arXiv subjects

Dafne Guetta

Publications and source records attributed to Dafne Guetta.

At least 73 records · Page 4Linked to original sources

arXiv2006

The redshift distribution of short gamma-ray bursts from dynamically formed neutron star binaries

Short-hard gamma-ray bursts (SHBs) may arise from gravitational wave (GW) driven mergers of double neutron star (DNS) systems. DNSs may be "primordial" or can form dynamically by binary exchange interactions in globular clusters during core-collapse. For primordial binaries, the time delay between formation and merger is expected to be short, tau~0.1 Gyr, implying that the redshift distribution of merger events should follow that of star-formation. We point out here that for dynamically formed DNSs, the time delay between star-formation and merger is dominated by the cluster core-collapse time, rather than by the GW inspiral time, yielding delays comparable to the Hubble time. We derive the redshift distribution of merger events of dynamically formed DNSs, and find it to differ significantly from that typically expected for primordial binaries. The observed redshift distribution of SHBs favors dynamical formation, although a primordial origin cannot be ruled out due to possible detection biases. Future red-shift observations of SHBs may allow to determine whether they are dominated by primordial or dynamically formed DNSs.

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arXiv2006

The rate and luminosity function of Short GRBs

We compare the luminosity function and rate inferred from the BATSE short hard bursts (SHBs) peak flux distribution with the redshift and luminosity distributions of SHBs observed by Swift/HETE II. The Swift/HETE II SHB sample is incompatible with SHB population that follows the star formation rate. However, it is compatible with a distribution of delay times after the SFR. This would be the case if SHBs are associated with binary neutron star mergers. The implied SHB rates that we find range from \sim 8 to \sim 30h_{70}^3Gpc^{-3}yr^{-1}. This rate is a much higher than what was previously estimated and, when beaming is taken into account, it is comparable to the rate of neutron star mergers estimated from statistics of binary pulsars. If GRBs are produced in mergers the implied rate practically guarantees detection by LIGO II and possibly even by LIGO I, if we are lucky. Our analysis, which is based on observed short hard burst is limited to bursts with luminosities above 10^{49}erg/sec. Weaker bursts may exist but if so they are hardly detected by BATSE or Swift and hence their rate is very weakly constrained by current observations. Thus the rate of mergers that lead to a detection of a gravitational radiation signal might be even higher.

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arXiv2006

The BATSE-Swift luminosity and redshift distributions of short-duration GRBs

We compare the luminosity function and rate inferred from the BATSE short hard bursts (SHBs) peak flux distribution with the redshift and luminosity distributions of SHBs observed by Swift/HETE II. While the Swift/HETE II SHB sample is incompatible with SHB population that follows the star formation rate, it is compatible with a SHB rate that reflect a distribution of delay times after the SFR. This would be the case if SHBs are associated with binary neutron star mergers. The available data allows, however, different interpretations. For example, a population whose rate is independent of the redshift fits the data very well. The implied SHB rates that we find range from $\sim 8$ to $\sim 30h_{70}^3$Gpc$^{-3}$yr$^{-1}$. This is a much higher rate than what was previously estimated. A detailed (2 dimensional) look at the best fit models shows, however, some discrepancy between the four Swift/HETE II SHBs and the models based on BATSE SHBs. This could be a statistical fluke. It could also arise from wrong estimates of the triggering criteria or from selection effects. If real it may indicate the existence of two SHB populations with different luminosity functions and redshift distributions.

