arXiv subjects

S. Kobayashi

Publications and source records attributed to S. Kobayashi.

At least 91 records · Page 5Linked to original sources

arXiv2010

Ten per cent polarized optical emission from GRB 090102

The nature of the jets and the role of magnetic fields in gamma-ray bursts (GRB) remains unclear. In a baryon-dominated jet only weak, tangled fields generated in situ through shocks would be present. In an alternative model, jets are threaded with large scale magnetic fields that originate at the central engine and which accelerate and collimate the jets. The way to distinguish between the models is to measure the degree of polarization in early-time emission, however previous claims of gamma-ray polarization have been controversial. Here we report that the early optical emission from GRB 090102 was polarized at the level of P=10+/-1%, indicating the presence of large-scale fields originating in the expanding fireball. If the degree of polarization and its position angle were variable on timescales shorter than our 60-s exposure, then the peak polarization may have been larger than 10 per cent.

astro-ph.HE↗

arXiv2010

GRB 090313 and the Origin of Optical Peaks in GRB Light Curves: Implications for Lorentz Factors and Radio Flares

We use a sample of 19 Gamma Ray Bursts (GRBs) that exhibit single-peaked optical light curves to test the standard fireball model by investigating the relationship between the time of the onset of the afterglow and the temporal rising index. Our sample includes GRBs and X-ray flashes for which we derive a wide range of initial Lorentz factors ($40 < Γ< 450$). Using plausible model parameters the typical frequency of the forward shock is expected to lie close to the optical band; within this low typical frequency framework, we use the optical data to constrain $ε_e$ and show that values derived from the early time light curve properties are consistent with published typical values derived from other afterglow studies. We produce expected radio light curves by predicting the temporal evolution of the expected radio emission from forward and reverse shock components, including synchrotron self-absorption effects at early time. Although a number of the GRBs in this sample do not have published radio measurements, we demonstrate the effectiveness of this method in the case of {\it Swift} GRB 090313, for which millimetric and centrimetric observations were available, and conclude that future detections of reverse-shock radio flares with new radio facilities such as the EVLA and ALMA will test the low frequency model and provide constraints on magnetic models.

astro-ph.HE↗

arXiv2010

GRB 090902B: afterglow observations and implications

The optical-infrared afterglow of the LAT-detected long duration burst, GRB 090902B, has been observed by several instruments. The earliest detection by ROTSE-IIIa occurred 80 minutes after detection by the GBM instrument onboard the Fermi Gamma-Ray Space Telescope, revealing a bright afterglow and a decay slope suggestive of a reverse shock origin. Subsequent optical-IR observations followed the light curve for 6.5 days. The temporal and spectral behavior at optical-infrared frequencies is consistent with synchrotron fireball model predictions; the cooling break lies between optical and XRT frequencies ~ 1.9 days after the burst. The inferred electron energy index is $p = 1.8 \pm 0.2$, which would however imply an X-ray decay slope flatter than observed. The XRT and LAT data have similar spectral indices and the observed steeper value of the LAT temporal index is marginally consistent with the predicted temporal decay in the radiative regime of the forward shock model. Absence of a jet break during the first 6 days implies a collimation-corrected $γ$-ray energy $E_γ > 2.2\times10^{52}\rm$ ergs, one of the highest ever seen in a long-duration GRBs. More events combining GeV photon emission with multi-wavelength observations will be required to constrain the nature of the central engine powering these energetic explosions and to explore the correlations between energetic quanta and afterglow emission.

astro-ph.HE↗

arXiv2009

Precise measurement of HFS of positronium

The ground state hyperfine splitting in positronium, $Δ_{\mathrm{HFS}}$, is sensitive to high order corrections of QED. A new calculation up to $O(α^3)$ has revealed a $3.9 σ$ discrepancy between the QED prediction and the experimental results. This discrepancy might either be due to systematic problems in the previous experiments or to contributions beyond the Standard Model. We propose an experiment to measure $Δ_{\mathrm{HFS}}$ employing new methods designed to remedy the systematic errors which may have affected the previous experiments. Our experiment will provide an independent check of the discrepancy. The measurement is in progress and a preliminary result of $Δ_{\mathrm{HFS}} = 203.399 \pm 0.029 \mathrm{GHz} (143 \mathrm{ppm})$ has been obtained. A measurement with a precision of O(1) ppm is expected within a few years.

