arXiv subjects

Tadayuki Takahashi

Publications and source records attributed to Tadayuki Takahashi.

121 records · Page 7Linked to original sources

arXiv2003

Evolution of the Synchrotron Spectrum in Mrk421 during the 1998 Campaign

The uninterrupted 7-day ASCA observations of the TeV blazar Mrk421 in 1998 have clearly revealed that X-ray flares occur repeatedly. In this paper, we present the results of the time-resolved spectral analysis of the combined data taken by ASCA, RXTE, BeppoSAX, and EUVE. In this object - and in many other TeV blazars - the precise measurement of the shape of the X-ray spectrum, which reflects the high energy portion of the synchrotron component, is crucial in determining the high energy cutoff of the accelerated electrons in the jet. Thanks to the simultaneous broadband coverage, we measured the 0.1-25 keV spectrum resolved on time scales as short as several hours, providing a great opportunity to investigate the detailed spectral evolution at the flares. By analyzing the time subdivided observations, we parameterize the evolution of the synchrotron peak, where the radiation power dominates, by fitting the combined spectra with a quadratic form (where the vFv flux at the energy E obeys log vFv(E)=log(vFv_peak) - const x (log E - log Epeak)^2). In this case, we show that there is an overall trend that the peak energy Ep and peak flux vFvp both increase or decrease together. The relation of the two parameters is best described as Ep \propto vFvp^{0.7} for the 1998 campaign. Similar results were derived for the 1997 observation, while the relation gave a smaller index when included both 1997 and 1998 data. On the other hand, we show that this relation, and also the detailed spectral variations, differ from flare to flare within the 1998 campaign. We suggest that the observed features are consistent with the idea that flares are due to a appearance of a new spectral component. With the availability of the simultaneous TeV data, we also show that there exists a clear correlation between the synchrotron peak flux and the TeV flux.

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arXiv2003

Detection of a Fully-resolved Compton Shoulder of the Iron K-alpha Line in the Chandra X-ray Spectrum of GX 301-2

We report the detection of a fully-resolved, Compton-scattered emission line in the X-ray spectrum of the massive binary GX 301-2 obtained with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory. The iron K-alpha fluorescence line complex observed in this system consists of an intense narrow component centered at an energy of E = 6.40 keV and a redward shoulder that extends down to ~6.24 keV, which corresponds to an energy shift of a Compton back-scattered iron K-alpha photon. From detailed Monte Carlo simulations and comparisons with the observed spectra, we are able to directly constrain the physical properties of the scattering medium, including the electron temperature and column density, as well as an estimate for the metal abundance.

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arXiv2003

Optical Identification of the ASCA Medium Sensitivity Survey in the Northern Sky: Nature of Hard X-ray-selected Luminous Active Galactic Nuclei

We present the results of optical spectroscopic identifications of a bright subsample of 2-10keV hard X-ray selected sources from the ASCA Medium Sensitivity Survey in the northern sky. The flux limit of the subsample is 3*10^-13 erg s^-1 cm^-2 in the 2-10keV band. All but one of the 87 hard X-ray selected sources are optically identified, with AGNs, 7 clusters of galaxies, and 1 galactic star. It is the largest complete sample of hard X-ray selected AGNs at the bright flux limit. Amounts of absorption to their nuclei are estimated to be hydrogen column densities (N_H) of up to ~3*10^23 cm^-2 from their X-ray spectra. Optical properties of X-ray absorbed AGNs with N_H > 1*10^22 cm^-2 indicate the effects of dust absorption: at redshifts, z<0.6, AGNs without broad Hβemission lines have significantly larger N_H value than AGNs with broad Hβemission lines. At z>0.6, the X-ray absorbed AGNs have a large hard X-ray to optical flux ratio (log f2-10keV/fR > +1). However, three X-ray absorbed z>0.6 AGNs show strong broad lines. In combination with hard X-ray selected AGN samples from the ASCA Large Sky Survey, the ASCA Deep Survey in the Lockman Hole and Chandra Deep Field North, the luminosity distributions of absorbed and less-absorbed AGNs are compared.

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arXiv2002

Implications of Variability Patterns observed in TeV Blazars on the Structure of the Inner Jet

The recent long look X-ray observations of TeV blazars have revealed many important new features concerning their time variability. In this paper, we suggest a physical interpretation for those features based on the framework of the internal and external shock scenarios. We present a simplified model applicable to TeV blazars, and investigate through simulations how each of the model parameters would affect to the observed light curve or spectrum. In particular, we show that the internal shock scenario naturally leads to all the observed variability properties including the structure function, but for it to be applicable, the fractional fluctuation of the initial bulk Lorentz factors must be small, with sigma_gamma / gamma_average < 0.01. This implies very low dynamical efficiency of the internal shock scenario. We also suggest that several observational quantities -- such as the characteristic time scale, the relative amplitude of flares as compared to the steady (``offset'') component, and the slope of the structure function -- can be used to probe the inner jet. The results are applied to the TeV blazar Mrk421, and this, within the context of the model, leads to the determination of several physical parameters: the ejection of a shell with average thickness of ~1E13 cm occurs on average every 10 minutes, and the shells collide ~1E17 cm away from the central source.

