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Tsvi Piran

Publications and source records attributed to Tsvi Piran.

At least 217 records · Page 12Linked to original sources
arXiv2007

A Canonical High Energy Afterglow Emission Light Curve?

We present self consistent calculations of Synchrotron self Compton (SSC) radiation that takes place within the afterglow blast wave and External inverse Compton (EIC) radiation that takes place when flare photons (produced by an internal process) pass through the blast wave. We show that if our current interpretations of the Swift XRT data are correct, there should be a canonical high energy afterglow emission light curve. We expect that GRBs with a long term X-ray flattening or X-ray flares should show similar high energy features. The EIC emission, however, is long lasting and weak and might be outshined by the SSC emission of the forward shock. The high energy emission could be well detected by the soon to be launched GLAST satellite. Its detection could shed new light on the conditions within the emitting regions of GRBs.

astro-ph↗
arXiv2007

Do long-duration GRBs follow star formation?

We compare the luminosity function and rate inferred from the BATSE long bursts peak flux distribution with those inferred from the Swift peak flux distribution. We find that both the BATSE and the Swift peak fluxes can be fitted by the same luminosity function and the two samples are compatible with a population that follows the star formation rate. The estimated local long GRB rate (without beaming corrections) varies by a factor of five from 0.05 Gpc^(-3)yr^(-1) for a rate function that has a large fraction of high redshift bursts to 0.27 Gpc^(-3)yr^(-1) for a rate function that has many local ones. We then turn to compare the BeppoSax/HETE2 and the Swift observed redshift distributions and compare them with the predictions of the luminosity function found. We find that the discrepancy between the BeppoSax/HETE2 and Swift observed redshift distributions is only partially explained by the different thresholds of the detectors and it may indicate strong selection effects. After trying different forms of the star formation rate (SFR) we find that the observed Swift redshift distribution, with more observed high redshift bursts than expected, is inconsistent with a GRB rate that simply follows current models for the SFR. We show that this can be explained by GRB evolution beyond the SFR (more high redshift bursts). Alternatively this can also arise if the luminosity function evolves and earlier bursts were more luminous or if strong selection effects affect the redshift determination.

astro-ph↗
arXiv2006

The Interpretation and Implication of the Afterglow of GRB 060218

The nearby GRB 060216/SN 2006aj was an extremely long, weak and very soft GRB. It was peculiar in many aspects. We show here that the X-ray, ultraviolet/optical and radio afterglow of GRB 060218 have to be attributed to different physical processes arising from different emission regions. From the several components in this burst's afterglow only the radio afterglow can be interpreted in terms of the common external shock model. We infer from the radio that the blast wave's kinetic energy was $\sim 10^{50}$ erg and the circumburst matter had a constant rather than a wind profile. The lack of a "jet break" up to 22 days implies that the outflow was wide $θ_j >1$. Even though the late X-ray afterglow decays normally it cannot result from an external shock because of its very steep spectrum. Furthermore, the implied kinetic energy would have produced far too much radio. We suggest that this X-ray afterglow could be attributed to a continued activity of the central engine that within the collapsar scenario could arise from fall-back accretion. "Central engine afterglow" may be common in under-luminous GRBs where the kinetic energy of the blast wave is small and the external shock does not dominate over this component. Such under-luminous GRBs might be very common but they are rarely recorded because they can be detected only from short distances.

astro-ph↗
arXiv2006

GRBs Neutrinos as a Tool to Explore Quantum Gravity induced Lorentz Violation

Lorentz Invariance Violation (LIV) arises in various quantum-gravity theories. As the typical energy for quantum gravity is the Planck mass, $M_{pl}$, LIV will, most likely, be manifested at very high energies that are not accessible on Earth in the foreseeable future. One has to turn to astronomical observations. Time of flight measurement from different astronomical sources set current limits on the energy scale of possible LIV to $> 0.01 M_{pl}$ (for n=1 models) and $> 10^{-9} M_{pl}$ (for n=2). According to current models Gamma-Ray Bursts (GRBs) are accompanied by bursts of high energy ($\gsim 100$TeV) neutrinos. At this energy range the background level of currently constructed neutrino detectors is so low that a detection of a single neutrino from the direction of a GRB months or even years after the burst would imply an association of the neutrino with the burst and will establish a measurement of a time of flight delay. Such time of flight measurements provide the best way to observe (or set limits) on LIV. Detection of a single GRB neutrino would open a new window on LIV and would improve current limits by many orders of magnitude.

