arXiv subjects

Paz Beniamini

Publications and source records attributed to Paz Beniamini.

At least 109 records · Page 6Linked to original sources

arXiv2019

Discovery and Identification of MAXI J1621-501 as a Type I X-ray Burster with a Super-Orbital Period

MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up with a multitude of space missions (NuSTAR, Swift, Chandra, NICER, INTEGRAL, and MAXI) and ground-based observatories (Gemini, IRSF, and ATCA). The source was discovered with MAXI on 2017 October 19 as a new, unidentified transient. Further observations with NuSTAR revealed 2 Type I X-ray bursts, identifying MAXI J1621-501 as a Low Mass X-ray Binary (LMXB) with a neutron star primary. Overall, 24 Type I bursts were detected from the source during a 15 month period. At energies below 10 keV, the source spectrum was best fit with three components: an absorbed blackbody with kT = 2.3 keV, a cutoff power law with index $Γ$ = 0.7, and an emission line centered on 6.3 keV. Timing analysis of the X-ray persistent emission and burst data has not revealed coherent pulsations from the source or an orbital period. We identified, however, a super-orbital period $\sim{}$78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of $\sim{}$82 days. Thus, MAXI J1621-501 joins a small group of sources characterized with super-orbital periods.

astro-ph.HE↗

arXiv2019

The Gravitational waves merger time distribution of binary neutron star systems

Binary neutron stars (BNS) mergers are prime sites for $r$-process nucleosynthesis. Their rate determines the chemical evolution of heavy elements in the Milky Way. The merger rate of BNS is a convolution of their birth rate and the gravitational radiation spiral-in delay time. Using the observed population of Galactic BNS we show here that the lifetimes of pulsars in observed BNSs are sufficiently short that the ages of BNSs have little to no effect on the observed merger time distribution. We find that at late times ($t\gtrsim 1$ Gyr) the gravitational wave delay time distribution (DTD) follows the expected $ t^{-1}$. However, a significant excess of rapidly merging systems (between $40-60\%$ of the entire population) is apparent at shorter times. Although the exact shape of the DTD cannot be determined with the existing data, in all models that adequately describe the data we find at least $40\%$ of BNSs with merger times less than 1Gyr. This population of rapid mergers implies a declining deposition rate of $r$-process materials that is consistent with several independent observations of heavy element abundances in the Milky Way. At the same time this population that requires initial binary separations of roughly one solar radius clearly indicates that these binaries had common envelope progenitors. Our results suggest that a significant fraction of future LIGO/Virgo BNS mergers would reside in star forming galaxies.

astro-ph.HE↗

arXiv2019

Evolution of the extended X-ray emission from the PSR B1259-63/LS 2883 binary in the 2014-2017 binary cycle

We have performed a series of Chandra X-ray Observatory observations of the gamma-ray binary LS 2883, which is comprised of a young pulsar (PSR B1259-63) orbiting a massive Be star with a period of 1236.7 days. The system was observed in 5 epochs, spanning a range from 352 to 1175 days after the periastron passage on 2014 May 4. The observations confirmed the recurrent nature of the high-speed ejecta that appear as an extended X-ray structure (clump) moving away from the binary. Compared to the results of the previous monitoring campaign (between the 2010 and 2014 periastron passages), this time we find evidence suggesting that the clump is accelerated to a projected velocity $ v_{\perp}\approx0.15c$ with an acceleration $a_{\perp}=47\pm2$ cm s$^{-2}$ (for uniformly accelerated motion), assuming that it was launched near periastron passage. The observed X-ray properties of the clump are consistent with synchrotron emission from pulsar wind particles accelerated at the interface between the pulsar wind and the clump. We have also performed contemporaneous observations with the {\sl Hubble Space Telescope}, which are used to set an upper limit on the optical flux of the extended emission.

