arXiv subjects

Shiho Kobayashi

Publications and source records attributed to Shiho Kobayashi.

At least 37 records · Page 2Linked to original sources

arXiv2018

First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B

We present detailed multi-wavelength observations of GRB 161219B at $z=0.1475$, spanning the radio to X-ray regimes, and the first ALMA light curve of a GRB afterglow. The cm- and mm-band observations before $8.5$ d require emission in excess of that produced by the afterglow forward shock (FS). These data are consistent with radiation from a refreshed reverse shock (RS) produced by the injection of energy into the FS, signatures of which are also present in the X-ray and optical light curves. We infer a constant-density circumburst environment with an extremely low density, $n_0\approx 3\times10^{-4}$ cm$^{-3}$ and show that this is a characteristic of all strong RS detections to date. The VLA observations exhibit unexpected rapid variability on $\sim$ minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, $t_{\rm jet}\approx32$ day, yielding a wide jet opening angle of $θ_{\rm jet}\approx13^{\circ}$, implying beaming corrected $γ$-ray and kinetic energies of $E_γ\approx4.9\times10^{48}$ erg and $E_{\rm K}\approx1.3\times10^{50}$ erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, $R_{\rm B}\approx1$, compared to the FS. These direct, multi-frequency measurements of a refreshed RS spanning the optical to radio bands highlight the impact of radio and millimeter data in probing the production and nature of GRB jets.

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arXiv2018

GRB 170817A as a jet counterpart to gravitational wave trigger GW 170817

{\it Fermi}/GBM (Gamma-ray Burst Monitor) and INTEGRAL (the International Gamma-ray Astrophysics Laboratory) reported the detection of the $γ$-ray counterpart, GRB 170817A, to the LIGO (Light Interferometer Gravitational-wave Observatory)/{\it Virgo} gravitational wave detected binary neutron star merger, GW 170817. GRB 170817A is likely to have an internal jet or another origin such as cocoon emission, shock-breakout, or a flare from a viscous disc. In this paper, we assume that the $γ$-ray emission is caused by energy dissipation within a relativistic jet and we model the afterglow synchrotron emission from a reverse- and forward-shock in the outflow. We show the afterglow for a low-luminosity $γ$-ray burst (GRB) jet with a high Lorentz-factor ($Γ$); a low-$Γ$ and low-kinetic energy jet; a low-$Γ$, high kinetic energy jet; structured jets viewed at an inclination within the jet-half-opening angle; and an off-axis `typical' GRB jet. All jet models will produce observable afterglows on various timescales. The late-time afterglow from 10-110 days can be fit by a Gaussian structured jet viewed at a moderate inclination, however, the GRB is not directly reproduced by this model. These jet afterglow models can be used for future GW detected NS merger counterparts with a jet afterglow origin.

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arXiv2018

Tidal Disruption of Inclined or Eccentric Binaries by Massive Black Holes

Binary stars that are on close orbits around massive black holes (MBH) such as Sgr A* in the center of the Milky Way are liable to undergo tidal disruption and eject a hypervelocity star. We study the interaction between such a MBH and circular binaries for general binary orientations and penetration depths (i.e. binaries penetrate into the tidal radius around the BH). We show that for very deep penetrators, all binaries are disrupted when the binary rotation axis is roughly oriented toward the BH or it is in the opposite direction. The surviving chance becomes significant when the angle between the binary rotation axis and the BH direction is between π/4 and 3 π/4. The surviving chance is as high as $\sim$ 20$\%$ when the binary rotation axis is perpendicular to the BH direction. The angular dependence is opposite for very shallow penetrators where coplanar prograde orbits have the lowest surviving chance (or equivalently most vulnerable). We provide numerical fits to the disruption probability and energy gain at the the BH encounter as a function of the penetration depth. The latter can be simply rescaled in terms of binary masses, their initial separation and the binary-to-BH mass ratio to evaluate the ejection velocity of a binary members in various systems. We also investigate the disruption of coplanar, eccentric binaries by a MBH. It is shown that for highly eccentric binaries retrograde orbits have a significantly increased disruption probability and ejection velocities compared to the circular binaries.