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arXiv2002

Chandra Observations and Optical Identification of Hard X-ray Sources Discovered with ASCA

We present the first results of the Chandra and optical follow-up observations of hard X-ray sources detected in the ASCA Medium Sensitivity Survey (AMSS). Optical identifications are made for five objects. Three of them show either weak or absent optical narrow emission lines and are at low redshift ~0.06. One of them is a broad line object at z=0.910 and one is a z=0.460 object with only narrow lines. All the narrow line objects show strong evidence for absorption in their X-ray spectra. Their line ratios are consistent with a Seyfert II/LINER identification as are the line widths. The three low redshift objects have the colors of normal galaxies and apparently the light is dominated by stars. This could be due to the extinction of the underlying nuclear continuum by the same matter that absorbs X-rays and/or due to the dilution of the central source by starlight. These results suggest that X-ray sources that appear as ``normal'' galaxies in optical and near-IR bands significantly contribute to the hard X-ray background. This population of objects has a high space density and probably dominates the entire population of active galaxies.

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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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arXiv2002

ASCA View of the Supernova Remnant Gamma Cygni (G78.2+2.1): Bremsstrahlung X-ray Spectrum from Loss-flattened Electron Distribution

We perform X-ray studies of the shell-type supernova remnant (SNR) gamma-Cygni associated with the brightest EGRET unidentified source 3EG J2020+4017. In addition to the thermal emissions with characteristic temperature of kT = 0.5-0.9 keV, we found an extremely hard X-ray component from several clumps localized in the northern part of the remnant. This component is described by a power-law with a photon index of 0.8-1.5. Both the absolute flux and the spectral shape of the nonthermal X-rays cannot be explained by the synchrotron or inverse-Compton mechanisms. We argue that the unusually hard X-ray spectrum can be naturally interpreted in terms of nonthermal bremsstrahlung from Coulomb-loss-flattened electron distribution in dense environs with the gas density about 10 to 100 cm^-3 . For given spectrum of the electron population, the ratio of the bremsstrahlung X- and gamma-ray fluxes depends on the position of the ``Coulomb break'' in the electron spectrum. The bulk of gamma-rays detected by EGRET would come from the radio-bright and X-ray dim cloud at southeast, where very dense gas and strong magnetic field would illuminate the cloud in the radio and gamma-ray bands, but suppress the bremsstrahlung X-ray emission due to the shift of the ``Coulomb break'' in the electron spectrum towards higher energies.

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arXiv2001

Variability Time Scales of TeV Blazars Observed in the ASCA Continuous Long-Look X-ray Monitoring

Three uninterrupted, long (lasting respectively 7, 10, and 10 days) ASCA observations of the well-studied TeV-bright blazars Mrk 421, Mrk 501 and PKS 2155-304 all show continuous strong X-ray flaring. Despite the relatively faint intensity states in 2 of the 3 sources, there was no identifiable quiescent period in any of the observations. Structure function analysis shows that all blazars have a characteristic time scale of ~ a day, comparable to the recurrence time and to the time scale of the stronger flares. On the other hand, examination of these flares in more detail reveals that each of the strong flares is not a smooth increase and decrease, but exhibits substructures of shorter flares having time scales of ~10 ks. We verify via simulations that in order to explain the observed structure function, these shorter flares ("shots") are unlikely to be fully random, but in some way are correlated with each other. The energy dependent cross-correlation analysis shows that inter-band lags are not universal in TeV blazars. This is important since in the past, only positive detections of lags were reported. In this work, we determine that the sign of a lag may differ from flare to flare; significant lags of both signs were detected from several flares, while no significant lag was detected from others. However, we also argue that the nature of the underlying component can affect these values. The facts that all flares are nearly symmetric and that fast variability shorter than the characteristic time scale is strongly suppressed, support the scenario where the light crossing time dominates the variability time scales of the day-scale flares.

