arXiv subjects
Publications and source records attributed to Takashi Nakamura.
We provide a consistency relation between cosmological observables in general relativity with the cosmological constant. Breaking of this relation at any redshift would imply the breakdown of the hypothesis of the cosmological constant as an explanation of the current acceleration of the universe.
We propose an emission mechanism of prompt gamma-ray bursts (GRBs) that can reproduce the observed non-thermal spectra with high radiative efficiencies, >50%. Internal dissipation below a photosphere can create a radiation-dominated thermal fireball. If electron-positron pairs outnumber protons, radiative acceleration of pairs drives the two-stream instabilities between pairs and protons, leading to the ``proton sedimentation'' in the accelerating pair frame. Pairs are continuously shock heated by proton clumps, scattering the thermal photons into the broken power-law shape, with the non-thermal energy that is comparable to the proton kinetic energy, consistent with observations. Pair photospheres become unstable around the radius of the progenitor star where strong thermalization occurs, if parameters satisfy the observed spectral (Yonetoku) relation. Pair annihilation lines are predicted above continua, which could be verified by GLAST.
The gamma-ray burst (GRB) 060218 has ~ 10^5 times lower luminosity than typical long GRBs, and is associated with a supernova (SN). The radio afterglow displays no jet break, so that this burst might arise from a mildly-relativistic spherical outflow produced by the SN shock sweeping the stellar surface. Since this model is energetically difficult, we propose that the radio afterglow is produced by a non-relativistic phase of an initially collimated outflow (jet). Our jet model is supported by the detection of optical linear polarization in the SN component. We also show analytically that the jet can penetrate a progenitor star. We analyzed the observational data of the prompt emission of this burst and obtained a smooth power-law light curve which might last longer than 10^6s. This behavior contrasts with the long intermittent activities with the X-ray flares of typical GRBs, implying that the central engine of this burst is different from those of typical GRBs. This argument is consistent with the analysis of the SN component of this burst, which suggests that the progenitor star was less massive and collapsed to a neutron star instead of a black hole. The collimation-corrected event rate of such low-luminosity GRBs is estimated to be ~ 10 times higher than that of typical long GRBs, and they might form a different GRB population: low-luminosity GRBs are produced by mildly-relativistic jets from neutron stars at the collapses of massive stars, while typical long GRBs by highly-relativistic jets from black holes. We suggest that the central engine of GRB 060218 is a pulsar (or a magnetar) with the initial rotation period P_0 \~ 10ms and the magnetic field B ~ 10^16G. A giant flare from the magnetar might be observed in future.
The recently discovered gamma-ray burst (GRB) GRB 060218/SN 2006aj is classified as an X-ray Flash with very long duration driven possibly by a neutron star. Since GRB 060218 is very near about 140 Mpc and very dim, one-year observation by Swift suggests that the rate of GRB 060218-like events might be very high so that such low luminosity GRBs (LL-GRBs) might form a different population from the cosmological high luminosity GRBs (HL-GRBs). We found that the high energy neutrino background from LL-GRBs could be comparable with that from HL-GRBs. If each neutrino event is detected by IceCube, later optical-infrared follow-up observations such as by Subaru and HST have possibilities to identify a Type Ibc supernova associated with LL-GRBs, even if gamma- and X-rays are not observed by Swift. This is in a sense a new window from neutrino astronomy, which might enable us to confirm the existence of LL-GRBs and to obtain information about their rate and origin. We also argue LL-GRBs as high energy gamma-ray and cosmic-ray sources.
Most X-ray afterglows of gamma-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase t^{-1/2} in the first few hours. This is not predicted by the standard afterglow model and needs an explanation. We discuss that the shallow decay requires an unreasonably high gamma-ray efficiency, >75-90%, within current models, which is difficult to produce by internal shocks. Such a crisis may be avoided if a weak relativistic explosion occurs ~10^3-10^6 s prior to the main burst or if the microphysical parameter of the electron energy increases during the shallow decay, ε_e ~ t^{1/2}. The former explanation predicts a very long precursor, while both prefer dim optical flashes from the reverse shock, as was recently reported. We also calculate the multi-wavelength afterglows and compare them with observations. No optical break at the end of the shallow X-ray decay indicates a preference for the time-dependent microphysics model with additionally decaying magnetic fields, ε_B ~ t^{-0.6}.