hep-ex↗

arXiv2009

Early time optical polarization of GRB Afterglows: GRB 060418 and GRB 090102

RINGO on the Liverpool Telescope has now measured the optical polarization of GRB 060418 (where a 2 sigma upper limit of P<8% was determined) and GRB 090102 (when a detection of P = 10 +/-1 % was made). We discuss the implications of these observations for the various competing models of GRB jet magnetization and describe a possible unified model that can explain both measurements.

astro-ph.HE↗

arXiv2009

Rise and fall of the X-ray flash 080330: an off-axis jet?

X-ray flashes (XRFs) are a class of gamma-ray bursts (GRBs) with the peak energy of the time-integrated spectrum, Ep, below 30 keV, whereas classical GRBs have Ep of a few hundreds keV. Apart from Ep and the lower luminosity, the properties of XRFs are typical of the classical GRBs. Yet, the nature of XRFs and the differences from that of GRBs are not understood. In addition, there is no consensus on the interpretation of the shallow decay phase observed in most X-ray afterglows of both XRFs and GRBs. We examine in detail the case of XRF 080330 discovered by Swift at the redshift of 1.51. This burst is representative of the XRF class and exhibits an X-ray shallow decay. The rich and broadband (from NIR to UV) photometric data set we collected across this phase makes it an ideal candidate to test the off-axis jet interpretation proposed to explain both the softness of XRFs and the shallow decay phase. We present prompt gamma-ray, early and late IR/visible/UV and X-ray observations of the XRF 080330. We derive a SED from NIR to X-ray bands across the plateau phase with a power-law index of 0.79 +- 0.01 and negligible rest-frame dust extinction. The multi-wavelength evolution of the afterglow is achromatic from ~10^2 s out to ~8x10^4 s. We describe the temporal evolution of the multi-wavelength afterglow within the context of the standard afterglow model and show that a single-component jet viewed off-axis explains the observations (abriged).

astro-ph.HE↗

arXiv2009

Evidence for energy injection and a fine-tuned central engine at optical wavelengths in GRB 070419A

We present a comprehensive multiwavelength temporal and spectral analysis of the FRED GRB 070419A. The early-time emission in the $γ$-ray and X-ray bands can be explained by a central engine active for at least 250 s, while at late times the X-ray light curve displays a simple power-law decay. In contrast, the observed behaviour in the optical band is complex (from 10$^2$ up to 10$^6$ s). We investigate the light curve behaviour in the context of the standard forward/reverse shock model; associating the peak in the optical light curve at $\sim$450 s with the fireball deceleration time results in a Lorenz factor $Γ\approx 350$ at this time. In contrast, the shallow optical decay between 450 and 1500 s remains problematic, requiring a reverse shock component whose typical frequency is above the optical band at the optical peak time for it to be explained within the standard model. This predicts an increasing flux density for the forward shock component until t $\sim$ 4 $\times$ 10$^6$ s, inconsistent with the observed decay of the optical emission from t $\sim$ 10$^4$ s. A highly magnetized fireball is also ruled out due to unrealistic microphysic parameters and predicted light curve behaviour that is not observed. We conclude that a long-lived central engine with a finely tuned energy injection rate and a sudden cessation of the injection is required to create the observed light curves - consistent with the same conditions that are invoked to explain the plateau phase of canonical X-ray light curves of GRBs.

astro-ph.HE↗

arXiv2009

Search for a light pseudoscalar particle in the decay $K^0_L \to π^0 π^0 X$

We performed a search for a light pseudoscalar particle $X$ in the decay $K_L^0->pi0pi0X$, $X->γγ$ with the E391a detector at KEK. Such a particle with a mass of 214.3 MeV/$c^2$ was suggested by the HyperCP experiment. We found no evidence for $X$ and set an upper limit on the product branching ratio for $K_L^0->pi0pi0X$, $X->γγ$ of $2.4 \times 10^{-7}$ at the 90% confidence level. Upper limits on the branching ratios in the mass region of $X$ from 194.3 to 219.3 MeV/$c^2$ are also presented.