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arXiv2018

Transient Survey Rates for Orphan Afterglows from Compact Merger Jets

Orphan afterglows from short $γ$-ray bursts (GRB) are potential candidates for electromagnetic (EM) counterpart searches to gravitational wave (GW) detected neutron star or neutron star black hole mergers. Various jet dynamical and structure models have been proposed that can be tested by the detection of a large sample of GW-EM counterparts. We make predictions for the expected rate of optical transients from these jet models for future survey telescopes, without a GW or GRB trigger. A sample of merger jets is generated in the redshift limits $0\leq z\leq 3.0$, and the expected peak $r$-band flux and timescale above the LSST or ZTF detection threshold, $m_r=24.5$ and $20.4$ respectively, is calculated. General all-sky rates are shown for $m_r\leq26.0$ and $m_r\leq21.0$. The detected orphan and GRB afterglow rate depends on jet model, typically $16\lesssim R\lesssim 76$ yr$^{-1}$ for the LSST, and $2\lesssim R \lesssim 8$ yr$^{-1}$ for ZTF. An excess in the rate of orphan afterglows for a survey to a depth of $m_r\leq26$ would indicate that merger jets have a dominant low-Lorentz factor population, or the jets exhibit intrinsic jet structure. Careful filtering of transients is required to successfully identify orphan afterglows from either short or long GRB progenitors.

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arXiv2017

Revealing Short GRB Jet Structure and Dynamics with Gravitational Wave Electromagnetic Counterparts

Compact object mergers are promising candidates for the progenitor system of short gamma-ray bursts (GRBs). Using gravitational wave (GW) triggers to identify a merger, any electromagnetic (EM) counterparts from the jet can be used to constrain the dynamics and structure of short GRB jets. GW triggered searches could reveal a hidden population of optical transients associated with the short-lived jets from the merger object. If the population of merger-jets is dominated by low-Lorentz-factors, then a GW triggered search will reveal the on-axis orphan afterglows from these failed GRBs. By considering the EM counterparts from a jet, with or without the prompt GRB, the jet structure and dynamics can be constrained. By modelling the afterglow of various jet structures with viewing angle, we provide observable predictions for the on- and off- axis EM jet counterparts. The predictions provide an indication for the various features expected from the proposed jet structure models.

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arXiv2017

Electromagnetic Counterparts to Structured Jets from Gravitational Wave Detected Mergers

We show the peak magnitude for orphan afterglows from the jets of gravitational wave (GW) detected black-hole/neutron star - neutron star (BH/NS-NS) mergers highly depends on the jet half-opening angle $θ_j$. Short $γ$-ray bursts (GRB) with a homogeneous jet structure and $θ_j>10^\circ$, the orphan afterglow viewed at the typical inclination for a GW detected event, 38$^\circ$, is brighter at optical frequencies than the comparable macronova emission. Structured jets, where the energetics and Lorentz factor $Γ$ vary with angle from the central axis, may have low-$Γ$ components where the prompt emission is suppressed; GW electromagnetic (EM) counterparts may reveal a population of failed-GRB orphan afterglows. Using a Monte Carlo method assuming a NS-NS detection limit we show the fraction of GW-EM counterparts from homogeneous, two-component, power-law structured, and Gaussian jets where the variable structure models include a wide low energy and $Γ$ component: for homogeneous jets, with a {$θ_j=6^\circ$ and typical short GRB parameters, we find {\it r}-band magnitude $m_r\leq21$ counterparts for $\sim 13.6\%$ of GW detected mergers; where jet structure extends to a half-opening angle of $25^\circ$, two-component jets produce $m_r\leq21$ counterparts in $\sim30\%$ of GW detected mergers; power-law structured jets result in $\sim37\%$; and Gaussian jets with our parameters $\sim13\%$.} We show the features in the lightcurves from orphan afterglows can be used to indicate the presence of extended structure.

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arXiv2017

Extending the "Energetic Scaling of Relativistic Jets From Black Hole Systems" to Include $γ$-ray-loud X-ray Binaries

We show that the jet power $P_j$ and geometrically corrected $γ$-ray luminosity $L_γ$ for the X-ray binaries (XRBs) Cygnus X-1, Cygnus X-3, and V404 Cygni, and $γ$-ray upper limits for GRS 1915+105 and GX339-4, follow the universal scaling for the energetics of relativistic jets from black hole (BH) systems found by Nemmen et al. (2012) for blazars and GRBs. The observed peak $γ$-ray luminosity for XRBs is geometrically corrected; and the minimum jet power is estimated from the peak flux density of radio flares and the flare rise time. The $L_γ-P_j$ correlation holds across $\sim 17$ orders of magnitude. The correlation suggests a jet origin for the high energy emission from X-ray binaries, and indicates a common mechanism or efficiency for the high energy emission 0.1-100 GeV from all relativistic BH systems.

