arXiv subjects

Rosalba Perna

Publications and source records attributed to Rosalba Perna.

At least 163 records · Page 9Linked to original sources

arXiv2007

How rapidly do neutron stars spin at birth? Constraints from archival X-ray observations of extragalactic supernovae

Traditionally, studies aimed at inferring the distribution of birth periods of neutron stars are based on radio surveys. Here we propose an independent method to constrain the pulsar spin periods at birth based on their X-ray luminosities. In particular, the observed luminosity distribution of supernovae poses a constraint on the initial rotational energy of the embedded pulsars, via the L_X-dot{E}_{rot} correlation found for radio pulsars, and under the assumption that this relation continues to hold beyond the observed range. We have extracted X-ray luminosities (or limits) for a large sample of historical SNe observed with Chandra, XMM and Swift, that have been firmly classified as core-collapse supernovae. We have then compared these observational limits with the results of Monte Carlo simulations of the pulsar X-ray luminosity distribution, for a range of values of the birth parameters. We find that a pulsar population dominated by millisecond periods at birth is ruled out by the data.

astro-ph↗

arXiv2007

Instabilities in the time-dependent neutrino disc in Gamma-Ray Bursts

We investigate the properties and evolution of accretion tori formed after the coalescence of two compact objects. At these extreme densities and temperatures, the accreting torus is cooled mainly by neutrino emission produced primarily by electron and positron capture on nucleons (beta reactions). We solve for the disc structure and its time evolution by introducing a detailed treatment of the equation of state which includes photodisintegration of helium, the condition of beta-equilibrium, and neutrino opacities. We self-consistently calculate the chemical equilibrium in the gas consisting of helium, free protons, neutrons and electron-positron pairs and compute the chemical potentials of the species, as well as the electron fraction throughout the disc. We find that, for sufficiently large accretion rates (> 10 solar masses per second), the inner regions of the disk become opaque and develop a viscous and thermal instability. The identification of this instability might be relevant for GRB observations.

astro-ph↗

arXiv2007

Dust Echoes from the Ambient Medium of Gamma-Ray Bursts

Long gamma-ray bursts (GRBs) are likely associated with the collapse of massive stars, which produce dust and are born in dusty environments. Absorption and scattering of ultraviolet/X-ray photons from the prompt, optical flash and afterglow emission of the GRB produce dust echoes. We perform time-dependent calculations of these echoes, accounting for the evolution of the dust grain distribution due to selective grain destruction by the GRB radiation, and for off-axis beaming. We explore cloud configurations of differing density and size -- the echo light curve and spectrum depend on the cloud radius, with larger clouds peaking at longer wavelengths. For a region ~ 3 pc in size with hydrogen density ~ 1000 per cubic centimeter, the echo spectrum peaks at ~ 3.6 microns and ~ 8.8 eV for thermal and scattered components, respectively. Dust echoes should be detectable with the Very Large Telescope up to z ~ 0.1, IRAC onboard the Spitzer Space Telescope up to z ~ 0.2, and NICMOS onboard the Hubble Space Telescope up to z ~ 0.3. Furthermore, the shape of the echo light curve allows one to infer: the jet opening angle; the inclination of the jet axis with respect to the line of sight; the size of the dust-emitting region. For sources with symmetric, bipolar jets, dust echoes exhibit two bumps in the light curve, making them easily distinguishable from the rebrightening due to an underlying supernova.

astro-ph↗

arXiv2006

X-ray flares and the duration of engine activity in gamma-ray bursts

The detection of bright X-ray flares superimposed on the regular afterglow decay in Swift gamma-ray bursts has triggered theoretical speculations on their origin. We study the temporal properties of flares due to internal dissipation and external shock mechanisms. We first show that at least a sizable fraction of the flares cannot be related to external shock mechanisms, since external shock flares evolve on much longer time scales than observed. We then study flares from internal dissipation, showing that the temporal properties allow us to distinguish the emission of slow early shells from that of late faster shells. We show that, due to the rapid evolution of the detected flares, it is most likely that the flares are produced by relatively fast shells ejected by the central engine shortly before they are observed. This implies that the central engine must be active for, in some cases, as long as one day. We finally discuss the constraints and implications that this observation has on the properties and physics of the inner engine, and we elaborate on possible future observational tests on the flare sample to further understand their origin and physics.