hep-ex↗

arXiv2008

GW approximation with LSDA+U method and applications to NiO, MnO, and V$_2$O$_3$

A GW approximation (GWA) method named U+GWA is proposed, where we can start GWA with more localized wave functions obtained by the local spin-density approximation (LSDA)+U method. Then GWA and U+GWA are applied to MnO, NiO, and V$_2$O$_3$ in antiferromagnetic phase. The band gaps and energy spectra show an excellent agreement with the experimentally observed results and are discussed in details. The calculated width of d-bands of V$_2$O$_3$ is much narrower than that of the observed one which may be a mixture of t$_{2g}^2$ multiplet and single electron t$_{2g}$ level. GWA or U+GWA does not work also in the paramagnetic phase of V$_2$O$_3$ and the reason for thois is clarified. The method of the unique choice of on-site Coulomb interaction is discussed in details. The criterion for whether we should adopt GWA or U+GWA is discussed and is assessed with the help of the off-diagonal elements of the self-energy.

cond-mat.str-el↗

arXiv2008

Multiwavelength analysis of the intriguing GRB 061126: the reverse shock scenario and magnetization

We present a detailed study of the prompt and afterglow emission from Swift GRB 061126 using BAT, XRT, UVOT data and multi-color optical imaging from ten ground-based telescopes. GRB 061126 was a long burst (T_90=191 s) with four overlapping peaks in its gamma-ray light curve. The X-ray afterglow, observed from 26 min to 20 days after the burst, shows a simple power-law decay with alpha_X=1.290 \pm 0.008. Optical observations presented here cover the time range from 258 s (Faulkes Telescope North) to 15 days (Gemini North) after the burst; the decay rate of the optical afterglow shows a steep-to-shallow transition (from alpha_1=1.48 \pm 0.06 to alpha_2=0.88 \pm 0.03) approximately 13 min after the burst. We suggest the early, steep component is due to a reverse shock and show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartion value, is a few ten times larger than in the forward shock in the early afterglow phase. The ejecta might be endowed with primordial magnetic fields at the central engine. The optical light curve implies a late-time break at about 1.5 days after the burst, while there is no evidence of the simultaneous break in the X-ray light curve. We model the broad band emission and show that some afterglow characteristics (the steeper decay in X-ray and the shallow spectral index from optical to X-ray) are difficult to explain in the framework of the standard fireball model. This might imply that the X-ray afterglow is due to an additional emission process, such as late time central engine activity rather than blast-wave shock emission. The possible chromatic break at 1.5 days after the burst would give support to the additional emission scenario.

astro-ph↗

arXiv2008

The Early-time Optical Properties of Gamma-Ray Burst Afterglows

We present a multiwavelength analysis of 63 Gamma-Ray Bursts observed with the world's three largest robotic optical telescopes, the Liverpool and Faulkes Telescopes (North and South). Optical emission was detected for 24 GRBs with brightnesses ranging from R = 10 to 22 mag in the first 10 minutes after the burst. By comparing optical and X-ray light curves from t = 100 to about 10^6 seconds, we introduce four main classes, defined by the presence or absence of temporal breaks at optical and/or X-ray wavelengths. While 15/24 GRBs can be modelled with the forward-shock model, explanation of the remaining nine is very challenging in the standard framework even with the introduction of energy injection or an ambient density gradient. Early X-ray afterglows, even segments of light curves described by a power-law, may be due to additional emission from the central engine. 39 GRBs in our sample were not detected and have deep upper limits (R < 22 mag) at early time. Of these, only ten were identified by other facilities, primarily at near infrared wavelengths, resulting in a dark burst fraction of about 50%. Additional emission in the early time X-ray afterglow due to late-time central engine activity may also explain some dark bursts by making the bursts brighter than expected in the X-ray band compared to the optical band.