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arXiv2005

The Luminosity and Redshift Distributions of Short-Duration GRB

Using the BATSE peak flux distribution we rederive the short GRBs luminosity function and compare it with the observed redshift distribution of long bursts. We show that both distributions are compatible with the assumption that short as well as long bursts follow the star formation rate. In this case the difference between the two observed distributions can be interpreted as arising mostly from differences in the detector's sensitivity to long and short bursts, while the local rate of short bursts is 0.1 h_70^3 Gpc^-3 yr^-1 . We also consider the possibility that short GRBs may be associated with binary neutron star mergers and estimate the effect of the merging time delay on the luminosity function and redshift distribution. We find that in this case the local rate of short GRBs is ~0.8 h_70^3 Gpc^-3 yr^-1. Assuming that all binary merging systems lead to short GRBs, we find a typical jet opening angle of 1.6^o

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arXiv2005

Constraining the Structure of GRB Jets Through the log(N)-log(S) Distribution

A general formalism is developed for calculating the luminosity function and the expected number $N$ of observed GRBs above a peak photon flux $S$ for any GRB jet structure. This new formalism directly provides the true GRB rate without the need for a `correction factor'. We apply it to the uniform jet (UJ) and universal structured jet (USJ) models for the structure of GRB jets and perform fits to the observed log(N)-log(S) distribution from the GUSBAD catalog which contains 2204 BATSE bursts. A core angle $θ_c$ and an outer edge at $θ_{max}$ are introduced for the structured jet, and a finite range of half-opening angles $θ_{min}\leqθ_j\leqθ_{max}$ is assumed for the uniform jets. The efficiency $ε_γ$ for producing gamma-rays, and the energy per solid angle $ε$ in the jet are allowed to vary with $θ_j$ (the viewing angle $θ_{obs}$) in the UJ (USJ) model, $ε_γ\proptoθ^{-b}$ and $ε\proptoθ^{-a}$. We find that a single power-law luminosity function provides a good fit to the data. Such a luminosity function arises naturally in the USJ model, while in the UJ model it implies a power-law probability distribution for $θ_j$, $P(θ_j)\proptoθ_j^{-q}$. The value of $q$ cannot be directly determined from the fit to the observed log(N)-log(S) distribution, and an additional assumption on the value of $a$ or $b$ is required. Alternatively, an independent estimate of the true GRB rate would enable one to determine $a$, $b$ and $q$. The implied values of $θ_c$ (or $θ_{min}$) and $θ_{max}$ are close to the current observational limits. The true GRB rate for the USJ model is found to be $R_{GRB}(z=0)=0.86^{+0.14}_{-0.05} Gpc^{-3} yr^{-1}$.

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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.

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arXiv2004

Testing the Predictions of the Universal Structured GRB Jet Model

The two leading models for the structure of GRB jets are the uniform jet model and the universal structured jet (USJ) model. In the latter, all GRB jets are intrinsically identical and the energy per solid angle drops as the inverse square of the angle from the jet axis. The simplicity of the USJ model gives it a strong predictive power, including a specific prediction for the observed GRB distribution as a function of both the redshift $z$ and the viewing angle $θ$. We show that the current sample of GRBs with known $z$ and estimated $θ$ does not agree with the predictions of the USJ model. This can be best seen for a relatively narrow range in $z$, in which the USJ model predicts that most GRBs should be near the upper end of the observed range in $θ$, while in the observed sample most GRBs are near the lower end of that range. Since the current sample is very inhomogeneous (i.e. involves many different detectors), it should be taken with care and cannot be used to rule out the USJ model. Nevertheless, this sample strongly disfavors the USJ model. Comparing the prediction for the observed GRB distribution both in $θ$ and in $z$, with a larger and more homogeneous GRB sample, like the one expected from Swift, would either clearly rule out the USJ model, or alternatively, provide a strong support for it. The test presented here is general, and can be used to test any model that predicts both a luminosity function and a luminosity-angle relation.

