arXiv subjects

Rosalba Perna

Publications and source records attributed to Rosalba Perna.

At least 91 records · Page 5Linked to original sources

arXiv2019

AGN (and other) astrophysics with Gravitational Wave Events

The stellar mass binary black hole (sBBH) mergers presently detected by LIGO may originate wholly or in part from binary black hole mergers embedded in disks of gas around supermassive black holes. Determining the contribution of these active galactic nucleus (AGN) disks to the sBBH merger rate enables us to uniquely measure important parameters of AGN disks, including their typical density, aspect ratio, and lifetime, thereby putting unique limits on an important element of galaxy formation. For the first time, gravitational waves will allow us to reveal the properties of the hidden interior of AGN disks, while electromagnetic radiation (EM) probes the disk photosphere. The localization of sBBH merger events from LIGO is generally insufficient for association with a single EM counterpart. However, the contribution to the LIGO event rate from rare source types (such as AGNs) can be determined on a statistical basis. To determine the contribution to the sBBH rate from AGNs in the next decade requires: {\em 1) a complete galaxy catalog for the LIGO search volume, 2) strategic multi-wavelength EM follow-up of LIGO events and 3) significant advances in theoretical understanding of AGN disks and the behavior of objects embedded within them.}

astro-ph.HE↗

arXiv2019

Limits on electromagnetic counterparts of gravitational wave-detected binary black hole mergers

Unlike mergers of two compact objects containing a neutron star (NS), binary black hole (BBH) mergers are not accompanied by the production of tidally disrupted material, and hence lack the most direct source of accretion to power a jet and generate electromagnetic (EM) radiation. However, following a tentative detection by the Fermi GBM of a $γ$-ray counterpart to GW150914, several alternative ideas were proposed for driving a jet and producing EM radiation. {\em If} such jets were in fact produced, they would however lack the bright cocoon emission that makes jets from binary NSs bright also at large viewing angles. Here, via Monte Carlo simulations of a population of BBH mergers with properties consistent with those inferred from the existing LIGO/Virgo observations, and the angular emission characteristic of jets propagating into the interstellar medium, we derive limits on the allowed energetics and Lorentz factors of such jets from EM follow ups to GW-detected BBH merger events to date, and we make predictions which will help tighten these limits with broadband EM follow ups to events in future LIGO/Virgo runs. The condition that $\lesssim 1$ event out of 10 GW-detected BBH mergers be above the Fermi/GBM threshold imposes that any currently allowed emission model has to satisfy the condition $E_{\rm iso}/(10^{49} {\rm erg})(θ_{jet}/20^\circ)\lesssim 1$.

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arXiv2019

Multi-Messenger Astronomy with Extremely Large Telescopes

The field of time-domain astrophysics has entered the era of Multi-messenger Astronomy (MMA). One key science goal for the next decade (and beyond) will be to characterize gravitational wave (GW) and neutrino sources using the next generation of Extremely Large Telescopes (ELTs). These studies will have a broad impact across astrophysics, informing our knowledge of the production and enrichment history of the heaviest chemical elements, constrain the dense matter equation of state, provide independent constraints on cosmology, increase our understanding of particle acceleration in shocks and jets, and study the lives of black holes in the universe. Future GW detectors will greatly improve their sensitivity during the coming decade, as will near-infrared telescopes capable of independently finding kilonovae from neutron star mergers. However, the electromagnetic counterparts to high-frequency (LIGO/Virgo band) GW sources will be distant and faint and thus demand ELT capabilities for characterization. ELTs will be important and necessary contributors to an advanced and complete multi-messenger network.