astro-ph↗

arXiv2007

The prompt to late-time multiwavelength analysis of GRB 060210

We present our analysis of the multiwavelength photometric & spectroscopic observations of GRB 060210 and discuss the results in the overall context of current GRB models. All available optical data underwent a simultaneous temporal fit, while X-ray and gamma-ray observations were analysed temporally & spectrally. The results were compared to each other and to possible GRB models. The X-ray afterglow is best described by a smoothly broken power-law with a break at 7.4 hours. The late optical afterglow has a well constrained single power-law index which has a value between the two X-ray indices, though it does agree with a single power-law fit to the X-ray. An evolution of the hardness of the high-energy emission is demonstrated and we imply a minimum host extinction from a comparison of the extrapolated X-ray flux to that measured in the optical. We find that the flaring gamma-ray and X-ray emission is likely due to internal shocks while the flat optical light curve at that time is due to the external shock. The late afterglow is best explained by a cooling break between the optical and X-rays and continued central engine activity up to the time of the break. The required collimation corrected energy of ~ 2x10^52 erg, while at the high end of the known energy distribution, is not unprecedented.

astro-ph↗

arXiv2007

The Remarkable Afterglow of GRB 061007: Implications for Optical Flashes and GRB Fireballs

We present a multiwavelength analysis of Swift GRB 061007. The 2-m robotic Faulkes Telescope South (FTS) began observing 137 s after the onset of the gamma-ray emission, when the optical counterpart was already decaying from R 10.3 mag, and continued observing for the next 5.5 hours. These observations begin during the final gamma-ray flare and continue through and beyond a long, soft tail of gamma-ray emission whose flux shows an underlying simple power-law decay identical to that seen at optical and X-ray wavelengths, with temporal slope alpha 1.7 This remarkably simple decay in all of these bands is rare for Swift bursts, which often show much more complex light curves. We suggest the afterglow emission begins as early as 30-100 s and is contemporaneous with the on-going variable prompt emission from the central engine, but originates from a physically distinct region dominated by the forward shock. The afterglow continues unabated until at least 10^5 s showing no evidence of a break. The observed multiwavelength evolution of GRB 061007 is explained by an expanding fireball whose optical, X-ray and late-time gamma-ray emission is dominated by emission from a forward shock with typical synchrotron frequency, nu_m, that is already below the optical band as early as t=137 s and a cooling frequency, nu_c, above the X-ray band to at least t=10^5 s. In contrast, the typical frequency of the reverse shock lies in the radio band at early time. We suggest that the unexpected lack of bright optical flashes from the majority of Swift GRBs may be explained with a low nu_m originating from small microphysics parameters, epsilon_e and epsilon_B. (Abridged)

astro-ph↗

arXiv2006

A Review of Early-Time Optical Follow-ups with 2-m Robotic Telescopes

We summarise recent deep, rapid GRB follow-up observations using the RoboNet-1.0 network which comprises three fully-robotic 2-m telescopes, the Liverpool Telescope and the Faulkes Telescopes North and South. Observations begin automatically within minutes of receipt of a GRB alert and may continue for hours or days to provide well-sampled multi-colour light curves or deep upper limits. Our light curves show a variety of early afterglow behaviour, from smooth, simple or broken power laws to 'bumpy', for a wide range of optical brightness (from the unprecedented faint detections of GRB 060108 and GRB 060510B to classical bright ones). We discuss GRB 051111 as an example of how the combination of optical and X-ray light curves can provide insight into the circumburst environment, in particular the role played by intrinsic extinction soon after the burst.

astro-ph↗

arXiv2006

The circumburst environment of a FRED GRB: study of the prompt emission and X-ray/optical afterglow of GRB 051111