Tail emission of the prompt gamma-ray burst (GRB) is discussed using a multiple emitting sub-shell (inhomogeneous jet, sub-jets or mini-jets) model, where the whole GRB jet consists of many emitting sub-shells. One may expect that such a jet with angular inhomogeneity should produce spiky tail emission. However, we found that the tail is not spiky but is decaying roughly monotonically. The global decay slope of the tail is not so much affected by the local angular inhomogeneity but affected by the global sub-shell energy distribution. The fact that steepening breaks appeared in some events prefers the structured jets. If the angular size of the emitting sub-shell is around 0.01-0.02 rad, some bumps or fluctuations appear in the tail emission observed frequently in long GRBs. If the parameter differences of sub-shell properties are large, the tail has frequent changes of the temporal slope observed in a few bursts. Therefore, the multiple emitting sub-shell model has the advantage of explaining the small-scale structure in the observed rapid decay phase.
Almost all the X-ray afterglows of gamma-ray bursts (GRBs) observed by the Swift satellite have a shallow decay phase in the first thousands of seconds. We show that in an inhomogeneous jet (multiple-subjet or patchy-shell) model the superposition of the afterglows of off-axis subjets (patchy shells) can have the shallow decay phase. The necessary condition for obtaining the shallow decay phase is that gamma-ray bright subjets (patchy shells) should have gamma-ray efficiency higher than previously estimated, and should be surrounded by gamma-ray dim subjets (patchy shells) with low gamma-ray efficiency. Our model predicts that events with dim prompt emission have the conventional afterglow light curve without the shallow decay phase like GRB 050416A.
We perform Monte Carlo simulations to study Ep-Eiso correlation in the context of a multiple subjet model (or inhomogeneous jet model) for gamma-ray bursts (GRBs), X-ray--rich GRBs (XRRs), and X-ray flashes (XRFs). For a single subjet, we find that Ep prop. Eiso^0.4 for large viewing angles. For the multiple subjet model in which all the subjets have the same intrinsic properties, off-axis events show Ep prop. Eiso^a with 0.4<a<0.5. If the intrinsic properties of the subjets are distributed so that on-axis emission of each subjet follows a correlation Ep prop. Liso^1/2, we obtain the Amati correlation (Ep prop. Eiso^1/2) over three orders of magnitude in Ep. Although the scatter around the Amati correlation is large in the simulation, the results are consistent with the observed properties of GRBs with known redshifts and the BASTE GRBs with pseudo redshifts derived from the lag-luminosity correlation. We also calculate the event rates, the redshift distributions, and the T90 duration distributions of GRBs, XRRs, and XRFs which can be detected by HETE-2, assuming that the source redshift distribution is in proportion to the cosmic star formation rate. It is found that the event rates of three classes are comparable, that the average redshift of the XRRs is a little larger than those of the GRBs and the XRFs, and that short XRRs arise when a single subjet is viewed off-axis or viewed on-axis with slightly high redshift.
We propose a method to estimate the pseudo jet opening half-angle of GRBs using the spectral peak energy ($\Ep$)--peak luminosity relation (so called Yonetoku relation) as well as the $\Ep$--collimation-corrected $γ$-ray energy relation (so called Ghirlanda relation). For bursts with known jet break times and redshifts, we compared the pseudo jet opening half-angle with the standard one and found that the differences are within a factor 2. We apply the method to 689 long GRBS. We found that the distribution function of the pseudo jet opening half-angle obeys $f(θ_j)\proptoθ_j^{-2.2 \pm 0.2}$ with possible cutoffs for $θ_j < 0.04$ and $θ_j > 0.3$ although the log-normal fit is also possible. $θ^{-2}$ distribution is compatible with the structured jet model. From the distribution function we found that the beaming correction for the rate of GRBs is $\sim 340$, which means $\sim 10^{-5}$ yr$^{-1}$ galaxy$^{-1}$ or only one in $10^2$ type Ib/c supernovae. We also found the evolution of the distribution function as a function of the redshift.
A possible unified model of short and long gamma-ray bursts (GRBs), X-ray rich GRBs, and X-ray flashes is proposed. It is assumed that the jet of a GRB consists of many emitting sub-shells (i.e., an inhomogeneous jet model). The multiplicity of the sub-shells along a line of sight n_s is an important parameter. If n_s is large (>>1) the event looks like a long GRB, while if n_s=1, the event looks like a short GRB. Finally, when n_s=0, the event looks like an X-ray flash or an X-ray rich GRB. Furthermore, our model may also explain the bimodal distributions of T90 duration of BATSE-GRBs. Clearly, our model predicts that short GRBs should be associated with energetic SNe.