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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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arXiv2001

X-Ray Probing of the Central Regions of Clusters of Galaxies

Results of ASCA X-ray study of central regions of medium-richness clusters of galaxies are summarized, emphasizing differences between cD and non-cD clusters. The intra-cluster medium (ICM) is likely to consist of two (hot and cool) phases within $\sim 100$ kpc of a cD galaxy, where the ICM metallicity is also enhanced. In contrast, the ICM in non-cD clusters appears to be isothermal with little metallicity gradient right to the center. The gravitational potential exhibits a hierarchical nesting around cD galaxies, while a total mass-density profile with a central cusp is indicated for a non-cD cluster Abell~1060. The iron-mass-to-light ratio of the ICM decreases toward the center in both types of clusters, although it is radially constant in peripheral regions. The silicon-to-iron abundance ratio in the ICM increases with the cluster richness, but remains close to the solar ratio around cD galaxies. These overall results are interpreted without appealing to the popular cooling-flow hypothesis. Instead, an emphasis is put on the halo-in-halo structure formed around cD galaxies.

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arXiv1999

The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Counts and Evidence for Emerging Population of Hard Sources

We present first results from the ASCA Medium Sensitivity Survey (AMSS; or the GIS catalog project). From the serendipitous fields amounting to 106 degE-2, we determined the Log N - Log S relations in the 0.7-7 keV and 2-10 keV bands with the best statistical accuracy obtained so far, over the flux range from 1 x 10E-11 to 5 x 10E-14 and 7 x 10E-14 erg sE-1 cmE-2, respectively. When the sources detected in the 0.7-7 keV band are divided into two subsamples with higher and lower spectral hardness, the former exhibits a significantly steeper slope than the latter at fluxes below ~10E-12 erg sE-1 cmE-2 (0.7-7 keV). The average spectrum of sources becomes continuously harder toward fainter fluxes, from a photon index of 2.1 in the 0.7-10 keV range at the flux of ~10E-11 to 1.6 at ~10E-13 erg sE-1 cmE-2 (0.7-7 keV). This is consistent with the comparison of source counts between the 2-10 keV and the 0.7-2 keV band, and solves the puzzle of their discrepancy reported previously. Our results demonstrate rapid emergence of hard X-ray sources with a decreasing flux from \~10E-12 to ~10E-13 erg sE-1 cmE-2 (2-10 keV).

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arXiv1999

X-ray Observations of TeV Blazars and Multi-Frequency Analysis

The non-thermal spectra of blazars, observed from radio to GeV/TeV gamma-rays, reveal two pronounced components, both produced by radiation by energetic particles. One peaks in the IR - to soft X-ray band, radiating via the synchrotron process; the other, peaking in the high-energy gamma-rays, is produced by the Compton process. These spectra -- and, in particular, the \asca data -- suggest that the origin of the seed photons for Comptonization is diverse. In the High-energy peaked BL Lac objects (HBLs), the dominant seed photons for Comptonization appear to be the synchrotron photons internal to the jet (SSC process). In the quasar-hosted blazars (QHBs), on the other hand, the X-ray band emission is still dominated by the SSC process, while the MeV to GeV range is produced by Comptonization of external photons such as the emission line light. In the context of this three-component model, we derive the magnetic field of 0.1 - 1 Gauss for all classes of blazars. Lorentz factors gamma_{peak} of electrons radiating at each peak of the nuFnu spectra are estimated to be ~10^{5}$ for HBLs; this is much higher than ~10^{3}$ for QHBs. This difference is consistent with the fact that the four sources that are known to emit TeV gamma-rays (TeV blazars) are all classified as HBLs. Among the TeV blazars, Mkn 421 is one of the brightest and most variable emitters from ultraviolet (eV) to hard gamma-ray (TeV) energies.The multi-frequency observations including TeV energy band provide the best opportunity to understand high-energy emission from blazar jets. In this paper, we discuss results of multi-frequency analysis and review the results of intensive campaigns for Mkn 421 from 1994 to 1998

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arXiv1998

The hardest X-ray source in the ASCA Large Sky Survey: Discovery of a new type 2 Seyfert

We present results of ASCA deep exposure observations of the hardest X-ray source discovered in the ASCA Large Sky Survey (LSS) project, designated as AX J131501+3141. We extract its accurate X-ray spectrum, taking account of the contamination from a nearby soft source (AX J131502+3142), separated only by 1'. AX J131501+3141 exhibits a large absorption of NH = (6 +4 -2)x 10^22 H/cm^2 with a photon index Γ= 1.5 +0.7 -0.6. The 2--10 keV flux was about 5 x 10^-13 erg/s/cm^2 and was time variable by a factor of 30% in 0.5 year. From the highly absorbed X-ray spectrum and the time variability, as well as the results of the optical follow-up observations (Akiyama et al. 1998, astro-ph/9801173), we conclude that AX J131501+3141 is a type 2 Seyfert galaxy. Discovery of such a low flux and highly absorbed X-ray source could have a significant impact on the origin of the cosmic X-ray background.

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