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arXiv2004

Internal shocks and the blazar sequence. Low and intermediate power BL Lac objects

We consider internal shocks as the main dissipation mechanism responsible for the emission in blazars and show that it can satisfactorily account for the properties of all blazars. In particular, we extend previous work (Spada et al. 2001) on powerful objects, to intermediate (BL Lac) and low power sources (Mkn 421), in order to reproduce the whole of the blazar sequence. The model self-consistently treats the dynamics, spectral emission and its variability. The key parameters driving the phenomenological sequence are the jet power and the properties of the broad line region, which regulate the cooling efficiency of the emitting particles and thus the shape of the spectral energy distribution. By assuming that the remaining parameters are similar for all objects it has been possible to reproduce the full range of the observed spectral "states". A more detailed comparison of the variability properties shows (for Mkn 421) a good agreement in the X-ray band, while in the optical the simulated flux appears to be too variable. For BL Lac lags (~10 days) are predicted between the gamma--rays and the infrared emission

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arXiv2003

Explaining the High Energy Spectral Component in GRB 941017

The gamma-ray burst (GRB) of October 17, 1994 (941017), showed a distinct high energy spectral component extending from $\lesssim$ a few MeV to $\gtrsim$ 200 MeV, in addition to the typical GRB emission which peaked at $\lesssim$ a few hundred keV. The high energy component carried at least $\sim$ 3 times more energy than the lower energy component. It displayed an almost constant flux with a rather hard spectrum ($F_ν\propto ν^{-α}$ with $α\sim 0$) from $\lesssim$ 20 s into the burst up to $\sim$ 200 s, while the duration of the GRB, where 90% of the energy in the lower energy component was emitted, was only 77 s. Such a high energy component was seen in only one out of $\sim$ 30 GRBs in which a similar component could have been detected, and thus appears to be quite rare. We examine possible explanations for this high energy spectral component and find that most models fail. The only emission region that provides the right temporal behavior is the reverse shock that goes into the GRB ejecta as it is decelerated by the ambient medium, or possibly the very early forward shock while the reverse shock is still going on. The best candidate for the emission mechanism is synchrotron self-Compton emission from the reverse shock. Even in this model the most natural spectral slope is only marginally consistent with the observed value, and some degree of fine tuning is required in order to improve the agreement. This might suggest that an additional or alternative emission mechanism is at work here. A prediction of this interpretation is that such a high energy component should be accompanied by a bright optical transient, similar to the one observed in GRB 990123.

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arXiv2003

Neutrinos from Gamma-Ray Bursts in Pulsar Wind Bubbles: \sim 10^{16} eV

The supranova model for Gamma-Ray Bursts (GRBs) is becoming increasingly more popular. In this scenario the GRB occurs weeks to years after a supernova explosion, and is located inside a pulsar wind bubble (PWB). Protons accelerated in the internal shocks that emit the GRB may interact with the external PWB photons producing pions which decay into \sim 10^{16} eV neutrinos. A km^2 neutrino detector would observe several events per year correlated with the GRBs.

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arXiv2003

Neutrinos from Pulsar Wind Bubbles as Precursors to Gamma-Ray Bursts

The supranova model for gamma-ray bursts (GRBs) has recently gained popularity. In this scenario the GRB occurs weeks to years after a supernova explosion, and is located inside a pulsar wind bubble (PWB). High energy protons from the PWB can interact with photons from the rich radiation field inside the PWB or collide with cold protons from the supernova remnant, producing pions which decay into \sim 10-10^3 TeV neutrinos. The predicted neutrino flux from the PWBs that host the GRBs should be easily detectable by planned 1 km^2 detectors.