astro-ph.HE↗

arXiv2019

Formation Rates and Evolution Histories of Magnetars

We constrain the formation rate of Galactic magnetars directly from observations. Combining spin-down rates, magnetic activity, and association with supernova remnants, we put a 2$σ$ limit on their Galactic formation rate at $2.3-20\mbox{kyr}^{-1}$. This leads to a fraction $0.4_{-0.28}^{+0.6}$ of neutron stars being born as magnetars. We study evolutionary channels that can account for this rate as well as for the periods, period derivatives and luminosities of the observed population. We find that their typical magnetic fields at birth are $3\times 10^{14}-10^{15}$G, and that those decay on a time-scale of $\sim 10^4$years, implying a maximal magnetar period of $P_{\rm max}\approx 13$s. A sizable fraction of the magnetars' energy is released in outbursts. Giant Flares with $E\geq 10^{46}$ erg are expected to occur in the Galaxy at a rate of $\sim 5\mbox{kyr}^{-1}$. Outside our Galaxy, such flares remain observable by {\it Swift} up to a distance of $\sim 100$~Mpc, implying a detection rate of $\sim 5\mbox{ yr}^{-1}$. The specific form of magnetic energy decay is shown to be strongly tied to the total number of observable magnetars in the Galaxy. A systematic survey searching for magnetars could determine the former and inform physical models of magnetic field decay.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2019

Observable Features of GW170817 Kilonova Afterglow

The neutron star merger, GW170817, was followed by an optical-infrared transient (a kilonova) which indicated that a substantial ejection of mass at trans-relativistic velocities occurred during the merger. Modeling of the kilonova is able to constrain the kinetic energy of the ejecta and its characteristic velocity, but not the high-velocity distribution of the ejecta. Yet, this distribution contains crucial information on the merger dynamics. In this work, we assume a power-law distribution of the form $E(>βΓ)\propto(βΓ)^{-α}$ for the energy of the kilonova ejecta and calculate the non-thermal signatures produced by the interaction of the ejecta with the ambient gas. We find that ejecta with minimum velocity $β_0\simeq 0.3$ and energy $E\sim 10^{51}$ erg, as inferred from kilonova modeling, has a detectable radio, and possibly X-ray, afterglow for a broad range of parameter space. This afterglow component is expected to dominate the observed emission on a timescale of a few years post merger and peak around a decade later. Its light curve can be used to determine properties of the kilonova ejecta and in particular the ejecta velocity distribution $α$, the minimum velocity $β_0$ and its total kinetic energy $E$. We also predict that an afterglow rebrightening, that is associated with the kilonova component, will be accompanied by a shift of the centroid of the radio source towards the initial position of the explosion.

astro-ph.HE↗

arXiv2018

A lesson from GW170817: most neutron star mergers result in tightly collimated successful GRB jets

The joint detection of gravitational waves (GWs) and $γ$-rays from a binary neutron-star (NS) merger provided a unique view of off-axis GRBs and an independent measurement of the NS merger rate. Comparing the observations of GRB170817 with those of the regular population of short GRBs (sGRBs), we show that an order unity fraction of NS mergers result in sGRB jets that breakout of the surrounding ejecta. We argue that the luminosity function of sGRBs, peaking at $\approx 2\times 10^{52}\, \mbox{erg s}^{-1}$, is likely an intrinsic property of the sGRB central engine and that sGRB jets are typically narrow with opening angles $θ_0 \approx 0.1$. We perform Monte Carlo simulations to examine models for the structure and efficiency of the prompt emission in off axis sGRBs. We find that only a small fraction ($\sim 0.01-0.1$) of NS mergers detectable by LIGO/VIRGO in GWs is expected to be also detected in prompt $γ$-rays and that GW170817-like events are very rare. For a NS merger rate of $\sim 1500$ Gpc$^{-3}$ yr$^{-1}$, as inferred from GW170817, we expect within the next decade up to $\sim 12$ joint detections with off-axis GRBs for structured jet models and just $\sim 1$ for quasi-spherical cocoon models where $γ$-rays are the result of shock breakout. Given several joint detections and the rates of their discoveries, the different structure models can be distinguished. In addition the existence of a cocoon with a reservoir of thermal energy may be observed directly in the UV, given a sufficiently rapid localisation of the GW source.

astro-ph.HE↗

arXiv2018

Observational constraints on the structure of gamma-ray burst jets

Motivated by GW170817 we examine constraints that observations put on the angular structure of long gamma-ray burst (GRB) jets. First, the relatively narrow observed distribution of $E_{X}/E_γ$ (the isotropic equivalent early X-ray afterglow to prompt $γ$-ray energy ratio) implies that at any angle that $γ$-rays are emitted the Lorentz factor must be high. Specifically, the Lorentz factor of $γ$-ray emitting material cannot drop rapidly with angle, and must be $Γ(θ)\gtrsim 50$ even if there are angles for which the gamma-ray received energy is lower by three orders of magnitude compared to the jet core. Second, jets with an angular structure of the $γ$-ray emission that over-produce events with a $γ$-ray luminosity below the peak of the observed luminosity function are ruled-out. This eliminates models in which the $γ$-ray energy angular distribution isn't sufficiently steep and the Lorentz factor distribution isn't sufficiently shallow. Finally, models with a steep structure (e.g. Gaussian) which are detected away from the jet core generate afterglow light-curves that were never observed. We conclude that even if the jet kinetic energy distribution drops continuously with latitude, efficient $γ$-ray emission seems to be restricted to material with $Γ\gtrsim 50$ and is most likely confined to a narrow region around the core. While our study is confined to long GRBs, where the observed sample is larger and more complete, there are indications that similar conclusions may be applicable also to short GRBs. We discuss the possible implications to the $γ$-rays observed in GRB 170817A.