hep-ph↗
arXiv2006

Constraining Lorentz violations with Gamma Ray Bursts

Gamma ray bursts are excellent candidates to constrain physical models which break Lorentz symmetry. We consider deformed dispersion relations which break the boost invariance and lead to an energy-dependent speed of light. In these models, simultaneously emitted photons from cosmological sources reach Earth with a spectral time delay that depends on the symmetry breaking scale. We estimate the possible bounds which can be obtained by comparing the spectral time delays with the time resolution of available telescopes. We discuss the best strategy to reach the strongest bounds. We compute the probability of detecting bursts that improve the current bounds. The results are encouraging. Depending on the model, it is possible to build a detector that within several years will improve the present limits of 0.015 m_pl.

astro-ph↗
arXiv2006

GRB efficiency and Possible Physical Processes Shaping the Early Afterglow

The discovery by Swift that a good fraction of Gamma Ray Bursts (GRBs) have a slowly decaying X-ray afterglow phase led to the suggestion that energy injection into the blast wave takes place several hundred seconds after the burst. This implies that right after the burst the kinetic energy of the blast wave was very low and in turn the efficiency of production of $γ$-rays during the burst was extremely high, rendering the internal shocks model unlikely. We re-examine the estimates of kinetic energy in GRB afterglows and show that the efficiency of converting the kinetic energy into gamma-rays is moderate and does not challenge the standard internal shock model. We also examine several models, including in particular energy injection, suggested to interpret this slow decay phase. We show that with proper parameters, all these models give rise to a slow decline lasting several hours. However, even those models that fit all X-ray observations, and in particular the energy injection model, cannot account self-consistently for both the X-ray and the optical afterglows of well monitored GRBs such as GRB 050319 and GRB 050401. We speculate about a possible alternative resolution of this puzzle.

astro-ph↗
arXiv2006

Sub-GeV flashes in $γ-$ray burst afterglows as probes of underlying bright UV flares

Bright optical and X-ray flares have been observed in many Gamma-ray Burst (GRB) afterglows. These flares have been attributed to late activity of the central engine. In most cases the peak energy is not known and it is possible and even likely that there is a significant far-ultraviolet component. These far-ultraviolet photons escape our detection because they are absorbed by the neutral hydrogen before reaching Earth. However, these photons cross the blast wave produced by the ejecta that have powered the initial GRB. They can be inverse Compton upscattered by hot electrons within this blast wave. This process will produce a strong sub-GeV flare that follows the high energy (soft X-ray) tail of the far-UV flare but lasts much longer and can be detected by the upcoming {\em Gamma-Ray Large Area Telescope} (GLAST) satellite. This signature can be used to probe the spectrum of the underlying far-ultraviolet flare. The extra cooling produced by this inverse Compton process can lower the X-ray emissivity of the forward shock and explain the unexpected low early X-ray flux seen in many GRBs.

astro-ph↗
arXiv2006

Pure and loaded fireballs in SGR giant flares

On December 27, 2004, a giant flare from SGR 1806$-$20 was detected on earth. Its thermal spectrum and temperature suggest that the flare resulted from an energy release of about $10^{47}$ erg/sec close to the surface of a neutron star in the form of radiation and/or pairs. This plasma expanded under its own pressure producing a fireball and the observed gamma-rays escaped once the fireball became optically thin. The giant flare was followed by a bright radio afterglow, with an observable extended size, implying an energetic relativistic outflow. We revisit here the evolution of relativistic fireballs and we calculate the Lorentz factor and energy remaining in relativistic outflow once the radiation escapes. We show that pairs that arise naturally in a pure pairs-radiation fireball do not carry enough energy to account for the observed afterglow. We consider various alternatives and we show that if the relativistic outflow that causes the afterglow is related directly to the prompt flare, then the initial fireball must be loaded by baryons or Poynting flux. While we focus on parameters applicable to the giant flare and the radio afterglow of SGR 1806$-$20 the calculations presented here might be also applicable to GRBs.