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arXiv2016

Low-Γ jets from Compact Binary Mergers as Candidate Electromagnetic Counterparts to Gravitational Wave Sources

Compact binary mergers, with neutron stars or neutron star and black-hole components, are thought to produce various electromagnetic counterparts: short gamma-ray bursts (GRBs) from ultra-relativistic jets followed by broadband afterglow; semi-isotropic kilonova from radioactive decay of r-process elements; and late time radio flares; etc. If the jets from such mergers follow a similar power-law distribution of Lorentz factors as other astrophysical jets then the population of merger jets will be dominated by low-Γ values. The prompt gamma-rays associated with short GRBs would be suppressed for a low-Γ jet and the jet energy will be released as X-ray/optical/radio transients when a shock forms in the ambient medium. Using Monte Carlo simulations, we study the properties of such transients as candidate electromagnetic counterparts to gravitational wave sources detectable by LIGO/Virgo. Approximately 78% of merger-jets result in failed GRB with optical peaks 14-22 magnitude and an all-sky rate of 2-3 per year.

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arXiv2016

Low Lorentz Factor Jets from Compact Stellar Mergers - Candidate Electromagnetic Counterparts to Gravitational Wave Sources

Short gamma-ray bursts (GRBs) are believed to be produced by relativistic jets from mergers of neutron-stars (NS) or neutron-stars and black-holes (BH). If the Lorentz-factors $Γ$ of jets from compact-stellar-mergers follow a similar power-law distribution to those observed for other high-energy astrophysical phenomena (e.g. blazars, AGN), the population of jets would be dominated by low-$Γ$ outflows. These jets will not produce the prompt gamma-rays, but jet energy will be released as x-ray/optical/radio transients when they collide with the ambient medium. Using Monte Carlo simulations, we study the properties of such transients. Approximately $78\%$ of merger-jets $<300~$Mpc result in failed-GRBs if the jet $Γ$ follows a power-law distribution of index $-1.75$. X-ray/optical transients from failed-GRBs will have broad distributions of their characteristics: light-curves peak $t_p\sim0.1-10~$days after a merger; flux peaks for x-ray $10^{-6}{\rm~mJy}\lesssim~F_x\lesssim10^{-2}~$mJy; and optical flux peaks at $14\lesssim~m_g\lesssim22$. X-ray transients are detectable by Swift XRT, and $\sim85\%$ of optical transients will be detectable by telescopes with limiting magnitude $m_g \gtrsim 21$, for well localized sources on the sky. X-ray/optical transients are followed by radio transients with peak times narrowly clustered around $t_p\sim10~$days, and peak flux of $\sim~10-100~$mJy at 10 GHz and $\sim~0.1~$mJy at 150 MHz. By considering the all-sky rate of short GRBs within the LIGO/Virgo range, the rate of on-axis orphan afterglows from failed-GRB would be 2.6(26) per year for NS-NS(NS-BH) mergers, respectively. Since merger jets from gravitational-wave (GW) trigger events tend to be directed to us, a significant fraction of GW events could be associated with the on-axis orphan afterglow.

astro-ph.HE↗

arXiv2016

A Reverse Shock in GRB 160509A

We present the second multi-frequency radio detection of a reverse shock in a $γ$-ray burst. By combining our extensive radio observations of the Fermi-LAT GRB 160509A at $z = 1.17$ up to $20$ days after the burst with Swift X-ray observations and ground-based optical and near-infrared data, we show that the afterglow emission comprises distinct reverse shock and forward shock contributions: the reverse shock emission dominates in the radio band at $\lesssim10~$days, while the forward shock emission dominates in the X-ray, optical, and near-infrared bands. Through multi-wavelength modeling, we determine a circumburst density of $n_0\approx10^{-3}~$cm$^{-3}$, supporting our previous suggestion that a low-density circumburst environment is conducive to the production of long-lasting reverse shock radiation in the radio band. We infer the presence of a large excess X-ray absorption column, $N_{\rm H} \approx 1.5\times10^{22}~$cm$^{-2}$, and a high rest-frame optical extinction, $A_{\rm V}\approx3.4~$mag. We identify a jet break in the X-ray light curve at $t_{\rm jet}\approx6~$d, and thus derive a jet opening angle of $θ_{\rm jet}\approx4~$deg, yielding a beaming-corrected kinetic energy and radiated $γ$-ray energy of $E_{\rm K}\approx4\times10^{50}~$erg and $E_γ\approx1.3\times10^{51}~$erg ($1$-$10^4~$keV, rest frame), respectively. Consistency arguments connecting the forward and reverse shocks suggest a deceleration time of $t_{\rm dec} \approx 460~$s$~\approx T_{90}$, a Lorentz factor of $Γ(t_{\rm dec})\approx330$, and a reverse shock to forward shock fractional magnetic energy density ratio of $R_{\rm B}\equivε_{\rm B,RS}/ε_{\rm B,FS}\approx8$.

