric Winds

Hot Jupiters, with atmospheric temperatures T 1000 K, have residual thermal ionization levels sufficient for the interaction of ions with the planetary magnetic field to result in a sizable magnetic drag on the (neutral) atmospheric winds. We evaluate the magnitude of magnetic drag in a representative three-dimensional atmospheric model of the hot Jupiter HD 209458b and find that it is a plausible mechanism to limit wind speeds in this class of atmospheres. Magnetic drag has a strong geometrical dependence, both meridionally and from the dayside to the nightside (in the upper atmosphere), which could have interesting consequences for the atmospheric flow pattern. By extension, close-in eccentric planets with transiently heated atmospheres will experience time-variable levels of magnetic drag. A robust treatment of magnetic drag in circulation models for hot atmospheres may require iterated solutions to the magnetic induction and Saha equations as the hydrodynamic flow is evolved.

GRB 050904 at redshift 6.3: observations of the oldest cosmic explosion after the Big Bang ⋆

We present optical and near-infrared observations of the afterglow of the gamma-ray burst GRB 050904. We derive a photometric redshift z = 6.3, estimated from the presence of the Lyman break falling between the I and J filters. This is by far the most distant GRB known to date. Its isotropic-equivalent energy is 3.4 × 10 53 erg in the rest-frame 110−1100 keV energy band. Despite the high redshift, both the prompt and the afterglow emission are not peculiar with respect to other GRBs. We find a break in the J-band light curve at tb = 2.6 ± 1.0 d (observer frame). If we assume this is the jet break, we derive a beaming-corrected energy Eγ ∼ (4 ÷ 12) × 10 51 erg. This limit shows that GRB 050904 is consistent with the Amati and Ghirlanda relations. This detection is consistent with the expected number of GRBs at z > 6 and shows that GRBs are a powerful tool to study the star formation history up to very high redshift.

Time-dependent Photoionization in a Dusty Medium. II. Evolution of Dust Distributions and Optical Opacities

The interaction of a radiation field with a dusty medium is a relevant issue in several astrophysical contexts. We use the time-dependent photoionization code in a dusty medium developed by Perna & Lazzati to study the modifications in the dust distribution and the relative optical opacities when a strong X-ray/UV radiation flux propagates into a medium. We find that silicates are preferentially destroyed with respect to graphite, and the extinction curve becomes significantly flatter (hence implying less reddening), with the characteristic bump at 2175 A being highly suppressed because of the destruction of the small graphite grains. This could explain the observational lack of such a feature in gamma-ray-burst afterglow and active galactic nuclei spectra. For a very intense and highly variable source irradiating a compact and dense region, time variability in the optical opacity resulting from dust destruction can be observed on a relatively short timescale. We show that, under these circumstances, monitoring the time variability of the opacity can yield powerful clues to the properties of dust in the environment of the source. In particular, it allows one to break the observational degeneracy as to whether a gray extinction is the result of a low dust-to-gas ratio or of a dust grain distribution that is skewed toward large grains.