arXiv subjects

Simone Dall'Osso

Publications and source records attributed to Simone Dall'Osso.

At least 37 records · Page 2Linked to original sources

arXiv2020

High latitude emission from structured jet of Gamma-Ray Bursts observed off-axis

The X-ray emission of gamma-ray bursts (GRBs) is often characterized by an initial steep decay, followed by a nearly constant emission phase (so called "plateau") which can extend up to thousands of seconds. While the steep decay is usually interpreted as the tail of the prompt gamma-ray flash, the long-lasting plateau is commonly associated to the emission from the external shock sustained by energy injection from a long lasting central engine. A recent study proposed an alternative interpretation, ascribing both the steep decay and the plateau to high-latitude emission (HLE) from a "structured jet" whose energy and bulk Lorentz factor depend on the angular distance from the jet symmetry axis. In this work we expand over this idea and explore more realistic conditions: (a) the finite duration of the prompt emission, (b) the angular dependence of the optical depth and (c) the lightcurve dependence on the observer viewing angle. We find that, when viewed highly off-axis, the structured jet HLE lightcurve is smoothly decaying with no clear distinction between the steep and flat phase, as opposed to the on-axis case. For a realistic choice of physical parameters, the effects of a latitude-dependent Thomson opacity and finite duration of the emission have a marginal effect on the overall lightcurve evolution. We discuss the possible HLE of GW170817, showing that the emission would have faded away long before the first Swift-XRT observations. Finally, we discuss the prospects for the detection of HLE from off-axis GRBs by present and future wide-field X-ray telescopes and X-ray surveys, such as eROSITA and the mission concept THESEUS.

astro-ph.HE↗

arXiv2020

Lunar Gravitational-Wave Antenna

Monitoring of vibrational eigenmodes of an elastic body excited by gravitational waves was one of the first concepts proposed for the detection of gravitational waves. At laboratory scale, these experiments became known as resonant-bar detectors first developed by Joseph Weber in the 1960s. Due to the dimensions of these bars, the targeted signal frequencies were in the kHz range. Weber also pointed out that monitoring of vibrations of Earth or Moon could reveal gravitational waves in the mHz band. His Lunar Surface Gravimeter experiment deployed on the Moon by the Apollo 17 crew had a technical failure rendering the data useless. In this article, we revisit the idea and propose a Lunar Gravitational-Wave Antenna (LGWA). We find that LGWA could become an important partner observatory for joint observations with the space-borne, laser-interferometric detector LISA, and at the same time contribute an independent science case due to LGWA's unique features. Technical challenges need to be overcome for the deployment of the experiment, and development of inertial vibration sensor technology lays out a future path for this exciting detector concept.

gr-qc↗

arXiv2020

Structured Jets and X-ray Plateaus in Gamma-Ray Burst Phenomena

The first multi-messenger detection of a binary neutron star merger, GW170817, brought to the forefront the structured jet model as a way to explain multi-wavelength observations taken more than a year after the event. Here we show that the high-latitude emission from a structured jet can naturally produce an X-ray plateau in gamma-ray burst (GRB) light curves, independent of the radiation from an external shock. We calculate the radiation from a switched-off shell featuring an angular structure in both its relativistic bulk motion and intrinsic brightness. Our model is able to explain the shallow decay phase (plateau) often observed in GRB X-ray light curves. We discuss the possible contribution of the structured jet high-latitude emission to other distinctive features of GRB X-ray light curves, and its capability to explain the chromatic optical/X-ray light curve properties.