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arXiv2003

On the generation of UHECRs in GRBs: a reappraisal

We re-examine critically the arguments raised against the theory that Ultra High Energy Cosmic Rays observed at Earth are produced in Gamma Ray Bursts. These include the limitations to the highest energy attainable by protons around the bursts' shocks, the spectral slope at the highest energies, the total energy released in non--thermal particles, the occurrence of doublets and triplets in the data reported by AGASA. We show that, to within the uncertainties in our current knowledge of GRBs, none of these objections is really fatal to the scenario. In particular, we show that the total energy budget of GRBs easily accounts for the energy injection rate necessary to account for UHECRs as observed at Earth. We also compute the expected particle spectrum at Earth, showing that it fits the HiRes and AGASA data to within statistical uncertainties. We consider the existence of multiplets in AGASA' data. To this end, we present a Langevin--like treatment for the motion of a charged particle in the IGM magnetic field, which allows us to estimate both the average and the rms timedelay for particles of given energy; we discuss when particles of identical energies reach the Earth in bunches, or spread over the rms timedelay, showing that multiplets pose no problem for an explosive model for the sources of UHECRs. We compare our model with a scenario where the particles are accelerated at internal shocks, underlining differences and advantages of particle acceleration at external shocks.

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arXiv2003

Gamma Ray Bursts in Pulsar Wind Bubbles: Observational implications

We present the main observational features expected for Gamma-Ray Bursts (GRBs) that occur inside pulsar wind bubbles (PWBs). This is the most natural outcome of the supranova model, where initially a supernova (SN) explosion takes place, leaving behind a supra-massive neutron star, which loses its rotational energy over a time t_sd and collapses to a black hole, triggering a GRB. We find that the time delay, t_sd, between the SN and GRB events is the most important parameter that determines the behavior of the system. We consider the afterglow, prompt GRB and PWB emission. Constraints on the model are derived for a spherical PWB, from current afterglow observations and the lack of direct detection of the PWB emission. We find that a simple spherical model cannot account for the X-ray features detected in some afterglows, together with the typical afterglow emission that was observed for the same GRBs. The discrepancies with observations may be reconciled by resorting to a non-spherical geometry, where the PWB is elongated along the polar axis. Finally, we predict that the inverse Compton upscattering of PWB photons by the relativistic electrons of the afterglow (external Compton) should lead to high energy emission during the early afterglow that may explain the GeV photons detected by EGRET for a few GRBs, and should be detectable by future missions such as GLAST.

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arXiv2002

Observational Implications of a Plerionic Environment for GRBs

We consider GRBs that occur inside pulsar wind bubbles (PWBs), in the context of the supranova model, where initially a supernova explosion takes place, leaving behind a supra-massive neutron star (SMNS), which eventually loses its rotational energy and collapses to a black hole, triggering a GRB explosion. The most natural mechanism by which the SMNS can lose its rotational energy is through a strong pulsar type wind, between the supernova and the GRB events, which is expected to create a PWB. We analyze in some detail the observational implications of such a plerionic environment on the afterglow (AG) and prompt GRB emissions, and the prospect for direct detection of the plerion emission. We find that for a simple spherical model, GRBs with iron lines detected in the X-ray AG should not have a detectable radio afterglow, and should have small jet break times and non-relativistic transition times, in disagreement with observations for some of the GRBs with X-ray lines. These discrepancies with the observations may be reconciled by resorting to a non-spherical geometry, where the PWB is elongated along the polar axis. We find that lack of detection of the plerion emission provides interesting constraints on the model parameters. Finally, we predict that the inverse Compton upscattering of the PWB photons by the relativistic electrons of the AG (external Compton, EC) should lead to high energy emission during the early AG that might explain the GeV photons detected by EGRET for a few GRBs, and should be detectable by future missions such as GLAST.

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arXiv2002

Precursor Plerionic Activity and High Energy Gamma-Ray Emission in the Supranova Model of Gamma-Ray Bursts

The supranova model of gamma-ray bursts (GRBs), in which the GRB event is preceded by a supernova (SN) explosion by a few months to years, has recently gained support from Fe line detections in X-ray afterglows. A crucial ingredient of this model yet to be studied is the fast-rotating pulsar that should be active during the time interval between the SN and the GRB, driving a powerful wind and a luminous plerionic nebula. We discuss some observational consequences of this precursor plerion, which should provide important tests for the supranova model: 1) the fragmentation of the outlying SN ejecta material by the plerion and its implications for Fe line emission; and 2) the effect of inverse Compton cooling and emission in the GRB external shock due to the plerion radiation field. The plerion-induced inverse Compton emission can dominate in the GeV-TeV energy range during the afterglow, being detectable by GLAST from redshifts $z \lesssim 1.5$ and distinguishable from self-Compton emission by its spectrum and light curve. The prospects for direct detection and identification of the precursor plerion emission are also briefly considered.