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arXiv2018

Biases in inferring dark matter profiles from dynamical and lensing measurements

The degeneracy between disc and halo contributions in spiral galaxy rotation curves makes it difficult to obtain a full understanding of the distribution of baryons and dark matter in disc galaxies like our own Milky Way. Using mock data, we study how constraints on dark matter profiles obtained from kinematics, strong lensing, or a combination of the two are affected by assumptions about the halo model. We compare four different models: spherical isothermal and Navarro-Frenk-White halos, along with spherical and elliptical Burkert halos. For both kinematics and lensing we find examples where different models fit the data well but give enclosed masses that are inconsistent with the true (i.e., input) values. This is especially notable when the input and fit models differ in having cored or cuspy profiles (such as fitting an NFW model when the underlying dark matter distribution follows a different profile). We find that mass biases are more pronounced with lensing than with kinematics, and using both methods can help reduce the bias and provide stronger constraints on the dark matter distributions.

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arXiv2018

Hypervelocity binaries from close encounters with a SMBH-IMBH binary: orbital properties and diagnostics

Hypervelocity binaries (HVBs) can be produced by a close encounter between stellar binaries and supermassive black hole (SMBH)-intermediate massive black hole (IMBH) binaries. We perform scattering experiments between tight stellar binaries and an SMBH-IMBH binary (within the observationally allowed parameters), using high-precision N-body integrations. Simulation results show that only a few HVBs can be produced in the Galactic center if it harbors a $4\times10^3M_\odot$ IMBH. However, we find that tight HVBs can be efficiently produced in prograde coplanar orbits with respect to the SMBH-IMBH binary. Therefore, the trajectories of the HVBs provide a promising way to constrain the IMBH orbital plane, if one exists. More importantly, we find that the ejected tight binaries are eccentric. For other galaxies hosting equal-mass SMBH binaries for which the HVB ejection rate is much higher relative to SMBH-IMBH binaries, these ejected compact binaries constitute promising extragalactic merger sources that could be detected by gravitational wave detectors.

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arXiv2018

Turbulence-driven thermal and kinetic energy fluxes in the atmospheres of hot Jupiters

We have performed high resolution $3-$dimensional compressible hydrodynamics simulations to investigate the effects of shocks and turbulence on energy transport into hot Jupiter atmospheres, under a variety of shear gradients. We focus on a local atmospheric region to accurately follow the small-scale structures of turbulence and shocks. We find that the effects of turbulence above and below a shear layer are different in scale and magnitude: below the shear layer, the effects of turbulence on the vertical energy transfer are local, generally $\lesssim2~\times(\text{scale height})$. However, turbulence can have a spatially and thermally-large influence on almost the entire region above the shear layer. We also find that shock formation is local and transient. Once the atmosphere becomes steady, the time-averaged heat energy flux at $P\sim 1$ bar is insignificant, on the order of 0.001\% of the incoming stellar flux with a shear motion at $P\sim 1$ mbar, and 0.1\% with a deeper shear layer at $P\sim 100$ mbar. Accordingly, the diffusion coefficient is higher for the deeper shear layer. Therefore, our results suggest that turbulence near less dense regions ($P\sim 1$ mbar) does not cause a sufficiently deep and large penetration of thermal energy to account for radius inflation in hot Jupiters, regardless of how violent the turbulence is. However, as the shear layer gets deeper, heat energy transfer becomes more effective throughout the atmosphere (upwards and downwards) due to a larger kinetic energy budget. Therefore, it is more important how deep turbulence occurs in the atmosphere, than how unstable the atmosphere is for effective energy transfer.

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arXiv2018

Late time afterglow observations reveal a collimated relativistic jet in the ejecta of the binary neutron star merger GW170817

The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multi-wavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma-rays proved that BNS mergers are associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubts on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multi-wavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30 degrees away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.