astro-ph↗
arXiv2006

Outliers to the Isotropic Energy - Peak Energy Relation in GRBs

The peak energy - isotropic energy (EpEi) relation is among the most intriguing recent discoveries concerning GRBs. It can have numerous implications on our understanding of the emission mechanism of the bursts and on the application of GRBs for cosmological studies. However, this relation was verified only for a small sample of bursts with measured redshifts. We propose here a test whether a burst with an unknown redshift can potentially satisfy the EpEi relation. Applying this test to a large sample of BATSE bursts we find that a significant fraction of those bursts cannot satisfy this relation. Our test is sensitive only to dim and hard bursts and therefore this relation might still hold as an inequality (i.e. there are no intrinsically bright and soft bursts). We conclude that the observed relation seen in the sample of bursts with a known redshift might be influenced by observational biases and from the inability to locate and well localize hard and weak bursts that have only a small number of photons. In particular we point out that the threshold for detection, localization and redshift measurement is essentially higher than the threshold for detection alone. We predict that Swift will detect some hard and weak bursts that would be outliers to the EpEi relation. However, we cannot quantify this prediction. We stress the importance of understanding the detection-localization-redshift threshold for the coming Swift detections.

astro-ph↗
arXiv2006

A remark on ``On the Formation and Progenitor of PSR J0737-3039: New Constraints on the Supernova Explosion Forming Pulsar B" by Willems et al., astro-ph/0602024

Shortly after the discovery of the binary system PSR J0737-3039 we predicted that it has a small proper motion and suggested that this implies that the progenitor mass of the younger pulsar (B) must be around 1.45M_sun. This is in contradiction to standard evolutionary scenarios that suggest a He-star progenitor with M >2.1-2.3M_sun and in sharp contrast to a velocity of ~100 km/s predicted by these models. It requires a new previously unseen type of gravitational collapse. Our (generally ignored) prediction of a low peculiar motion was confirmed later by pulsar timing observations that found v_\bot < 30 km/s. Recently Willems et al. put forward an elaborate simulation suggesting that in spite of these observations, the most likely natal kick is large (50-170 km/s) and the corresponding most likely progenitor mass is large as well. We refute this claim and show that Willems et al. implicitly assume the result that they obtain.

astro-ph↗
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.

astro-ph↗
arXiv2006

GRB 051221A and Tests of Lorentz Symmetry

Various approaches to quantum gravity suggest the possibility of violation of Lorentz symmetry at very high energies. In these cases we expect a modification at low energies of the dispersion relation of photons that contains extra powers of the momentum suppressed by a high energy scale. These terms break boost invariance and can be tested even at relatively low energies. We use the light curves of the very bright short Gamma-Ray Burst GRB 051221A and compare the arrival times of photons at different energies with the expected time delay due to a modified dispersion relation. As no time delay was observed, we set a lower bound of 0.0066 E_{pl} \sim 0.66 10^{17} GeV on the scale of Lorentz invariance violation.

astro-ph↗
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.

astro-ph↗
arXiv2006

Implications of the Early X-Ray Afterglow Light Curves of Swift GRBs

According to current models, gamma-ray bursts (GRBs) are produced when the energy carried by a relativistic outflow is dissipated and converted into radiation. The efficiency of this process, $ε_γ$, is one of the critical factors in any GRB model. The X-ray afterglow light curves of Swift GRBs show an early stage of flat decay. This has been interpreted as reflecting energy injection. When combined with previous estimates, which have concluded that the kinetic energy of the late ($\gtrsim 10$ hr) afterglow is comparable to the energy emitted in gamma-rays, this interpretation implies very high values of $ε_γ\gtrsim 0.9$. Such a high efficiency is hard to reconcile with most models, including in particular the popular internal-shocks model. We re-analyze the derivation of the kinetic energy from the afterglow X-ray flux and re-examine the resulting estimates of the efficiency. We confirm that, if the flatt decay arises from energy injection and the pre-Swift broad-band estimates of the kinetic energy are correct, then $ε_γ\gtrsim 0.9$. We point out, however, that another interpretation of the flattish decay - a variable X-ray afterglow efficiency (e.g., due to a time dependence of afterglow shock microphysical parameters) - is possible. We also show that direct estimates of the kinetic energy from the late X-ray afterglow flux are sensitive to the assumed values of the shock microphysical parameters and suggest that broad-band afterglow fits might have underestimated the kinetic energy (e.g., by overestimating the fraction of electrons that are accelerated to relativistic energies). Either one of these possibilities implies a lower gamma-ray efficiency, and their joint effect could conceivably reduce the estimate of the typical $ε_γ$ to $\sim 0.1-0.5$.

