arXiv subjects

Fumio Takahara

Publications and source records attributed to Fumio Takahara.

At least 55 records · Page 3Linked to original sources

arXiv2006

TeV Gamma-Rays from Old Supernova Remnants

We study the emission from an old supernova remnant (SNR) with an age of around 10^5 yrs and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 10^5 yrs, proton acceleration is efficient enough to emit TeV gamma-rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV gamma-rays and X-rays are dominated by hadronic processes, pi^0-decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 10^5 yrs, there are few high-energy particles emitting TeV gamma-rays because of the energy loss effect and/or the wave damping effect occurring at low-velocity isothermal shocks. For old SNRs or SNR-GMC interacting systems capable of generating TeV gamma-ray emitting particles, we calculated the ratio of TeV gamma-ray (1-10 TeV) to X-ray (2-10 keV) energy flux and found that it can be more than ~10^2. Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.

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arXiv2006

A Structured Leptonic Jet Model of the "Orphan" TeV Gamma-Ray Flares in TeV Blazars

The emission spectra of TeV blazars extend up to tens of TeV and the emission mechanism of the TeV $γ$-rays is explained by synchrotron self-Compton scattering in leptonic models. In these models the time variabilities of X-rays and TeV $γ$-rays are correlated. However, recent observations of 1ES 1959+650 and Mrk 421 have found the ``orphan'' TeV $γ$-ray flares, i.e., TeV $γ$-ray flares without simultaneous X-ray flares. In this paper we propose a model for the ``orphan'' TeV $γ$-ray flares, employing an inhomogeneous leptonic jet model. After a primary flare that accompanies flare-up both in X-rays and TeV $γ$-rays, radiation propagates in various directions in the comoving frame of the jet. When a dense region in the jet receives the radiation, X-rays are scattered by relativistic electrons/positrons to become TeV $γ$-rays. These $γ$-ray photons are observed as an ``orphan'' TeV $γ$-ray flare. The observed delay time between the primary and ``orphan'' flares is about two weeks and this is accounted for in our model for parameters such as $Γ= 20$, $d = 4 \times 10^{17}$cm, $α= 3$, and $η= 1$, where $Γ$ is the bulk Lorentz factor of the jet, $d$ is the distance between the central black hole and the primary flare site, $α/Γ$ is the angle between the jet axis and the direction of the motion of the dense region that scatters incoming X-rays produced by the primary flare, and $η/Γ$ is the angle between the jet axis and the line of sight.

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arXiv2005

The X-ray Jet in Centaurus A: Clues on the Jet Structure and Particle Acceleration

We report detailed studies of the X-ray emission from the kpc scale jet in the nearest active galaxy, Cen A. 41 compact sources were found within the jet, 13 of which were newly identified. We construct the luminosity function for the detected jet-knots and argue that the remaining emission is most likely to be truly diffuse, rather than resulting from the pile-up of unresolved faint knots. The transverse jet profile reveals that the extended emission has the intensity peak at the jet boundaries. We note that limb-brightened jet morphologies have been observed previously at radio frequencies in some jet sources, but never so clearly at higher photon energies. Our result therefore supports a stratified jet model, consisting of a relativistic outflow including a boundary layer with a velocity shear. In addition, we found that the X-ray spectrum of the diffuse component is almost uniform across and along the jet. We discuss this spectral behavior within a framework of shock and stochastic particle acceleration processes. We note some evidence for a possible spectral hardening at the outer sheath of the jet. Due to the limited photon statistics of the present data, further deep observations of Cen A are required to determine the reality of this finding, however we note that the existence of the hard X-ray features at outer jet boundaries would provide an important challenge to theories for the evolution of ultra-relativistic particles within the jets.