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arXiv2018

Binary Black Hole Mergers within the LIGO Horizon: Statistical Properties and prospects for detecting Electromagnetic Counterparts

Binary black holes (BBHs) are one of the endpoints of isolated binary evolution, and their mergers a leading channel for gravitational wave events. Here, using the evolutionary code \textsc{StarTrack}, we study the statistical properties of the BBH population from isolated binary evolution for a range of progenitor star metallicities and BH natal kicks. We compute the mass function and the distribution of the primary BH spin $a$ as a result of mass accretion during the binary evolution, and find that this is not an efficient process to spin up BHs, producing an increase by at most $a\sim$~0.2--0.3 for very low natal BH spins. We further compute the distribution of merger sites within the host galaxy, after tracking the motion of the binaries in the potentials of a massive spiral, a massive elliptical, and a dwarf galaxy. We find that a fraction of 70-90\% of mergers in massive galaxies and of 40-60\% in dwarfs (range mostly sensitive to the natal kicks) is expected to occur inside of their hosts. The number density distribution at the merger sites further allows us to estimate the broadband luminosity distribution that BBH mergers would produce, \textit{if} associated with a kinetic energy release in an outflow, {which, as a reference, we assume at the level inferred for the \textit{Fermi} GBM counterpart to GW150914, with the understanding that current limits from the O1 and O2 runs would require such emission to be produced within a jet of angular size within $\lesssim 50^\circ$.}

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arXiv2018

Ultra Long Gamma-Ray Bursts from the collapse of Blue Super Giant stars: an end-to-end simulation

Ultra-long gamma ray bursts (ULGRBs) are a distinct class of GRBs characterized by durations of several thousands of seconds, about two orders of magnitude longer than those of standard long GRBs (LGRBs). The driving engine of these events has not been uncovered yet, and ideas range from magnetars, to tidal disruption events, to extended massive stars, such as blue super giants (BSG). BSGs, a possible endpoint of stellar evolution, are attractive for the relatively long free-fall times of their envelopes, allowing accretion to power a long-lasting central engine. At the same time, their large radial extension poses a challenge to the emergence of a jet. Here we perform an end-to-end simulation aimed at assessing the viability of BSGs as ULGRB progenitors. The evolution to core collapse of a BSG star model is calculated with the MESA code. We then compute the accretion rate for the fraction of envelope material with enough angular momentum to circularize and form an accretion disk, and input the corresponding power into a jet which we evolve through the star envelope with the FLASH code. Our simulation shows that the jet can emerge, and the resulting light curves resemble those observed in ULGRBs, with durations $T_{90}$ ranging from $\approx$4000~s to $\approx10^4$ s depending on the viewing angle.

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arXiv2018

When do star clusters become multiple star systems? II. Toward a half-life formalism with four bodies

We present a half-life formalism for describing the disruption of gravitationally-bound few-body systems, with a focus on binary-binary scattering. For negative total encounter energies, the four-body problem has three possible decay products in the point particle limit. For each decay product and a given set of initial conditions, we obtain directly from numerical scattering simulations the half-life for the distribution of disruption times. As in radioactive decay, the half-lives should provide a direct prediction for the relative fractions of each decay product. We test this prediction with simulated data and find good agreement with our hypothesis. We briefly discuss applications of this feature of the gravitational four-body problem to populations of black holes in globular clusters. This paper, the second in the series, builds on extending the remarkable similarity between gravitational chaos at the macroscopic scale and radioactive decay at the microscopic scale to larger-N systems.

astro-ph.SR↗

arXiv2017

The fate of close encounters between binary stars and binary supermassive black holes

The evolution of main sequence binaries that reside in the Galactic Centre can be heavily influenced by the central supermassive black hole (SMBH). Due to these perturbative effects, the stellar binaries in dense environments are likely to experience mergers, collisions or ejections through secular and/or non-secular interactions. More direct interactions with the central SMBH are thought to produce hypervelocity stars (HVSs) and tidal disruption events (TDEs). In this paper, we use N-body simulations to study the dynamics of stellar binaries orbiting a central SMBH primary with an outer SMBH secondary orbiting this inner triple. The effects of the secondary SMBH on the event rates of HVSs, TDEs and stellar mergers are investigated, as a function of the SMBH-SMBH binary mass ratio. Our numerical experiments reveal that, relative to the isolated SMBH case, the TDE and HVS rates are enhanced for, respectively, the smallest and largest mass ratio SMBH-SMBH binaries. This suggests that the observed event rates of TDEs and HVSs have the potential to serve as a diagnostic of the mass ratio of a central SMBH-SMBH binary. The presence of a secondary SMBH also allows for the creation of hypervelocity binaries. Observations of these systems could thus constrain the presence of a secondary SMBH in the Galactic Centre.