astro-ph↗

arXiv2006

Constraining the environment of GRB 990712 through emission line fluxes

The energy output in the gamma-ray burst (GRB) prompt emission and afterglow phase is expected to photoionize the surrounding medium out to large radii. Cooling of this gas produces line emission, particularly strong in the optical, whose variability is a strong diagnostics of the gas density and geometry in the close environment of the burst. We present the results of a spectral time series analysis of the host galaxy of GRB 990712 observed up to ~6 years after the burst. We analyze the emission line fluxes together with those of the previous observations of the same GRB, in search for photoionization signatures. We find that the emission line fluxes show no variation within the uncertainities up to 6 years after the burst, and we use the measured line intensities to set a limit on the density of the gas within a few parsecs of the burst location. This is the first time that emission from cooling GRB remnants is probed on years time scales.

astro-ph↗

arXiv2006

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-II 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)

astro-ph↗

arXiv2005

Flares in long and short Gamma-Ray Bursts: a common origin in a hyperaccreting accretion disk

Early-time X-ray observations of 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 time scales 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-time scale 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.

astro-ph↗

arXiv2005

On the spin-up/spin-down transitions in accreting X-ray binaries

Accreting X-Ray Binaries display a wide range of behaviours. Some of them are observed to spin up steadily, others to alternate between spin-up and spin-down states, sometimes superimposed on a longer trend of either spin up or spin down. Here we interpret this rich phenomenology within a new model of the disk-magnetosphere interaction. Our model, based on the simplest version of a purely material torque, accounts for the fact that, when a neutron star is in the propeller regime, a fraction of the ejected material does not receive enough energy to completely unbind, and hence falls back into the disk. We show that the presence of this feedback mass component causes the occurrence of multiple states available to the system, for a given, constant value of the mass accretion rate dot{M}_* from the companion star. If the angle chi of the magnetic dipole axis with respect to the perpendicular to the disk is larger than a critical value chi_crit, the system eventually settles in a cycle of spin-up/spin-down transitions for a constant value of dot{M}_* and independent of the initial conditions. No external perturbations are required to induce the torque reversals. The transition from spin up to spin down is often accompanied by a large drop in luminosity. The frequency range spanned in each cycle and the timescale for torque reversals depend on dot{M}_*, the magnetic field of the star, the magnetic colatitude chi, and the degree of elasticity regulating the magnetosphere-disk interaction. The critical angle chi_crit ranges from \~25-30 deg for a completely elastic interaction to ~40-45 deg for a totally anelastic one. For chi ~< chi_crit, cycles are no longer possible and the long-term evolution of the system is a pure spin up. We specifically illustrate our model in the cases of the X-ray binaries GX 1+4 and 4U 1626-67.

astro-ph↗

arXiv2005

Discovery and Monitoring of the likely IR Counterpart of SGR 1806-20 during the 2004 gamma-ray burst-active state

The sky region including the Chandra position of SGR 1806-20 was monitored in the IR band during 2004, following its increased high energy bursting activity. Observations were performed using NAOS-CONICA, the adaptive optics IR camera mounted on Yepun VLT, which provided images of unprecedented quality (FWHM better than 0.1"). After the 2004 December 27th giant flare, the source position has been nailed by VLA observations of its radio counterpart, reducing the positional uncertainty to 0.04". Using IR data from our monitoring campaign, we discovered the likely IR counterpart to SGR 1806-20 based on positional coincidence with the Chandra and VLA uncertainty regions and flux variability of a factor of about 2 correlated with that at higher energies. We compare our findings with other isolated neutron star classes thought to be related, at some level, with SGRs.

astro-ph↗

arXiv2005

Constraints on the small-scale power spectrum of density fluctuations from high-redshift gamma-ray bursts

Cosmological models that include suppression of the power spectrum of density fluctuations on small scales exhibit an exponential reduction of high-redshift, non-linear structures, including a reduction in the rate of gamma ray bursts (GRBs). Here we quantify the constraints that the detection of distant GRBs would place on structure formation models with reduced small-scale power. We compute the number of GRBs that could be detectable by the Swift satellite at high redshifts (z > 6), assuming that the GRBs trace the cosmic star formation history, which itself traces the formation of non-linear structures. We calibrate simple models of the intrinsic luminosity function of the bursts to the number and flux distribution of GRBs observed by the Burst And Transient Source Experiment (BATSE). We find that a discovery of high-z GRBs would imply strong constraints on models with reduced small-scale power. For example, a single GRB at z > 10, or 10 GRBs at z > 5, discovered by Swift during its scheduled two-year mission, would rule out an exponential suppression of the power spectrum on scales below R_c=0.09 Mpc (exemplified by warm dark matter models with a particle mass of m_x=2 keV). Models with a less sharp suppression of small-scale power, such as those with a red tilt or a running scalar index, n_s, are more difficult to constrain, because they are more degenerate with an increase in the power spectrum normalization, sigma_8, and with models in which star-formation is allowed in low-mass minihalos. We find that a tilt of δn_s ~ 0.1 is difficult to detect; however, an observed rate of 1 GRB/yr at z > 12 would yield an upper limit on the running of the spectral index, alpha = d(n_s)/d(ln k) > -0.05.