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arXiv2005

Constraining the Emission Properties of TeV Blazar H1426+428 by the Synchrotron-Self-Compton Model

H1426+428 is one of blazars that are observed by $γ$-rays in the TeV region. Because TeV $γ$-rays from distant sources are subject to attenuation by the extragalactic background light (EBL) via electron-positron pair production, the intrinsic spectrum of the TeV $γ$-rays should be inferred by using the models of radiation processes and EBL spectrum. We set constraints on the physical condition of H1426+428 with the synchrotron-self-Compton model applying several EBL models. We find that the emission region of H1426+428 is moving toward us with the bulk Lorentz factor of $\sim 20$ and that its magnetic field strength is $\sim 0.1$ G. These properties are similar to other TeV blazars such as Mrk 421 and Mrk 501. However, the ratio of the energy density of nonthermal electrons to that of the magnetic fields is about 190 and fairly larger than those of Mrk 421 and Mrk 501, which are about 5 -- 20. It is also found that the intensity of EBL in the middle and near infrared wavelengths should be low, i.e., the intensity at 10 $μ$m is about 1 nW m$^{-2}$ sr$^{-1}$ to account for the observed TeV $γ$-ray flux. Because the spectral data of H1426+428 in X-rays and $γ$-rays used in our analysis were not obtained simultaneously, further observations of TeV blazars are necessary to make the constraints on EBL more stringent.

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arXiv2005

Coherent Curvature Radiation and Proton Counterflow in the Pulsar Magnetosphere

In the proton counterflow model of a pulsar magnetosphere that we have recently proposed, non-relativistic protons are supplied from the magnetosphere to flow toward the pulsar surface and screen an electric field above the polar cap region. In this Letter, we show that the proton counterflow is also suitable for the bunching of pair plasma. The two-stream instability is easily excited and can produce bunches of pairs with a relevant length scale to emit coherent curvature radiation.

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arXiv2005

Giant flare of SGR 1806-20 from a relativistic jet

Japanese magnetospheric explorer GEOTAIL recorded a detailed light curve during the initial 600 msec of a giant flare from SGR 1806-20 on December 27, 2004. We show that the observed light curve is well explained by an emission from relativistically expanding fireballs, like those of gamma-ray bursts (GRBs). Especially, the observed rapid fading after 500msec suggests that ejecta is collimated in a jet. We derive an upper limit on the jet opening half-angle of 0.2 radian that is as narrow as those of GRBs.

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arXiv2005

Compton scattering in the Klein-Nishina Regime Revisited

In blazars such as 3C 279, GeV gamma-rays are thought to be produced by inverse Compton scattering of soft photons injected from external sources into the jet. Because of the large bulk Lorentz factor of the jet, the energy of soft photons is Doppler shifted in the comoving frame of the jet, and the scattering is likely to occur in the Klein-Nishina regime. Although the Klein-Nishina effects are well known, the properties of the electron and emission spectra have not been studied in detail in the environment of blazars. We solve the kinetic equation of electrons with the spatial escape term of the electrons to obtain the electron energy spectrum in the jet and calculated the observed emission spectrum. In calculations of the Compton losses in the Klein-Nishina regime, we use the discrete loss formalism to take into account the significant energy loss in a single scattering. Although the scattering cross section decreases because of the Klein-Nishina effects, ample gamma rays are emitted by inverse Compton scattering. When the injection spectrum of electrons obeys a power law, the electron spectrum does not follow a broken power law, as a result of the Klein-Nishina effects, and a large number of high-energy electrons remain in the emitting region.

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arXiv2004

Electric field screening by a proton counterflow in the pulsar polar cap

We propose a new mechanism to screen the electric field in the pulsar polar cap. Previous studies have shown that if an electron beam from the stellar surface is accelerated to energies high enough to create electron-positron pairs, the required electric field parallel to the magnetic field lines is too strong to be screened out by the produced pairs. We argue here that if non-relativistic protons are supplied from the magnetosphere to flow towards the stellar surface, they can provide an anode to screen out such a strong electric field. Injected electron-positron pairs yield an asymmetry of the electrostatic potential around the screening point. The required pair creation rate in this model is consistent with the conventional models.