astro-ph.HE↗

arXiv2018

Neutron star bulk viscosity, "spin-flip" and GW emission of newly born magnetars

The viscosity-driven "spin-flip" instability in newborn magnetars with interior toroidal magnetic fields is re-examined. We calculate the bulk viscosity coefficient ($ζ$) of cold, $npe μ$ matter in neutron stars (NS), for selected values of the nuclear symmetry energy and in the regime where $β$-equilibration is slower than characteristic oscillation periods. We show that: i) $ζ$ is larger than previously assumed and the instability timescale correspondingly shorter; ii) for a magnetically-induced ellipticity $ε_B \lesssim 4 \times 10^{-3}$, typically expected in newborn magnetars, spin-flip occurs for initial spin periods $\lesssim 2-3$ ms, with some dependence on the NS equation of state (EoS). We then calculate the detectability of GW signals emitted by newborn magnetars subject to "spin-flip", by accounting also for the reduction in range resulting from realistic signal searches. For an optimal range of $ε_B \sim (1-5) \times 10^{-3}$, and birth spin period $\lesssim 2$ ms, we estimate an horizon of $\gtrsim 4$ Mpc, and $\gtrsim 30$ Mpc, for Advanced and third generation interferometers at design sensitivity, respectively. A supernova (or a kilonova) is expected as the electromagnetic counterpart of such GW events. Outside of the optimal range for GW emission, EM torques are more efficient in extracting the NS spin energy, which may power even brighter EM transients.

astro-ph.HE↗

arXiv2017

The distribution of tilt angles in newly born NSs: role of interior viscosity and magnetic field

We study how the viscosity of neutron star (NS) matter affects the distribution of tilt angles ($χ$) between the spin and magnetic axes in young pulsars. Under the hypothesis that the NS shape is determined by the magnetically-induced deformation, and that the toroidal component of the internal magnetic field exceeds the poloidal one, we show that the dissipation of precessional motions by bulk viscosity can naturally produce a bi-modal distribution of tilt angles, as observed in radio/$γ$-ray pulsars, with a low probability of achieving $χ\sim (20^\circ - 70^\circ)$ if the interior B-field is $\sim (10^{11} - 10^{15})$~G and the birth spin period is $\sim 10 - 300$~ms. As a corollary of the model, the idea that the NS shape is solely determined by the poloidal magnetic field, or by the centrifugal deformation of the crust, is found to be inconsistent with the tilt angle distribution in young pulsars. When applied to the Crab pulsar, with $χ\sim 45^\circ - 70^\circ$ and birth spin $\sim$ 20 ms, our model implies that: (i) the magnetically-induced ellipticity is $ε_B \gtrsim 3 \times 10^{-6}$; (ii) the measured positive $\dotχ \sim 3.6 \times 10^{-12}$ rad s$^{-1}$ requires an additional viscous process, acting on a timescale $\lesssim 10^4$ yrs. We interpret the latter as crust-core coupling via mutual friction in the superfluid NS interior. One critical implication of our model is a GW signal at (twice) the spin frequency of the NS, due to $ε_B \sim 10^{-6}$. This could be detectable by Advanced LIGO/Virgo operating at design sensitivity.

astro-ph.HE↗

arXiv2016

Flares in Gamma Ray Bursts: Disc Fragmentation and Evolution

Flaring activity following gamma-ray bursts (GRBs), observed in both long and short GRBs, signals a long-term activity of the central engine. However, its production mechanism has remained elusive. Here we develop a quantitative model of the idea proposed by Perna et al. of a disc whose outer regions fragment due to the onset of gravitational instability. The self-gravitating clumps migrate through the disc and begin to evolve viscously when tidal and shearing torques break them apart. Our model consists of two ingredients: theoretical bolometric flare lightcurves whose shape (width, skewness) is largely insensitive to the model parameters, and a spectral correction to match the bandpass of the available observations, that is calibrated using the observed spectra of the flares. This simple model reproduces, with excellent agreement, the empirical statistical properties of the flares as measured by their width-to-arrival time ratio and skewness (ratio between decay and rise time). We present model fits to the observed lightcurves of two well-monitored flares, of GRB 060418 and of GRB 060904B. To the best of our knowledge, this is the first quantitative model able to reproduce the flare lightcurves and explain their global statistical properties.

astro-ph.HE↗

arXiv2015

The accretion regimes of a highly magnetised NS: the unique case of NuSTAR J095551+6940.8