astro-ph.HE↗

arXiv2018

Dark Passengers in Stellar Surveys

We develop stellar population models to predict the number of binaries with a single luminous member is Gaia and Hipparcos. Our models yield dozens of detections of black hole - luminous companion binaries (BHLC) and hundreds to thousands of neutron star - luminous companion binaries (NSLC) with Gaia. Interestingly, our models also yield a single detection of BHLC binary with Hipparcos, and a few NSLC binaries. We also show how the statistical distribution of detected binaries with a single luminous companion can be used to constrain the formation process of neutron stars and black holes.

astro-ph.SR↗

arXiv2018

Retainment of r-process material in dwarf galaxies

The synthesis of $r$-process elements is known to involve extremely energetic explosions. At the same time, recent observations find significant $r$-process enrichment even in extremely small ultra-faint dwarf (UFD) galaxies. This raises the question of retainment of those elements within their hosts. We estimate the retainment fraction and find that it is large $\sim 0.9$, unless the $r$-process event is very energetic ($\gtrsim 10^{52}$erg) and / or the host has lost a large fraction of its gas prior to the event. We estimate the $r$-process mass per event and rate as implied by abundances in UFDs, taking into account imperfect retainment and different models of UFD evolution. The results are consistent with previous estimates (Beniamini et al. 2016) and with the constraints from the recently detected macronova accompanying a neutron star merger (GW170817). We also estimate the distribution of abundances predicted by these models. We find that $\sim 0.07$ of UFDs should have $r$-process enrichment. The results are consistent with both the mean values and the fluctuations of [Eu/Fe] in galactic metal poor stars, supporting the possibility that UFDs are the main 'building blocks' of the galactic halo population.

astro-ph.HE↗

arXiv2018

The Mouse Pulsar Wind Nebula

The young energetic pulsar J1747-2958 ($τ=26$ kyr, $\dot{E}=2.5\times 10^{36}$ erg s$^{-1}$) powers the Mouse pulsar wind nebula (PWN), famous for its spectacular tail spanning 45$"$ in X-rays and 12$'$ in radio ($d\sim5$ kpc). We present the results of Chandra observations of the PWN and the analysis of archival lower-frequency data. The Chandra HRC image reveals a point-like source at the pulsar position, $\approx$1$'$ behind the bow shock apex of the PWN. The flattened appearance of the compact nebula is consistent with an equatorial outflow deformed by the ram pressure, implying that the angle between the pulsar's spin axis and line of sight is $\sim70^\circ$ (in agreement with the radio and $γ$-ray pulse profiles). The spatially-resolved spectroscopy with Chandra ACIS shows that the power-law (PL) spectrum steepens from $Γ=1.65\pm0.02$ to $3.0\pm0.1$ over the 45$'$ extent of the X-ray tail. However, the tail's X-ray spectrum integrated over its 45$"$ length fits a single absorbed PL with $Γ=2.09\pm0.03$. We also found the Mouse PWN in 150 MHz GMRT data, and a possible counterpart in 24 ${\rm μ}$m Spitzer data. The multiwavelength data suggest that, at low frequencies, the spectrum of the X-ray-emitting tail region can be described by a broken PL with at least one turnover between radio and X-rays. This is consistent with synchrotron cooling of electrons injected at the termination shock (with an SED slope of 2.2) in an equipartition magnetic field $B\sim200$ ${\rm μG}$ and a bulk flow speed $v\sim4000$ km s$^{-1}$.