We report a multi-wavelength analysis of the prompt emission and early afterglow of GRB051111 and discuss its properties in the context of current fireball models. The detection of GRB051111 by the Burst Alert Telescope on-board Swift triggered early BVRi' observations with the 2-m robotic Faulkes Telescope North in Hawaii, as well as X-ray observations with the Swift X-Ray Telescope. The prompt gamma-ray emission shows a classical FRED profile. The optical afterglow light curves are fitted with a broken power law, with alpha_1=0.35 to alpha_2=1.35 and a break time around 12 minutes after the GRB. Although contemporaneous X-ray observations were not taken, a power law connection between the gamma-ray tail of the FRED temporal profile and the late XRT flux decay is feasible. Alternatively, if the X-ray afterglow tracks the optical decay, this would represent one of the first GRBs for which the canonical steep-shallow-normal decay typical of early X-ray afterglows has been monitored optically. We present a detailed analysis of the intrinsic extinction, elemental abundances and spectral energy distribution. From the absorption measured in the low X-ray band we find possible evidence for an overabundance of some alpha elements such as oxygen, [O/Zn]=0.7+/-0.3, or, alternatively, for a significant presence of molecular gas. The IR-to-X-ray Spectral Energy Distribution measured at 80 minutes after the burst is consistent with the cooling break lying between the optical and X-ray bands. Extensive modelling of the intrinsic extinction suggests dust with big grains or grey extinction profiles. The early optical break is due either to an energy injection episode or, less probably, to a stratified wind environment for the circumburst medium.

astro-ph↗

arXiv2006

Minority Spin Condensate in Spin-Polarized Superfluid $^3$He-A$_1$ Phase

The spin-polarized superfluid $^3$He A$_1$ phase, which forms below 3 mK in external magnetic field, serves as a material in which theoretical notions of fundamental magnetic processes and macroscopic quantum spin phenomena may be tested. Conventionally, the superfluid component of the A$_1$ phase is understood to contain only the majority spin condensate having energetically-favoured paired spins directed along the external field and no minority spin condensate having paired spins in the opposite direction. We have developed a novel mechanical spin density detector to measure the spin relaxation in the A$_1$ phase as functions of temperature, pressure and magnetic field. The mechanical spin detector is based in principle on the unique magnetic fountain effect occurring only in the A$_1$ phase (occurring between two transition temperatures, T$_{c1}$ and T$_{c2}$). Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards T$_{c2}$. Our measurements, together with Leggett-Takagi and Monien-Tewordt theories, demonstrate that a minute presence of minority spin pairs is responsible for the unexpected spin relaxation phenomena in A$_1$ phase.

cond-mat.other↗

arXiv2006

Very Early Optical Afterglows of Gamma-Ray Bursts: Evidence for Relative Paucity of Detection

Very early observations with the Swift satellite of gamma-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g. GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt gamma-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal gamma-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include: foreground extinction, circumburst absorption, Ly-alpha blanketing and absorption due to high redshift, low density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the non-detections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting-flux-dominated flow or a pure non-relativistic hydrodynamical reverse shock.

astro-ph↗

arXiv2006

Anatomy of A Dark Burst - The Afterglow of GRB 060108

We report the first detection of an optical afterglow of a GRB (060108) that would have been classified as 'dark' in the absence of deep, rapid ground-based optical imaging with the 2-m robotic Faulkes Telesscope (FTN). Our multiwavelength analysis reveals an X-ray light curve typical of many Swift long GRBs (3-segments plus flare). Its optical afterglow, however, was already fainter than the detection limit of the UVOT within 100s of the burst. Optical imaging in BVRi' filters with the FTN began 2.75 minutes after the burst and resulted in the detection of the optical afterglow at 5.3 minutes, with a UKIRT K-band identification at ~45 mins. R and i'-band light curves are consistent with a single power law decay in flux, F(t) prop t^-a where a=0.43+/-0.08, or a 2-segment light curve with a steep decay a_1 <0.88, flattening to a_2 ~ 0.31, with evidence for rebrightening at i' band. Deep VLT R-band imaging at ~12 days reveals a faint, extended object (R ~23.5 mag) at the location of the afterglow. Although the brightness is compatible with the extrapolation of the a_2 slow decay, significant flux is likely due to a host galaxy. This implies that the optical light curve had a break before 12 days, akin to what observed in the X-rays. We derive a maximum photometric redshift z<3.2 for GRB 060108 and a best-fitting optical-to-X-ray SED at 1000 s after the burst consistent with a power law with index beta_OX = 0.54 and a small amount of extinction. The unambiguous detection at B-band and the derived photometric redshift rule out a high redshift as the reason for the optical faintness of GRB 060108. Instead, the hard opt/X-ray spectral index confirms it as one of the optically-darkest bursts detected and with modest host extinction explains the UVOT non-detection (abridged).

astro-ph↗