We analyze archival Chandra HRC observations of the ultra luminous accreting pulsar M82-X2 (NuSTAR J095551+6940.8), and determine an upper limit of $< 1.7\times 10^{38}$~erg/s to its luminosity at an epoch at which it was undetected. Combined with other recent measurements, this confirms that the source X-ray emission has been highly variable during the last 15 years, ranging from a maximum of $10^{40}$ erg/s through intermediate values $\sim$ a few $\times 10^{39}$ erg/s, and down to a minimum that must be below the current detection threshold $\sim (2-3) \times 10^{38}$ erg/s . We interpret these results by means of a magnetically-threaded disk model: when at peak luminosity, the neutron star (NS) is close to spin equilibrium, its inner disk edge r_m ~ 10^8 cm is approximately half the corotation radius r_{co}, and radiation pressure dominates the disk out to r_{tr} ~ 10^9 cm. In the radiation pressure-dominated regime, r_m grows very slowly as the mass inflow rate drops: as a result, r_m < r_{co} remains valid until the mass accretion rate becomes ~ the Eddington accretion rate, allowing a wide range of accretion luminosities to the NS. Once the mass accretion rate is below Eddington, accretion onto the NS is inhibited because r_m > r_{co}, and the source luminosity is expected to drop by a large factor. We conclude that a magnetically threaded, radiation pressure-dominated disk, around a highly magnetized NS (B~10^{13} G) offers the best intepretation for all the currently observed properties of NuSTAR J095551+6940.8. This source offers an unprecedented opportunity to study the disk-magnetosphere interaction in a new regime of supercritical accretion, and across the transition between-radiation pressure and gas-pressure dominance inside the disk.

astro-ph.HE↗

arXiv2015

NuSTAR J095551+6940.8: a highly magnetised neutron star with super-Eddington mass accretion

The identification of the Ultraluminous X-ray source (ULX) X-2 in M82 as an accreting pulsar has shed new light on the nature of a subset of ULXs, while rising new questions on the nature of the super-Eddington accretion. Here, by numerically solving the torque equation of the accreting pulsar within the framework of the magnetically threaded-disk scenario, we show that three classes of solutions, corresponding to different values of the magnetic field, are mathematically allowed. We argue that the highest magnetic field one, corresponding to B $\sim 10^{13}$ G, is favoured based on physical considerations and the observed properties of the source. In particular, that is the only solution which can account for the observed variations in $\dot{P}$ (over four time intervals) without requiring major changes in $\dot{M}$, which would be at odds with the approximately constant X-ray emission of the source during the same time. For this solution, we find that the source can only accomodate a moderate amount of beaming, 0.5 $\lesssim b < 1$. Last, we show that the upper limit on the luminosity, L$_X < 2.5 \times 10^{38}$ erg s$^{-1}$ from archival observations, is consistent with a highly-magnetized neutron star being in the propeller phase at that time.

astro-ph.HE↗

arXiv2014

Gravitational waves from massive magnetars formed in binary neutron star mergers

Binary neutron star (NS) mergers are among the most promising sources of gravitational waves (GWs), as well as candidate progenitors for short Gamma-Ray Bursts (SGRBs). Depending on the total initial mass of the system, and the NS equation of state (EOS), the post-merger phase can see a prompt collapse to a black hole, or the formation of a supramassive NS, or even a stable NS. In the case of post-merger NS (PMNS) formation, magnetic field amplification during the merger will produce a magnetar with a large induced mass quadrupole moment, and millisecond spin. If the timescale for orthogonalization of the magnetic symmetry axis with the spin axis is sufficiently short the NS will radiate its spin down energy primarily via GWs. Here we study this scenario for various outcomes of NS formation: we generalise the set of equilibrium states for a twisted torus magnetic configuration to include solutions that, at a fixed exterior dipole field, carry a larger magnetic energy reservoir; we hence compute their magnetic ellipticity and the strength of the expected GW signal as a function of the magnitude of the dipole and toroidal field. The relative number of GW detections from PMNSs and from binary NSs is a strong function of the NS equation of state (EOS), being higher (~ 1%) for the stiffest EOSs and negligibly small for the softest ones. For intermediate-stiffness EOSs, such as the n=4/7 polytrope recently used by Giacomazzo \& Perna or the GM1 used by Lasky et al., the relative fraction is ~0.3%; correspondingly we estimate a GW detection rate from stable PMNSs of ~ (0.1-1) yr$^{-1}$ with Advanced detectors, and of ~ (100-1000) yr$^{-1}$ with third generation detectors such as the Einstein Telescope. Measurement of such GW signal would provide strong constraints on the NS EOS and on the nature of the binary progenitors giving rise to SGRBs.