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arXiv2017

Population III X-Ray Binaries

Understanding of the role of X-rays for driving the thermal evolution of the intergalactic medium (IGM) at high redshifts is one of important questions in astrophysics. High-mass X-ray binaries (HMXBs) in early stellar populations are prime X-ray source; however, their formation efficiency is not well understood. Using $N$-body simulations, we estimate the HMXB formation rate via mutual gravitational interactions of nascent, small groups of the Population~III stars. We find that HMXBs form at a rate of one per $\gtrsim 10^{4}M_{\odot}$ in newly born stars, and that they emit with a power of $\sim 10^{41} {\rm erg}~{\rm s}^{-1}$ in the $2-10$ keV band per star formation rate (SFR). This value is a factor $\sim 10^{2}$ larger than what is observed in star forming galaxies at lower redshifts; the X-ray production from early HMXBs would have been even more copious, if they also formed \textit{in situ} or via migration in protostellar disks. Combining our results with earlier studies suggests that early HMXBs were highly effective at heating the IGM and leaving a strong 21 cm signature. We discuss broader implications of our results, such as the rate of long gamma-ray bursts from Population~III stars and the direct collapse channel for massive black hole formation.

astro-ph.GA↗

arXiv2017

Interactions between multiple supermassive black holes in galactic nuclei: a solution to the final parsec problem

Using few-body simulations, we investigate the evolution of supermassive black holes (SMBHs) in galaxies ($M_{\star}=10^{10}-10^{12}{\rm M}_{\odot}$ at $z=0$) at $0<z<4$. Following galaxy merger trees from the Millennium simulation, we model BH mergers with two extreme binary decay scenarios for the `hard binary' stage: a full or an empty loss cone. These two models should bracket the true evolution, and allow us to separately explore the role of dynamical friction and that of multi-body BH interactions on BH mergers. Using the computed merger rates, we infer the stochastic gravitational wave background (GWB). Our dynamical approach is a first attempt to study the dynamical evolution of multiple SMBHs in the host galaxies undergoing mergers with various mass ratios ($10^{-4} < q_{\star} < 1$). Our main result demonstrates that SMBH binaries are able to merge in both scenarios. In the empty loss cone case, we find that BHs merge via multi-body interactions, avoiding the `final parsec' problem, and entering the PTA band with substantial orbital eccentricity. Our full loss cone treatment, albeit more approximate, suggests that the eccentricity becomes even higher when GWs become dominant, leading to rapid coalescences (binary lifetime $\lesssim1 {\rm ~Gyr}$). Despite the lower merger rates in the empty loss cone case, due to their higher mass ratios and lower redshifts, the GWB in the full/empty loss cone models are comparable ($0.70\times10^{-15}$ and $0.53\times10^{-15}$ at a frequency of $1~{\rm yr}^{-1}$, respectively). Finally, we compute the effects of high eccentricities on the GWB spectrum.

astro-ph.GA↗

arXiv2017

Off-axis prompt X-ray transients from the cocoon of short gamma-ray bursts

We present the results of numerical simulations of the prompt emission of short-duration gamma-ray bursts. We consider emission from the relativistic jet, the mildly relativistic cocoon, and the non-relativistic shocked ambient material. We find that the cocoon material is confined between off-axis angles 15<theta<45 degrees and gives origin to X-ray transients with a duration of a few to ~10 seconds, delayed by a few seconds from the time of the merger. We also discuss the distance at which such transients can be detected, finding that it depends sensitively on the assumptions that are made about the radiation spectrum. Purely thermal cocoon transients are detectable only out to a few Mpc, Comptonized transients can instead be detected by the FERMI GBM out to several tens of Mpc.