astro-ph↗

arXiv2005

Afterglow Observations Shed New Light on the Nature of X-ray Flashes

X-ray flashes (XRFs) and X-ray rich gamma-ray bursts (XRGRBs) share many observational characteristics with long duration GRBs, but the reason for which their prompt emission peaks at lower photon energies, $E_p$, is still under debate. Although many different models have been invoked in order to explain the lower $E_p$ values, their implications for the afterglow emission were not considered in most cases, mainly because observations of XRF afterglows have become available only recently. Here we examine the predictions of the various XRF models for the afterglow emission, and test them against the observations of XRF 030723 and XRGRB 041006, the events with the best monitored afterglow light curves in their respective class. We show that most existing XRF models are hard to reconcile with the observed afterglow light curves, which are very flat at early times. Such light curves are, however, naturally produced by a roughly uniform jet with relatively sharp edges that is viewed off-axis (i.e. from outside of the jet aperture). This type of model self consistently accommodates both the observed prompt emission and the afterglow light curves of XRGRB 041006 and XRF 030723, implying viewing angles $θ_{obs}$ from the jet axis of $(θ_{obs}-θ_0)\sim 0.15θ_0$ and $\sim θ_0$, respectively, where $θ_0\sim 3$ deg is the jet half-opening angle. This suggests that GRBs, XRGRBs and XRFs are intrinsically similar relativistic jets viewed from different angles, corresponding to $γ(θ_{obs}-θ_0)$ of less than 1, between 1 and a few, and more than a few, respectively, where $γ$ is the Lorentz factor. Future observations with Swift could help test this unification scheme in which GRBs, XRGRBs and XRFs share the same basic physics and differ only by their orientation relative to our line of sight.

astro-ph↗

arXiv2005

Dynamical evolution of intermediate mass black holes and their observable signatures in the nearby Universe

We investigate the consequences of a model of the assembly and growth of massive black holes from primordial seeds, remnants of the first generation of stars in a hierarchical structure formation scenario. Our model traces the build-up of MBHs from an early epoch, and follows the merger history of dark matter halos and their associated holes via Monte Carlo realizations of the merger hierarchy from early times to the present time. The sequence of minor and major mergers experienced by galactic halos in their hierarchical growth affects the merger history of MBHs embedded in their nuclei. So, if the formation route for the assembly of SMBHs dates back to the early universe, a large number of BH interactions is inevitable. Binary black holes coalescence timescales can be long enough for a third BH to fall in and interact with the central binary. These BH triple interactions lead typically to the final expulsion of one of the three bodies and to the recoil of the binary. Also, asymmetric emission of gravitational waves in the last stages of the black hole merging can give a recoil velocity to the centre of mass of the coalescing binary. This scenario leads to the prediction of a population of intermediate mass BHs (IMBHs) wandering in galaxy halos at the present epoch. We compute the luminosity distribution produced by these IMBHs accreting from their circumstellar medium. We find that in a Milky Way-sized galaxy they are unable to account for sources with luminosities > 1e39 erg/s unless they carry a baryonic remnant from which they are able to accrete for a long time. We also find that, for typical spiral galaxies, the bright end of the point source distribution correlates with the mass of the galaxy, and the most luminous sources are expected to be found in the disk.

astro-ph↗

arXiv2004

Are all Gamma Ray Bursts like GRB 980425, GRB 030329 and GRB 031203?

We study the probability that three GRBs (980425, 030329, 031203) are found within z=0.17, given the luminosity functions consistent with the log N-log S relationship for classical cosmological bursts (i.e., those observed by BATSE). We show that, in order for the probability of these three low-z events to be non-negligible (thus making it more likely that they belong to the same class of the classical cosmological bursts), the bursts' luminosity function must be a broken powerlaw. By reasoning in analogy with beamed AGNs, we show that observations are consistent with the expectations if GRB 980425 and GRB 031203 are indeed normal bursts seen sideways. Within this model, no bright burst within z=0.17 should be observed by a HETE--like instrument within the next \sim 20 yr.