astro-ph.HE↗

arXiv2013

Constraining white dwarf viscosity through tidal heating in detached binary systems

Although the internal structure of white dwarfs is considered to be generally well understood, the source and entity of viscosity is still very uncertain. We propose here to study white dwarf viscous properties using short period (< 1 hr), detached white dwarf binaries, such as the newly discovered ~12.8 min system. These binaries are wide enough that mass transfer has not yet started but close enough that the least massive component is subject to a measurable tidal deformation. The associated tidal torque transfers orbital energy, which is partially converted into heat by the action of viscosity within the deformed star. As a consequence, its outer non-degenerate layers expand, and the star puffs up. We self-consistently calculate the fractional change in radius, and the degree of asynchronism (ratio of stellar to orbital spin) as a function of the viscous time. Specializing our calculations to J0651, we find that the discrepancy between the measured radius of the secondary star and He white dwarf model predictions can be interpreted as tidal inflation if the viscous timescale is either ~2 10^5 yr or ~10^4 yr. Such values point to a non-microscopic viscosity, possibly given by tidally induced turbulence, or by magnetic field stresses with a magnetic field strength of 10-100 Gauss. Fortunately, these two timescales produce very different degree of asynchronism, with the shortest one, bringing the system much closer to synchronisation. A measurement of the stellar spin can thus univocally determined the mean viscosity. Extrapolating the secondary's radial expansion, we predict that the star will fill is Roche lobe at a separation which is 1.2-1.3 smaller than the current one. Applying this method to a future sample of systems can allow us to learn whether viscosity changes with mass and/or nuclear composition.

astro-ph.HE↗

arXiv2013

Binary Pulsar J0737-3039: Evidence for a new core collapse and neutron star formation mechanism

The binary pulsar J0737-3039 is the only known system having two observable pulsars, thus offering a unique laboratory to test general relativity and explore pulsar physics. Based on the low eccentricity and the position within the galactic plane, Piran & Shaviv (2004, 2005) argued that pulsar B had a non-standard formation scenario with little or no mass ejection. They have also predicted that the system would have a very slow proper motion. Pulsar timing measurements (Kramer et al. 2006; Deller et al. 2009) confirmed this prediction. The recent observations of the alignment between the spin of pulsar A and the binary orbit is also in agreement with this scenario. Detailed simulations of the formation process of pulsar B enable us to show that its progenitor, just before the collapse, was a massive O-Ne-Mg white dwarf surrounded by a tenuous, 0.1-0.16 M_sun, envelope. This envelope was ejected when the white dwarf collapsed to form a neutron star. Pulsar B was born as a slow rotator (spin period ~ 1 s) and a kick received when the pulsar formed changed its spin direction to the current one. This realization sheds light on the angular momentum evolution of the progenitor star, a process which is strongly affected by interaction with the binary companion. The slow proper motion of the system also implies that the system must have undergone a phase of mass transfer in which Star A shed a significant fraction of its mass onto B.