Double-differential cross sections of neutron production at angles from 0 to 110 degrees from many reactions induced by light and medium nuclei on targets from 12-C to 208-Pb, at several incident energies from 95 to 600 MeV/A have been measured recently at the Institute of Physical and Chemical Research (RIKEN) Ring Cyclotron in Japan and at the Heavy-Ion Medical Accelerator of the National Institute of Radiological Science in Chiba, Japan using the time-of-flight technique. We have analyzed all these new measurements using the Quantum Molecular Dynamics (QMD) model, the Oak Ridge intranuclear cascade model HIC, the ISABEL intranuclear cascade model from LAHET, and the Los Alamos version of the Quark-Gluon String Model code LAQGSM03. On the whole, all four models used here describe reasonably well most of the measured neutron spectra, although different models agree differently with data from specific reactions and some serious discrepances are observed for some reactions. We present here some illustrative results from our study, discuss possible reasons for some of the observed discrepancies and try to outline ways to further improve the tested codes in order to address these problems.
We study the distribution of the durations of gamma-ray bursts (GRBs) in the unified model of short and long GRBs recently proposed by Yamazaki, Ioka, and Nakamura. Monte Carlo simulations show clear bimodal distributions, with lognormal-like shapes for both short and long GRBs, in a power-law as well as a Gaussian angular distribution of the subjets. We find that the bimodality comes from the existence of the discrete emission regions (subjets or patchy shells) in the GRB jet. To explain other temporal properties of short and long GRBs, the subjet parameters should depend on the angle in the whole jet.
We show that ten(one) years operation of the ultimate DECIGO (DECihertz Interferometer Gravitational wave Observatory) can determine the cosmic equation of the state with such accuracy that 0.06%(3%), 0.08%(4%) and 0.06%(3%) for $Ω_m$, $Ω_w$ and $w$, respectively. In more realistic case of practical DECIGO or BBO (Big Bang Observer), $w$ will be determined within $\sim 10%$ by ten years observation assuming the flat universe model. Hence, the DECIGO or BBO will give an independent determination of the cosmic equation of the state.
Gamma-Ray Bursts (GRBs) are now considered as relativistic jets. We analyze the gravitational waves from the acceleration stage of the GRB jets. We show that (i) the point mass approximation is not appropriate if the opening half-angle of the jet is larger than the inverse of the Lorentz factor of the jet, (ii) the gravitational waveform has many step function like jumps, and (iii) the practical DECIGO and BBO may detect such an event if the GRBs occur in Local group of galaxy. We found that the light curve of GRBs and the gravitational waveform are anti-correlated so that the detection of the gravitational wave is indispensable to determine the structure of GRB jets.
We propose a possible unified model of short and long gamma-ray bursts (GRBs), X-ray rich GRBs, and X-ray flashes. The jet of a GRB is assumed to consist of multiple sub-jets or sub-shells (i.e., an inhomogeneous jet model). The multiplicity of the sub-jets along a line of sight n_s is an important parameter. If n_s is large (>> 1) the event looks like a long GRB, while if n_s=1, the event looks like a short GRB. Finally, when n_s=0, the event looks like an X-ray flash or an X-ray rich GRB. Furthermore, our model may also explain the bimodal distributions of T_{90} duration of BATSE-GRBs.
Using a simple uniform jet model of prompt emissions of gamma-ray bursts (GRBs), we reproduce the observed peak energy--isotropic energy relation. A Monte Carlo simulation shows that the low-isotropic energy part of the relation is dominated by events viewed from off-axis directions, and the number of the off-axis events is about one-third of the on-axis emissions. We also compute the observed event rates of the GRBs, the X-ray-rich GRBs, and the X-ray flashes detected by HETE-2, and we find that they are similar.
Taking into account the recent suggestion that a short gamma-ray burst (GRB) looks like the first 1 sec of a long GRB, we propose that the jet of a GRB consists of multiple sub-jets or sub-shells (i.e., an inhomogeneous jet model). The multiplicity of the sub-jets along a line of sight n_s is an important parameter. If n_s is large (>> 1) the event looks like a long GRB, while if n_s is small (~1) the event looks like a short GRB. If our line of sight is off-axis to any sub-jets, the event looks like an X-ray flash or an X-ray rich GRB. The log-normal distribution of durations of short and long GRBs are also suggested in the same model.
The of the cosmological X-ray flashes detected by WFC/BeppoSAX is calculated theoretically in a simple jet model. The total emission energy from the jet is assumed to be constant. We find that if the jet opening half-angle is smaller than 0.03 radian, off-axis emission from sources at z<~4 can be seen. The theoretical is less than 0.4, which is consistent with the observational result of 0.27+/-0.16 at the 1-sigma level. This suggests that the off-axis GRB jet with the small opening half-angle at the cosmological distance can be identified as the cosmological X-ray flash.