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arXiv2002

Gamma-Ray Bursts in Pulsar Wind Bubbles: Putting the Pieces Together

We present the main observational features expected for GRBs that occur inside pulsar wind bubbles (PWBs). This is the most natural outcome of supranova model where initially a supernova explosion takes place, leaving a supra-massive NS, which loses its rotational energy over a time t_sd and collapses to a BH, triggering a GRB explosion. We find that the time delay t_sd between the supernova and GRB events is the most important parameter that determines the behavior of the system. We consider the afterglow, prompt GRB and direct emission from the PWB. The observational signatures for different t_sd are described and joined together into one coherent framework. Constraints on the model are derived for a spherical PWB, from the lack of detection of the PWB emission and afterglow observations. For very low values of t_sd<1 hr the supranova model reduces to the collapsar model; 0.4<t_sd/1yr<1 is required to produce the iron lines seen in some X-ray afterglows. However, we find that for a spherical PWB, this implies no detectable radio afterglow, a small jet break time and non-relativistic transition time, in disagreement with observations for some of these GRBs. These discrepancies with the observations may be reconciled by resorting to a non-spherical geometry. We find that light element lines, recently detected in a few X-ray afterglows, are expected to dominate over iron lines for small t_sd, while for large t_sd the situation is reversed. Finally, we predict that external Compton scattering of the PWB photons by the afterglow relativistic electrons, should lead to high energy emission during the early afterglow that may explain the GeV photons detected by EGRET in some GRBs, and should be detectable by future missions (GLAST).

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arXiv2002

High Energy Emission from the Prompt Gamma-Ray Burst

We study the synchrotron and synchrotron self-Compton (SSC) emission from internal shocks that are responsible for the prompt gamma-ray emission in GRBs, and consider the relation between these two components, taking into account the high energy (HE) cutoff due to pair production and Thomson scattering. We find that in order for the peak energy of the synchrotron to be E_p\sim 300 keV with a variability time t_v>1 ms, a Lorentz factor Γ<350 is needed, implying no HE emission above \sim 30 MeV and the synchrotron component would dominate at all energies. If we want both E_p\sim 300 keV and prompt HE emission up to 2 GeV, as detected by EGRET for GRB 940217, we need Γ\sim 600 and t_v\sim 0.1 ms, which might be resolved by super AGILE. If such prompt HE emission is common in GRBs, as may be tested by GLAST, then for t_v\gtrsim 1 ms we need Γ\gtrsim 350, which implies E_p\lesssim 100 keV. Therefore if X-ray flashes are GRBs with high values of t_v and Γ, they should produce \gtrsim 1 GeV emission. For an electron power law index p>2, the SSC component dominates the emission above 100 MeV. Future observations by GLAST may help determine the value of p and whether the HE emission is consistent with a single power law (one component--the synchrotron, dominates) or has a break where the \nuF_νslope turns from negative to positive, implying that the SSC component becomes dominant above \sim 100 MeV. The HE emission is expected to show similar variability and time structure to that of the soft gamma-ray emission. Finally, we find that in order to see delayed HE emission from the prompt GRB due to pair production with the cosmic IR background, extremely small intergalactic magnetic fields (\lessim 10^{-22} G) are required.

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arXiv2002

Neutrino flux predictions for Galactic plerions

We consider a class of plerions that have been detected in the TeV range, and investigate the possibility that the emission is due to π^0 decay. From the TeV flux we derive what is the expected νflux at Earth and find that this is likely to exceed the detection threshold of the upcoming km^2 neutrino detectors.

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