astro-ph.HE↗

arXiv2012

Tidal torque induced by orbital decay in compact object binaries

As we observe in the moon-earth system, tidal interactions in binary systems can lead to angular momentum exchange. The presence of viscosity is generally regarded as the condition for such transfer to happen. In this paper, we show how the orbital evolution can cause a persistent torque between the binary components, even for inviscid bodies. This preferentially occurs at the final stage of coalescence of compact binaries, when the orbit shrinks successively by gravitational waves and plunging on a timescale shorter than the viscous timescale. The total orbital energy transferred to the secondary by this torque is ~0.01 of its binding energy. We further show that this persistent torque induces a differentially rotating quadrupole perturbation. Specializing to the case of a secondary neutron star, we find that this non equilibrium state has an associated free energy of 10^47-10^48 erg, just prior to coalescence. This energy is likely stored in internal fluid motions, with a sizable amount of differential rotation. By tapping this free energy reservoir, a preexisting weak magnetic field could be amplified up to a strength of ~10^15 Gauss. Such a dynamically driven tidal torque can thus recycle an old neutron star into a highly magnetized neutron star, with possible observational consequences at merger.

astro-ph.HE↗

arXiv2012

Generation of strong magnetic fields by r-modes in millisecond accreting neutron stars: induced deformations and gravitational wave emission

Differential rotation induced by the r-mode instability can generate very strong toroidal fields in the core of accreting, millisecond spinning neutron stars. We introduce explicitly the magnetic damping term in the evolution equations of the r-modes and solve them numerically in the Newtonian limit, to follow the development and growth of the internal magnetic field. We show that the strength of the latter can reach large values, $B \sim 10^{14} $ G, in the core of the fastest accreting neutron stars. This is strong enough to induce a significant quadrupole moment of the neutron star mass distribution, corresponding to an ellipticity $|ε_B}| \sim 10^{-8}$. If the symmetry axis of the induced magnetic field is not aligned with the spin axis, the neutron star radiates gravitational waves. We suggest that this mechanism may explain the upper limit of the spin frequencies observed in accreting neutron stars in Low Mass X-Ray Binaries. We discuss the relevance of our results for the search of gravitational waves.

astro-ph.HE↗

arXiv2010

GRB Afterglows with Energy Injection from a spinning down NS

We investigate a model for the shallow decay phases of Gamma-ray Burst (GRB) afterglows discovered by Swift/XRT in the first hours following a GRB event. In the context of the fireball scenario, we consider the possibility that long-lived energy injection from a millisecond spinning, ultramagnetic neutron star (magnetar) powers afterglow emission during this phase. We consider the energy evolution in a relativistic shock subject to both radiative losses and energy injection from a spinning down magnetar in spherical symmetry. We model the energy injection term through magnetic dipole losses and discuss an approximate treatment for the dynamical evolution of the blastwave. We obtain an analytic solution for the energy evolution in the shock and associated lightcurves. To fully illustrate the potential of our solution we calculate lightcurves for a few selected X-ray afterglows observed by Swift and fit them using our theoretical lightcurves. Our solution naturally describes in a single picture the properties of the shallow decay phase and the transition to the so-called normal decay phase. In particular, we obtain remarkably good fits to X-ray afterglows for plausible parameters of the magnetar. Even though approximate, our treatment provides a step forward with respect to previously adopted approximations and provides additional support to the idea that a millisecond spinning (1-3 ms), ultramagnetic (B$\sim 10^{14}-10^{15}$ G) neutron star loosing spin energy through magnetic dipole radiation can explain the luminosity, durations and shapes of X-ray GRB afterglows.

astro-ph.HE↗

arXiv2010

White Dwarfs in Ultrashort Binary Systems

This contribution summarizes, as of early 2008, the observational and theoretical understanding of the physics, and emission properties of two Double Degenerate Binaries with the shortest orbital period known to date. In particular, the Unipolar Inductor Model and its coupling to GW emission have been introduced to explain a number of puzzling features that these two sources have in common and that are thought difficult to reconcile with models of mass transfer in such systems. Emphasis was put on the relevant new physical features that characterize the model. The role of spin-orbit coupling through the Lorentz torque and the role of GW emission in keeping the electric interaction active at all times has been thoroughly discussed in all their implications. Application of the model to both HM Cnc and V407 Vul accounts in a natural way for their main observational properties. Constraints on physical parameters are derived in order for the model to work, and can be verified by future observations.