astro-ph.HE↗

arXiv2015

A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst

A new class of ultra-long duration (>10,000 s) gamma-ray bursts has recently been suggested. They may originate in the explosion of stars with much larger radii than normal long gamma-ray bursts or in the tidal disruptions of a star. No clear supernova had yet been associated with an ultra-long gamma-ray burst. Here we report that a supernova (2011kl) was associated with the ultra-long duration burst 111209A, at z=0.677. This supernova is more than 3 times more luminous than type Ic supernovae associated with long gamma-ray bursts, and its spectrum is distinctly different. The continuum slope resembles those of super-luminous supernovae, but extends farther down into the rest-frame ultra-violet implying a low metal content. The light curve evolves much more rapidly than super-luminous supernovae. The combination of high luminosity and low metal-line opacity cannot be reconciled with typical type Ic supernovae, but can be reproduced by a model where extra energy is injected by a strongly magnetized neutron star (a magnetar), which has also been proposed as the explanation for super-luminous supernovae.

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arXiv2014

The Velocity Distribution of Hypervelocity Stars

We consider the process of stellar binaries tidally disrupted by a supermassive black hole. For highly eccentric orbits, as one star is ejected from the three-body system, the companion remains bound to the black hole. Hypervelocity stars (HVSs) observed in the Galactic halo and S-stars observed orbiting the central black hole may originate from such mechanism. In this paper, we predict the velocity distribution of the ejected stars of a given mass, after they have travelled out of the Galactic potential. We use both analytical methods and Monte Carlo simulations. We find that each part of the velocity distribution encodes different information. At low velocities < 800 km/s, the Galactic Potential shapes universally the observed distribution, which rises towards a peak, related to the Galactic escape velocity. Beyond the peak, the velocity distribution depends on binary mass and separation distributions. Finally, the finite star life introduces a break related to their mass. A qualitative comparison of our models with current observations shows the great potential of HVSs to constrain bulge and Galactic properties. Standard choices for parameter distributions predict velocities below and above ~800 km/s with equal probability, while none are observed beyond ~700 km/s and the current detections are more clustered at low velocities 300-400 km/s. These features may indicate that the separation distribution of binaries that reach the tidal sphere is not flat in logarithmic space, as observed in more local massive binaries, but has more power towards larger separations, enhancing smaller velocities. In addition, the binary formation/evolution process or the injection mechanism might also induce a cut-off at ~ 10 solar radii in the separation distribution.

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arXiv2013

GRB 081007 and GRB 090424: the surrounding medium, outflows and supernovae

We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum luminosity is only about half as large as that of SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a GRB clearly associated with a massive star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be Γ~ 200, while for GRB 090424 a lower limit of Γ> 170 is derived. We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux dominated outflow or to a dissipative photosphere.

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arXiv2013

Magnetization Degree of Gamma-Ray Burst Fireballs: Numerical Study

The relative strength between forward and reverse shock emission in early gamma-ray burst afterglow reflects that of magnetic energy densities in the two shock regions. We numerically show that with the current standard treatment, the fireball magnetization is underestimated by up to two orders of magnitude. This discrepancy is especially large in the sub-relativistic reverse shock regime (i.e. the thin shell and intermediate regime) where most optical flashes were detected. We provide new analytic estimates of the reverse shock emission based on a better shock approximation, which well describe numerical results in the intermediate regime. We show that the reverse shock temperature at the onset of afterglow is constant, $(\barΓ_d-1)\sim 8\times10^{-2}$, when the dimensionless parameter $ξ_{0}$ is more than several. Our approach is applied to case studies of GRB 990123 and 090102, and we find that magnetic fields in the fireballs are even stronger than previously believed.