astro-ph↗

arXiv2004

Evolution of a neutrino-cooled disc in Gamma-Ray Bursts

Rapid, hyper-Eddington accretion is likely to power the central engines of gamma-ray bursts (GRBs). In the extreme conditions of densities and temperatures the accreting torus is cooled by neutrino emission rather than by radiation. Another important cooling mechanism is the advection of energy into the central black hole. We compute the time evolution of a neutrino-dominated disc that proceeds during the burst and investigate the changes in its density and temperature. The discrimination between short and long bursts is made on the basis of the different rates of material inflow to the outer parts of the disc, thus favoring the binary merger scenario for the short GRBs and the collapsar scenario for the long ones. Within the context of the collapsar model, we also study the evolution of the photon luminosity of the remnant disc up to times of the order of 1 day, and discuss its implications for the production of emission lines in GRB spectra.

astro-ph↗

arXiv2004

Accretion flows in early-type galaxies and CMB experiments

We investigate the possible contribution from the emission of accretion flows around supermassive black holes in early type galaxies to current measurements of the Cosmic Microwave Background (CMB) at radio frequencies. We consider a range of luminosities suggested by targeted radio observations and accretion models and compute the residual contribution of these sources to the spectrum and bispectrum of the observed CMB maps. As for high-resolution CMB experiments, we find that the unresolved component of these sources could make up to ~40-50% of the observed CBI and BIMA power spectrum at l > 2000. As a consequence, the inferred sigma_8^{SZ} value could be biased high by up to 6-7%. As for all sky experiments, we find that the contribution of accretion-flow sources to the WMAP bispectrum is at the 2-3 per cent level at most. At the flux limit that Planck will achieve, however, these sources may contribute up to 15 per cent of the bispectrum in the 60-100 GHz frequency range. Moreover, Planck should detect hundreds of these sources in the 30-300 GHz frequency window. These detections, possibly coupled with galaxy type confirmation from optical surveys, will allow number counts to put tighter constraints on early-type galaxies radio luminosity and accretion flows properties. These sources may also contribute up to the 30 per cent level to the residual radio sources power spectrum in future high-resolution SZ surveys (like ACT or APEX) reaching mJy flux limits.

astro-ph↗

arXiv2004

Young Crab-like pulsars and luminous X-ray sources in starbursts and optically dull galaxies

Recent Chandra observations of nearby galaxies have revealed a number of ultraluminous X-ray sources (ULXs) with super-Eddington luminosities, away from the central regions of non-active galaxies. The nature of these sources is still debated. We argue that a fraction of them could be young, Crab-like pulsars, the X-ray luminosity of which is powered by rotation. We use the pulsar birth parameters estimated from radio pulsar data to compute the steady-state pulsar X-ray luminosity distribution as a function of the star formation rate (SFR) in the galaxy. We find that ~10% of optically dull galaxies are expected to have a source with L_x >~ 10^{39} erg/s, while starbursts galaxies should each have several of these sources. We estimate that the X-ray luminosity of a few percents of galaxies is dominated by a single bright pulsar with L_x >~10^{39} erg/s, roughly independently of its SFR. We discuss observational diagnostics that can help distinguish the young pulsar population in ULXs.

astro-ph↗

arXiv2004

Are Supershells Powered by Multiple Supernovae? Modeling the Radio Pulsar Population Produced by OB Associations

Traditional searches for radio pulsars have targeted individual small regions such as supernova remnants or globular clusters, or have covered large contiguous regions of the sky. None of these searches has been specifically directed towards giant supershells, some of which are likely to have been produced by multiple supernova (SN) explosions from an OB association. Here we perform a Montecarlo simulation of the pulsar population associated with supershells powered by multiple SNe. We predict that several tens of radio pulsars could be detected with current instruments associated with the largest Galactic supershells (with kinetic energies >~ 10^{53} ergs), and a few pulsars with the smaller ones. We test these predictions for some of the supershells which lie in regions covered by past pulsar surveys. For the smaller supershells, our results are consistent with the few detected pulsars per bubble. For the giant supershell GSH 242-03+37, we find the multiple SN hypothesis inconsistent with current data at the 95% level. We stress the importance of undertaking deep pulsar surveys in correlation with supershells. Failure to detect any pulsar enhancement in the largest of them would put serious constraints on the multiple SN origin for them. Conversely, the discovery of the pulsar population associated with a supershell would allow a different/independent approach to the study of pulsar properties.

astro-ph↗

arXiv2003

Pulsars With Jets May Harbor Dynamically Important Accretion Disks

For many astrophysical sources with jets, there is evidence for the contemporaneous presence of disks. In contrast, pulsars such as the Crab and Vela show jets but have not yet revealed direct evidence for accretion disks. Here we show that for such pulsars, an accretion disk radiating below detectable thresholds may simultaneously account for (1) observed deviations in the braking indices from that of the simple dipole, (2) observed pulsar timing ages, and (3) possibly even the jet morphology via a disk outflow that interacts with the pulsar wind within, collimating and/or redirecting it.

astro-ph↗