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arXiv2004

Electrostatic Instability in Electron-Positron Pairs Injected in an External Electric Field

Motivated by the particle acceleration problem in pulsars, we numerically investigate electrostatic instability of electron-positron pairs injected in an external electric field. The electric field is expected to be so strong that we cannot neglect effects of spatial variation in the 0-th order distribution functions on the scale of the plasma oscillation. We assume that pairs are injected mono-energetically with 4-velocity $u_0>0$ in a constant external electric field by which electrons (positrons) are accelerated (decelerated). By solving linear perturbations of the field and distribution functions of pairs, we find a new type of electrostatic instability. The properties of the instability are characterized by $u_0$ and the ratio $R$ of the braking time-scale (determined by the external electric field) to the time-scale of the plasma oscillation. The growth rate is as large as a few times the plasma frequency. We discuss the possibility that the excited waves prevent positrons from returning to the stellar surface.

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arXiv2003

Constraints on the energetics and plasma composition of relativistic jets in FR II sources

We explore the energetics and plasma composition in FR II sources using a new simple method of combining shock dynamics and radiation spectrum. The hot spots are identified with the reverse shocked region of jets. With the one-dimensional shock jump conditions taking account of the finite pressure of hot ICM, we estimate the rest mass and energy densities of the sum of thermal and non-thermal particles in hot spots. Independently, based on the Synchrotron Self-Compton (SSC) model, we estimate the number and energy densities of {\it non-thermal} electrons using the multi-frequency radiation spectrum of hot spots. We impose the condition that the obtained rest mass, internal energy, and number densities of non-thermal electrons should be lower than those of the total particles determined by shock dynamics. We apply this method to Cygnus A. We examine three extreme cases of pure electron-positron pair plasma (Case I), pure electron-proton plasma with separate thermalization (Case II), and pure electron-proton plasma in thermal-equilibrium (Case III). By detailed SSC analysis for Cygnus A and 3C123, we find that the energy density of non-thermal electrons is about 10 times larger than that of magnetic field. We find that the Case III is not acceptable because predicted photon spectra do not give a good fit to the observed one. We find that Case II can also be ruled out since the number density of non-thermal electrons exceeds that of the total number density. Hence, we find that only pure $e^{\pm}$ plasma (Case I) is acceptable among the three cases. Total kinetic power of jet and electron acceleration efficiency are also constrained by internal energy densities of non-thermal and total particles.

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arXiv2003

The Electron Spectrum in 3C279 and the Observed Emission Spectrum

The emission mechanisms of the blazar 3C 279 are studied by solving the kinetic equations of electrons and photons in a relativistically moving blob. The gamma-ray spectral energy distribution (SED) is fitted by inverse Compton scattering of external photons. The bulk Lorentz factor of the emitting blob is found to be 25, and the magnetic field is found to be 0.3 G. GeV gamma-rays are well explained by inefficiently cooled electrons because of the Klein-Nishina effects. The electron spectrum is not a broken power law with a steeper spectrum above a break energy, which is often used to fit the observed SED. The kinetic energy density of the nonthermal electrons dominates the magnetic energy density; this result is qualitatively the same as that for TeV blazars such as Mrk 421 and Mrk 501. The gamma-ray luminosity of 3C 279 is often observed to increase rapidly. We show that one of the better sampled gamma-ray flares can be explain by the internal shock model.

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arXiv2003

Photon Emission in a Cascade from Relativistic Protons Initiated by Residual Thermal Photons in Gamma-Ray Bursts

Gamma-ray bursts are generally considered to be the result of internal shocks generated in an inhomogeneous relativistic outflow that arises from a fireball. In such shocks, the Fermi acceleration of protons is naturally expected to be at least as efficient as that of electrons. We investigate the consequences of proton acceleration in the residual thermal photon field of a fireball, especially those on the emission spectra of photons. In contrast to most other studies, we do not invoke a direct electron acceleration in shock waves. We show that the residual radiation field of the fireball can ignite the photopion production and subsequent cascade, and that the photons emitted in this process further enhance the photon-initiated cascade. We find that Fermi accelerated protons with $\gtrsim 10^{13}$ eV efficiently bring their energy into pion production and subsequent photon emission. The particles cascading from the pions emit photons over a wide energy range. The photons emitted from electron--positron pairs distribute continuously from the GeV range down to the X-ray range, while muons can emit gamma-rays by synchrotron radiation with a break at around 1--10 MeV. We also discuss several radiation processes which may possibly produce a break feature in the MeV range, as observed in addition to muon synchrotron radiation.