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↗

arXiv2017

Formation of runaway stars in a star-cluster potential

We study the formation of runaway stars due to binary-binary (2+2) interactions in young star-forming clusters and/or associations. This is done using a combination of analytic methods and numerical simulations of 2+2 scattering interactions, both in isolation and in a homogeneous background potential. We focus on interactions that produce two single stars and a binary, and study the outcomes as a function of the depth of the background potential, within a range typical of cluster cores. As reference parameters for the observational properties, we use those observed for the system of runaway stars AE Aur and $μ$ Col and binary $ι$ Ori. We find that the outcome fractions have no appreciable dependence on the depth of the potential, and neither do the velocities of the ejected single stars. However, as the potential gets deeper and a larger fraction of binaries remain trapped, two binary populations emerge, with the escaped component having higher speeds and shorter semi-major axes than the trapped one. Additionally, we find that the relative angles between the ejected products are generally large. In particular, the angle between the ejected fastest star and the escaped binary is typically $\gtrsim 120-135^{\circ}$, with a peak at around $160^{\circ}$. However, as the potential gets deeper, the angle distribution becomes broader. Finally, we discuss the implications of our results for the interpretation of the properties of the runaway stars AE Aur and $μ$ Col.

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arXiv2017

An analytic method for identifying dynamically-formed runaway stars

In this paper, we study the 3-body products (two single stars and a binary) of binary-binary (2+2) scattering interactions. This is done using a combination of analytic methods and numerical simulations of 2+2 scattering interactions, both in isolation and in a homogeneous background potential. We derive analytically a simple formula relating the angle between the velocity vectors of the two ejected single stars and the orbital separation of the remaining binary. We compare our analytic formulation to numerical scattering simulations, and illustrate that the agreement is excellent, both in isolation and in a homogeneous background potential. Our results are ideally suited for application to the GAIA database, which is expected to identify many hundred runaway stars. The analytic relation presented here has the potential to identify runaway stars formed dynamically with high confidence. Finally, by applying our method to the runaways AE Aur and $μ$ Col, we illustrate that it can be used to constrain the history of the background potential, which was denser than the presently observed density in the case of the Trapezium cluster.

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arXiv2017

General relativistic magnetohydrodynamic simulations of binary neutron star mergers forming a long-lived neutron star

Merging binary neutron stars (BNSs) represent the ultimate targets for multimessenger astronomy, being among the most promising sources of gravitational waves (GWs), and, at the same time, likely accompanied by a variety of electromagnetic counterparts across the entire spectrum, possibly including short gamma-ray bursts (SGRBs) and kilonova/macronova transients. Numerical relativity simulations play a central role in the study of these events. In particular, given the importance of magnetic fields, various aspects of this investigation require general relativistic magnetohydrodynamics (GRMHD). So far, most GRMHD simulations focused the attention on BNS mergers leading to the formation of a hypermassive NS, which, in turn, collapses within few tens of ms into a black hole surrounded by an accretion disk. However, recent observations suggest that a significant fraction of these systems could form a long-lived NS remnant, which will either collapse on much longer timescales or remain indefinitely stable. Despite the profound implications for the evolution and the emission properties of the system, a detailed investigation of this alternative evolution channel is still missing. Here, we follow this direction and present a first detailed GRMHD study of BNS mergers forming a long-lived NS. We consider magnetized binaries with different mass ratios and equations of state and analyze the structure of the NS remnants, the rotation profiles, the accretion disks, the evolution and amplification of magnetic fields, and the ejection of matter. Moreover, we discuss the connection with the central engine of SGRBs and provide order-of-magnitude estimates for the kilonova/macronova signal. Finally, we study the GW emission, with particular attention to the post-merger phase.

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