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arXiv2013

The Transient Gravitational-Wave Sky

Interferometric detectors will very soon give us an unprecedented view of the gravitational-wave sky, and in particular of the explosive and transient Universe. Now is the time to challenge our theoretical understanding of short-duration gravitational-wave signatures from cataclysmic events, their connection to more traditional electromagnetic and particle astrophysics, and the data analysis techniques that will make the observations a reality. This paper summarizes the state of the art, future science opportunities, and current challenges in understanding gravitational-wave transients.

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arXiv2012

Ejection and Capture Dynamics in Restricted Three-Body Encounters

We study the tidal disruption of binaries by a massive point mass (e.g. the black hole at the Galactic center), and we discuss how the ejection and capture preference between unequal-mass binary members depends on which orbit they approach the massive object. We show that the restricted three-body approximation provides a simple and clear description of the dynamics. The orbit of a binary with mass m around a massive object M should be almost parabolic with an eccentricity |1-e| < (m/M)^{1/3} << 1 for a member to be captured, while the other is ejected. Indeed, the energy change of the members obtained for a parabolic orbit can be used to describe non-parabolic cases. If a binary has an encounter velocity much larger than (M/m)^{1/3} times the binary rotation velocity, it would be abruptly disrupted, and the energy change at the encounter can be evaluated in a simple disruption model. We evaluate the probability distributions for the ejection and capture of circular binary members and for the final energies. In principle, for any hyperbolic (elliptic) orbit, the heavier member has more chance to be ejected (captured), because it carries a larger fraction of the orbital energy. However, if the orbital energy is close to zero, the difference between the two members becomes small, and there is practically no ejection and capture preference. The preference becomes significant when the orbital energy is comparable to the typical energy change at the encounter. We discuss its implications to hypervelocity stars and irregular satellites around giant planets.

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arXiv2009

Hyper Velocity Stars and the Restricted Parabolic 3-body Problem

Motivated by detections of hypervelocity stars that may originate from the Galactic Center, we revist the problem of a binary disruption by a passage near a much more massive point mass. The six order of magnitude mass ratio between the Galactic Center black hole and the binary stars allows us to formulate the problem in the restricted parabolic three-body approximation. In this framework, results can be simply rescaled in terms of binary masses, its initial separation and binary-to-black hole mass ratio. Consequently, an advantage over the full three-body calculation is that a much smaller set of simulations is needed to explore the relevant parameter space. Contrary to previous claims, we show that, upon binary disruption, the lighter star does not remain preferentially bound to the black hole. In fact, it is ejected exactly in 50% of the cases. Nonetheless, lighter objects have higher ejection velocities, since the energy distribution is independent of mass. Focusing on the planar case, we provide the probability distributions for disruption of circular binaries and for the ejection energy. We show that even binaries that penetrate deeply into the tidal sphere of the black hole are not doomed to disruption, but survive in 20% of the cases. Nor do these deep encounters produce the highest ejection energies, which are instead obtained for binaries arriving to 0.1-0.5 of the tidal radius in a prograde orbit. Interestingly, such deep-reaching binaries separate widely after penetrating the tidal radius, but always approach each other again on their way out from the black hole.[shortened]

astro-ph.HE↗

arXiv2009

Optical flashes, reverse shocks and magnetization

Despite the pre-Swift expectation that bright optical flashes from reverse shocks would be prevalent in early-time afterglow emission, rapid response observations show this not to be the case. Although very bright at early times, some GRBs such as GRB 061007 and GRB 060418, lack the short-lived optical flash from the reverse shock within minutes after the GRB. In contrast, other optical afterglows, such as those of GRB 990123, GRB 021211, GRB 060111B, GRB 060117, GRB 061126, and recently GRB 080319B, show a steep-to-flat transition within first 10^3 s typical of a rapidly evolving reverse + forward shock combination. We review the presence and absence of the reverse shock components in optical afterglows and discuss the implications for the standard model and the magnetization of the fireball. We show that the previously predicted optical flashes are likely to occur at lower wavelengths, perhaps as low as radio wavelengths and, by using the case of GRB 061126 we show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartion value, is a key physical parameter.

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