astro-ph.HE↗

arXiv2018

Compton echoes from nearby Gamma-Ray Bursts

The recent discovery of gravitational waves from GW170817, associated with a short Gamma-Ray Burst (GRB) at a distance of $40$Mpc, has demonstrated that short GRBs can occur locally and at a reasonable rate. Furthermore, gravitational waves enable us to detect close by GRBs, even when we are observing at latitudes far from the jet's axis. We consider here Compton echoes, the scattered light from the prompt and afterglow emission. Compton echoes, an as yet undetected counterpart of GRBs, peak in X-rays and maintain a roughly constant flux for hundreds to thousands of years after the burst. Though too faint to be detected in typical cosmological GRBs, a fraction of close by bursts with a sufficiently large energy output in X-rays, and for which the surrounding medium is sufficiently dense, may indeed be observed in this way. The detection of a Compton echo could provide unique insight into the burst properties and the environment's density structure. In particular, it could potentially determine whether or not there was a successful jet that broke through the compact binary merger ejecta. We discuss here the properties and expectations from Compton echoes and suggest methods for detectability.

astro-ph.HE↗

arXiv2018

Effects of Fall-Back Accretion on Proto-Magnetar Outflows in Gamma-Ray Bursts and Superluminous Supernovae

Rapidly spinning, strongly magnetized proto-neutron stars ("millisecond proto-magnetars") are candidate central engines of long-duration gamma-ray bursts (GRB), superluminous supernovae (SLSNe), and binary neutron star mergers. Magnetar birth may be accompanied by the fall-back of stellar debris, lasting for seconds or longer following the explosion. Accretion alters the magnetar evolution by (1) providing an additional source of rotational energy (or a potential sink, if the propeller mechanism operates); (2) enhancing the spin-down luminosity above the dipole rate by compressing the magnetosphere and expanding the polar cap region of open magnetic field lines; (3) supplying an additional accretion-powered neutrino luminosity that sustains the wind baryon-loading, even after the magnetar's internal neutrino luminosity has subsided. The more complex evolution of the jet power and magnetization of an accreting magnetar more readily accounts for the high 56Ni yields GRB SNe and irregular time evolution of some GRB light curves (e.g.~bursts with precursors followed by a long quiescent interval before the main emission episode). Additional baryon-loading from accretion-powered neutrino irradiation of the magnetar polar cap lengthens the timeframe over which the jet magnetization is in the requisite range sigma ~< 1e3 for efficient gamma-ray emission, thereby accommodating GRBs with ultra-long durations. Though accretion does not significantly raise the maximum energy budget from the limit of <~ few 1e52 ergs for an isolated magnetar, it greatly expands the range of magnetic field strengths and birth spin periods capable of powering GRB jets, reducing the differences between the magnetar properties normally invoked to explain GRBs versus SLSNe.

astro-ph.HE↗

arXiv2018

Marginally fast cooling synchrotron models for prompt GRBs

Previous studies have considered synchrotron as the emission mechanism for prompt Gamma-Ray Bursts (GRBs). These works have shown that the electrons must cool on a timescale comparable to the dynamic time at the source in order to satisfy spectral constraints while maintaining high radiative efficiency. We focus on conditions where synchrotron cooling is balanced by a continuous source of heating, and in which these constraints are naturally satisfied. Assuming that a majority of the electrons in the emitting region are contributing to the observed peak, we find that the energy per electron has to be $E\gtrsim 20$ GeV and that the Lorentz factor of the emitting material has to be very large $10^3\lesssim Γ_{\rm em} \lesssim 10^4$, well in excess of the bulk Lorentz factor of the jet inferred from GRB afterglows. A number of independent constraints then indicate that the emitters must be moving relativistically, with $Γ'\approx 10$, relative to the bulk frame of the jet and that the jet must be highly magnetized upstream of the emission region, $σ_{\rm up}\gtrsim 30$. The emission radius is also strongly constrained in this model to $R\gtrsim 10^{16}$cm. These values are consistent with magnetic jet models where the dissipation is driven by magnetic reconnection that takes place far away from the base of the jet.

astro-ph.HE↗

arXiv2018

Neutron Star Mergers as sites of r-process Nucleosynthesis and Short Gamma-Ray Bursts

Neutron star mergers have been long considered as promising sites of heavy $r$-process nucleosynthesis. We overview observational evidence supporting this scenario including: the total amount of $r$-process elements in the Galaxy, extreme metal poor stars, geological radioactive elemental abundances, dwarf galaxies, and short gamma-ray bursts (sGRBs). Recently, the advanced LIGO and Virgo observatories discovered a gravitational-wave signal of a neutron star merger, GW170817, as well as accompanying multi-wavelength electromagnetic (EM) counterparts. The ultra-violet, optical, and near infrared observations point to $r$-process elements that have been synthesized in the merger ejecta. The rate and ejected mass inferred from GW170817 and the EM counterparts are consistent with other observations. We find however that, within simple one zone chemical evolution models (based on merger rates with reasonable delay time distributions as expected from evolutionary models, or from observations of sGRBs), it is difficult to reconcile the current observations of the europium abundance history of Galactic stars for [Fe/H] $\gtrsim -1$. This implies that to account for the role of mergers in the Galactic chemical evolution, we need a Galactic model with multiple populations that have different spatial distributions and/or varying formation rates.