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arXiv2003

Probabilistic description of the first-order Fermi acceleration in shock waves: Time-dependent solution by single-particle approach

We give a new coherent description of the first-order Fermi acceleration of particles in shock waves from the point of view of stochastic process of the individual particles, under the test particle approximation. The time development of the particle distribution function can be dealt with by this description, especially for relativistic shocks. We formulate the acceleration process of a particle as a two-dimensional Markov process in a logarithmic momentum-time space, and relate the solution of the Markov process with the particle distribution function at the shock front, for both steady and time-dependent case. For the case where the probability density function of the energy gain and cycle-time at each shock crossing of the particles obeys a scaling law in momentum, which is usually assumed in the literature, it is confirmed in more general form that the energy distribution of particles has the power-law feature in steady state. The equation to determine the exact power-law index which is applicable for any shock speed is derived and it is shown that the power-law index, in general, depends on the shape of the probability density function of the energy gain at each shock crossing; in particular for relativistic shocks, the dispersion of the energy gain can influence the power-law index. It is also shown that the time-dependent solution has a self-similarity for the same case.

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arXiv2002

Chandra Detection of Hotspot and Knots of 3C 303

We report the detection at X-rays of the radio/optical hotspot and knots of 3C 303 from a short (15 ksec) Chandra exposure in 2001 March. The X-ray morphology is similar to that of the radio/optical emission with peaks in the X-ray emission found at 5.5''(knot B), 9'' (knot C) and 17'' (hotspot) from the core of 3C 303. Despite the limited signal-to-noise ratio of the short Chandra exposure, the X-ray photon spectrum was measured for the hotspot. We construct the spectral energy distribution (SED) and find that the X-ray flux is well below the extrapolation of the radio-to-optical continuum, which we interpret as resulting from the production of X-rays via inverse Compton scattering of both synchrotron photons (SSC) and cosmic microwave background photons (EC/CMB). The magnetic field strength, region size, and the maximum energy of electrons are self-consistently determined for the hotspot to be B ~ 4.3 μG, R ~ 6.5x10^{21} cm, and G_max ~ 1.4x10^7. This implies a magnetic field strength a factor of ~30 below the equipartition value; B_eq ~ 150 μG. The origin of this large departure from equipartition is still uncertain, but the discrepancy is reduced if the plasma in the hotspot is moving with mildly relativistic speeds. Our observation of 3C 303, as well as recent Chandra detections of large scale jets and hotspots in a number of radio galaxies, confirm that particles are accelerated very efficiently in radio galaxies.

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arXiv2002

Properties of the Brightest Cluster Galaxy and Its Host Cluster

We investigate the relation between the properties of Brightest Cluster Galaxies (BCGs) and those of their host clusters. To quantify the properties of cluster hot gas, we employ the parameter $Z$ of the fundamental plane of X-ray clusters. It is found that the offset of the BCG from the peak of cluster X-ray emission is larger for smaller $Z$ clusters. The parameter $Z$ (not the redshift {\it z}), which mainly depends on virial density $ρ_{\rm {vir}}$, is considered to represent the formation epoch of a cluster. We thus consider that the offset of the BCG is correlated with the dynamical equilibrium state of its host cluster. On the contrary, no significant correlation is found between the absolute optical magnitude of the BCG and the parameter $Z$. If the extreme brightness of the BCG is mainly acquired in the course of cluster evolution by environmental effect, BCGs are expected to be brighter in large $Z$ clusters. Our result is not consistent with this simplified view. On the contrary, it is possible that the extreme brightness of the BCG is likely to be determined in the early history of cluster collapse.