astro-ph.HE↗

arXiv2007

Transient Phenomena in Anomalous X-ray Pulsars

In 2003 a previously unpulsed Einstein and ROSAT source cataloged as soft and dim (Lx of few 10^33 ergs) thermal emitting object, namely XTE J1810-197, was identified as the first unambiguous transient Anomalous X-ray Pulsar. Two years later this source was also found to be a bright highly polarized transient radio pulsar, a unique property among both AXPs and radio pulsars. In September 2006 the Swift Burst Alert Telescope (BAT) detected an intense burst from the candidate AXP CXOU J164710.2-455216, which entered in an outburst state reaching a peak emission of at least a factor of 300 higher than quiescence. Here, we briefly outline the recent results concerning the outburst phenomena observed in these two AXPs. In particular, XTE J1810-197 has probed to be a unique laboratory to monitor the timing and spectral properties of a cooling/fading AXP, while new important information have been inferred from X-ray and radio band simultaneous observations. CXOU J164710.2-455216 has been monitored in X-rays and radio bands since the very beginning of its outbursting state allowing us to cover the first phases of the outburst and to study the timing and spectral behavior during the first months.

astro-ph↗

arXiv2007

Newborn Magnetars as sources of Gravitational Radiation: constraints from High Energy observations of Magnetar Candidates

Soft Gamma Repeaters and the Anomalous X-ray Pulsars are believed to contain slowly spinning "magnetars". The enormous energy liberated in the 2004 Dece 27 giant flare from SGR 1806-20, together with the likely recurrence time of such events, points to an internal magnetic field strength ~ 10^{16} G. Such strong fields are expected to be generated by a coherent alpha-Omega dynamo in the early seconds after the Neutron Star formation, if its spin period is of a few milliseconds at most. A substantial deformation of the NS is caused by such fields and a newborn millisecond-spinning magnetar would thus radiate for a few days a strong gravitational wave signal. Such a signal may be detected with Advanced LIGO-class detectors up to the distance of the Virgo cluster, where ~ 1 magnetar per year are expected to form. Recent X-ray observations reveal that SNRs around magnetar candidates do not show evidence for a larger energy content than standard SNRs (Vink & Kuiper 2006). This is at variance with what would be expected if the spin energy of the young, millisecond NS were radiated away as electromagnetic radiation andd/or relativistic particle winds and, thus, transferred quickly to the expanding gas shell. We show here that these recent findings can be reconciled with the idea of magnetars being formed with fast spins, if most of their initial spin energy is radiated thorugh GWs. In particular, we find that this occurs for essentially the same parameter range that would make such objects detectable by Advanced LIGO-class detectors up to the Virgo Cluster.

astro-ph↗

arXiv2005

Gravitational Radiation from Newborn Magnetars

There is growing evidence that two classes of high-energy sources, the Soft Gamma Repeaters and the Anomalous X-ray Pulsars contain slowly spinning ``magnetars'', i.e. neutron stars whose emission is powered by the release of energy from their extremely strong magnetic fields (>10^15 G. We show here that the enormous energy liberated in the 2004 December 27 giant flare from SGR1806-20 (~5 10^46 erg), together with the likely recurrence time of such events, requires an internal field strength of > 10^16 G. Toroidal magnetic fields of this strength are within an order of magnitude of the maximum fields that can be generated in the core of differentially-rotating neutron stars immediately after their formation, if their initial spin period is of a few milliseconds. A substantial deformation of the neutron star is induced by these magnetic fields and, provided the deformation axis is offset from the spin axis, a newborn fast-spinning magnetar would radiate for a few weeks a strong gravitational wave signal the frequency of which (0.5-2 kHz range) decreases in time. The signal from a newborn magnetar with internal field > 10^16.5 G could be detected with Advanced LIGO-class detectors up to the distance of the Virgo cluster (characteristic amplitude h_c about 10^-21). Magnetars are expected to form in Virgo at a rate approx. 1/yr. If a fraction of these have sufficiently high internal magnetic field, then newborn magnetars constitute a promising new class of gravitational wave emitters.

astro-ph↗