astro-ph.HE↗

arXiv2017

Electrons' energy in GRB afterglows implied by radio peaks

Gamma-ray burst (GRB) afterglows have been observed across the electromagnetic spectrum, and physical parameters of GRB jets and their surroundings have been derived using broadband modeling. While well-sampled lightcurves across the broadband spectrum are necessary to constrain all the physical parameters, some can be strongly constrained by the right combination of just a few observables, almost independently of the other unknowns. We present a method involving the peaks of radio lightcurves to constrain the fraction of shock energy that resides in electrons, $ε_e$. This parameter is an important ingredient for understanding the microphysics of relativistic shocks; Based on a sample of 36 radio afterglows, we find $ε_e$ has a narrow distribution centered around $0.13-0.15$. Our method is suggested as a diagnostic tool for determining $ε_e$, and to help constrain the broadband modeling of GRB afterglows. Some earlier measurements of the spreads in parameter values for $ε_e$, the kinetic energy of the shock, and the density of the circumburst medium, based on broadband modeling across the entire spectrum, are at odds with our analysis of radio peaks. This could be due to different modeling methods and assumptions, and possibly missing ingredients in past and current modeling efforts. Furthermore, we show that observations at $\gtrsim10$~GHz performed $0.3-30$ days after the GRB trigger, are best suited for pinpointing the synchrotron peak frequency, and consequently $ε_e$. At the same time, observations at lower radio frequencies can pin down the synchrotron self-absorption frequency and help constrain the other physical parameters of GRB afterglows.

astro-ph.HE↗

arXiv2017

Constraints on millisecond magnetars as the engines of prompt emission in gamma-ray bursts

We examine millisecond magnetars as central engines of Gamma Ray Bursts' (GRB) prompt emission. Using the proto-magnetar wind model of Metzger et al. 2011, we estimate the temporal evolution of the magnetization and power injection at the base of the GRB jet and apply these to different prompt emission models to make predictions for the GRB energetics, spectra and lightcurves. We investigate both shock and magnetic reconnection models for the particle acceleration, as well as the effects of energy dissipation across optically thick and thin regions of the jet. The magnetization at the base of the jet, $σ_0$, is the main parameter driving the GRB evolution in the magnetar model and the emission is typically released for $100\lesssim σ_0 \lesssim 3000$. Given the rapid increase in $σ_0$ as the proto-magnetar cools and its neutrino-driven mass loss subsides, the GRB duration is typically limited to $\lesssim 100$ s. This low baryon loading at late times challenges magnetar models for ultra-long GRBs, though black hole models likely run into similar difficulties without substantial entrainment from the jet walls. The maximum radiated gamma-ray energy is $\lesssim 5 \times 10^{51}$erg, significantly less than the magnetar's total initial rotational energy and in strong tension with the high end of the observed GRB energy distribution. However, the gradual magnetic dissipation model (Beniamini & Giannios 2017) applied to a magnetar central engine, naturally explains several key observables of typical GRBs, including energetics, durations, stable peak energies, spectral slopes and a hard to soft evolution during the burst.

astro-ph.HE↗

arXiv2017

Supernovae generated High Velocity Compact Clouds

It has been proposed that an intermediate mass black hole can produce compact molecular clouds with large velocity dispersions. This model is called into question due to the discovery of several such short lived clouds, which suggests they are common, and hence it is unlikely that their formation involves an exotic object such as an intermediate mass black hole. In this paper we propose an alternative model, where supernovae produce such clouds. We apply our model to a such a cloud called CO-0.40-0.22. A previous work pointed out the fact that the kinetic energy of this cloud is comparable to that from a supernova, but disqualified the supernova scenario due to lack of thermal emission. We explain the lack of thermal emission as a consequence of the large density of the medium. In such a medium, the supernova shock is radiation dominated, and hence the initial temperatures are lower by many orders of magnitude compared with a typical supernova. Finally, we discuss some of the consequences of having a significant population of intermediate mass black holes in the galactic centre.

astro-ph.HE↗