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arXiv2002

RXTE Observations of 3C273 between 1996 and 2000:Variability Timescale and Jet Power

We present the results of a long-look monitoring of 3C273 with RXTE between 1996 and 2000. A total of 230 observations amounts to a net exposure of 845ksec, with this spectral and variability analysis of 3C273 covering the longest observation period available at hard X-ray energies. Flux variations by a factor of 4 have been detected over 4years, whereas less than 30% flux variations have been observed for individual flares on time-scales of 3 days. Two temporal methods, the power spectrum density (PSD) and the structure function (SF), have been used to study the variability characteristics of 3C273. The hard X-ray photon spectra generally show a power-law shape with a differential photon index of G = 1.6+-0.1. In 10 of 261 data segments, exceptions to power-law behaviour have been found: (i) an additional soft excess below 4keV, and (ii) a broad Fe fluorescent line feature with EW = 100-200 eV. Our new observations of these previously reported X-ray features may imply that 3C273 is a unique object whose hard X-ray emission occasionally contains a component which is not related to a beamed emission (Seyfert like), but most hard X-rays are likely to originate in inverse Compton radiation from the relativistic jet (blazar like). Multi-frequency spectra from radio to gamma-ray are presented in addition to our RXTE results. The X-ray time variability and spectral evolution are discussed in the framework of beamed, synchrotron self-Compton picture. We consider the ``power balance'' (both radiative and kinetic) between the accretion disk, sub-pc-scale jet, and the 10kpc-scale jet.

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arXiv2001

Energetics of Tev Blazars and Physical Constraints on their Emission Regions

Using multi-frequency spectra from TeV blazars in quiescent states, we obtain the physical parameters of the emission region of blazars within the framework of the one-zone synchrotron self-Compton (SSC) model. We numerically calculate the steady-state energy spectra of electrons by self-consistently taking into account the effects of radiative cooling with a proper account of the Klein-Nishina effects. Here electrons are assumed to be injected with a power-law spectrum and to escape on a finite time scale, which naturally leads to the existence of a break energy scale. Although we do not use time variabilities but utilize a model of electron escape to constrain the size of the emission region, the resultant size turns out to be similar to that obtained based on time variabilities. Through detailed comparison of the predicted emission spectra with observations, we find that for Mrk 421, Mrk 501, and PKS 2155--304, the energy density of relativistic electrons is about an order of magnitude larger than that of magnetic fields with an uncertainty within a factor of a few.

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arXiv2001

Characteristic X-ray Variability of TeV Blazars: Probing the Link between the Jet and the Central Engine

We have studied the rapid X-ray variability of three extragalactic TeV gamma-ray sources; Mrk 421, Mrk 501 and PKS 2155-304. Analyzing the X-ray light curves obtained from ASCA and/or RXTE observations between 1993 and 1998, we have investigated the variability in the time domain from 10^3 to 10^8 sec. For all three sources, both the power spectrum density (PSD) and the structure function (SF) show a roll-over with a time-scale of the order of 1 day or longer, which may be interpreted as the typical time-scale of successive flare events. Although the exact shape of turn-over is not well constrained and the low-frequency (long time-scale) behavior is still unclear, the high-frequency (short time-scale) behavior is clearly resolved. We found that, on time-scales shorter than 1 day, there is only small power in the variability, as indicated by a steep power spectrum density of f^{-2~-3}. This is very different from other types of mass-accreting black-hole systems for which the short time-scale variability is well characterized by a fractal, flickering-noise PSD (f^{-1~-2}). The steep PSD index and the characteristic time-scale of flares imply that the X-ray emitting site in the jet is of limited spatial extent; D > 10^{17} cm distant from the base of the jet, which corresponds to > 100 Schwarzschild radii for 10^{7-10} M_solar black-hole systems.

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