astro-ph↗
arXiv2005

The Evidence of Absence: Galaxy Voids in the Excursion Set Formalism

We present an analytic model for the sizes of voids in the galaxy distribution. Peebles and others have recently emphasized the possibility that the observed characteristics of voids may point to a problem in galaxy formation models, but testing these claims has been difficult without any clear predictions for their properties. In order to address such questions, we build a model to describe the distribution of galaxy underdensities. Our model is based on the "excursion set formalism," the same technique used to predict the dark matter halo mass function. We find that, because of bias, galaxy voids are typically significantly larger than dark matter voids and should fill most of the universe. We show that voids selected from catalogs of luminous galaxies should be larger than those selected from faint galaxies: the characteristic radii range from ~5-10h^{-1} Mpc for galaxies with absolute r-band magnitudes M_r-5 log(h) < -16 to -20. These are reasonably close to, though somewhat smaller than, the observed sizes. The discrepancy may result from the void selection algorithm or from their internal structure. We also compute the halo populations inside voids. We expect small haloes (M<10^{11} Msun) to be up to a factor of two less underdense than the haloes of normal galaxies. Within large voids, the mass function is nearly independent of the size of the underdensity, but finite-size effects play a significant role in small voids (R<7h^{-1} Mpc).

astro-ph↗
arXiv2005

The Origin of the Binary Pulsar J0737-3039

It is generally accepted that neutron stars form in core collapse events that are accompanied by a supernovae (types II or Ib or Ic). Typical progenitors are, therefore, larger than $\sim 2.1 \Ms$. We suggest \cite{PS04,PS05} that the binary pulsar J0737-3039 provides evidence for a new formation channel: collapse of a light progenitor. This binary pulsar J0737-3039 has several remarkable features including among others: a very tight orbit with a Keplerian velocity of ~600km/sec, a low eccentricity, and a location ~50pc from the Galactic plane implying that the system has, at high likelihood, a small (compared to Keplerian) center of mass velocity. A significant mass loss during the formation of the second pulsar would have lead either to an eccentric orbit or to a large center of mass velocity or to both. Therefore, we can set a strong upper limit on the progenitor's mass. A progenitor more massive than 1.9$\Ms$ is ruled out (at 97% confidence). The kinematically favored option is of a progenitor mass around 1.45$\Ms$. Recent evidence for a rather low velocity proper motion supports this prediction and decreases the likelihood of the standard (high mass progenitor) scenario. Lack of variations in the pulses' profiles (which indicate no significant geodetic precession) provides further support for the no kick and low progenitor mass formation scenario.

astro-ph↗
arXiv2005

The distances of short-hard GRBs and the SGR connection

We present a search for nearby (D<100 Mpc) galaxies in the error boxes of six well-localized short-hard gamma-ray bursts (GRBs). None of the six error boxes reveals the presence of a plausible nearby host galaxy. This allows us to set lower limits on the distances and, hence, the isotropic-equivalent energy of these GRBs. Our lower limits are around $1 \times 10^{49}$ erg (at $2σ$ confidence level); as a consequence, some of the short-hard GRBs we examine would have been detected by BATSE out to distances greater than 1 Gpc and therefore constitute a bona fide cosmological population. Our search is partially motivated by the December 27, 2004 hypergiant flare from SGR 1806-20, and the intriguing possibility that short-hard GRBs are extragalactic events of a similar nature. Such events would be detectable with BATSE to a distance of \~50 Mpc, and their detection rate should be comparable to the actual BATSE detection rate of short-hard GRBs. The failure of our search, by contrast, suggests that such flares constitute less than 15% of the short-hard GRBs (<40% at 95% confidence). We discuss possible resolutions of this discrepancy.

astro-ph↗