arXiv subjects

N. Kawai

Publications and source records attributed to N. Kawai.

123 records · Page 7Linked to original sources

arXiv2001

Non-Equilibrium Ionization States of GRB Environments

Iron spectral features are thought to be the best tracer of a progenitor of gamma-ray bursts (GRBs). The detections of spectral features such as an iron line and/or a Radiative Recombination edge and Continuum (RRC) were reported in four X-ray afterglows of GRBs. However their properties were different each other burst by burst. For example, Chandra observation of GRB 991216 reported both the strong H-like iron line together with its RRC. On the contrary, Yoshida et al. (2001) report only a detection of the strong RRC in GRB 970828 with ASCA. Since it is difficult to produce the strong RRC, we have to consider special condition for the line and/or the RRC forming region. In this paper, we point out a possibility of a ``non-equilibrium ionization state'' for the line and the RRC forming region.

astro-ph↗

arXiv2001

A Possible Emission Feature in an X-ray Afterglow of GRB970828 as a Radiative Recombination Edge

A gamma-ray burst of 28 August 1997 was localized by the All-Sky Monitor on the Rossi XTE satellite and its coordinates were promptly disseminated. An ASCA followup started 1.17 days after the burst as a Target of Opportunity Observation and detected an X-ray afterglow. The spectral data displayed a hump around ~5 keV and an absorption column of 7.1 x 10^21 cm^{-2}. This hump structure is likely a recombination edge of iron in the vicinity of the source, taking account of the redshift z = 0.9578 found for the likely host galaxy of the associated radio flare. Radiative Recombination edge and Continuum model can interpret the spectrum from highly ionized plasma in a non equilibrium ionization state. The absorption could be also due to the medium presumably in the vicinity of the GRB.

astro-ph↗

arXiv2001

Chandra Observations of G11.2-0.3: Implications for Pulsar Ages

We present Chandra X-ray Observatory imaging observations of the young Galactic supernova remnant G11.2-0.3. The image shows that the previously known young 65-ms X-ray pulsar is at position (J2000) RA 18h 11m 29.22s, DEC -19o 25' 27.''6, with 1 sigma error radius 0.''6. This is within 8'' of the geometric center of the shell. This provides strong confirming evidence that the system is younger, by a factor of ~12, than the characteristic age of the pulsar. The age discrepancy suggests that pulsar characteristic ages can be poor age estimators for young pulsars. Assuming conventional spin down with constant magnetic field and braking index, the most likely explanation for the age discrepancy in G11.2-0.3 is that the pulsar was born with a spin period of ~62 ms. The Chandra image also reveals, for the first time, the morphology of the pulsar wind nebula. The elongated hard-X-ray structure can be interpreted as either a jet or a Crab-like torus seen edge on. This adds to the growing list of highly aspherical pulsar wind nebulae and argues that such structures are common around young pulsars.

astro-ph↗

arXiv2001

Five-year Monitorings of TeV Blazars with ASCA and RXTE

We study the temporal/spectral variability of two extragalactic TeV sources, Mrk 421 and Mrk 501, based on 5-year observations with the ASCA and RXTE satellites. We found that the peak of the synchrotron emission exists just in the X-ray band and its position shifted from lower to higher energy when the source became brighter. Relationship between the peak energy and peak luminosity showed quite different behavior in the two sources; Mrk 421 showed very little change in the peak position (0.5-2 keV), while Mrk 501 revealed the largest shift ever observed in blazars (1-100 keV). We analyze these X-ray data with the flux changes in TeV band, which are obtained from 35 truly simultateneous observations. Very different spectral evolution of both objects indicates some differences in the electron acceleration mechanism at work during the flares. We argue that the flux variability of Mrk 421 is associated with an increase in the number of electrons, while the flare of Mrk 501 is mostly due to the large changes in maximum energy of electrons. We also discuss the characteristic temporal variability of TeV sources, and implications for the X-ray emitting site in the relativistic jet.

astro-ph↗

arXiv2001

ASCA Detection of Pulsed X-ray Emission from PSR J0631+1036

ASCA's long look at the 288 millisecond radio pulsar, PSR J0631+1036, reveals coherent X-ray pulsation from this source for the first time. The source was first detected in the serendipitous Einstein observation and later identified as a radio pulsar. Possible pulsation in the gamma-ray band has been detected from the CGRO EGRET data (Zepka, et al. 1996). The X-ray spectrum in the ASCA band is characterized by a hard power-law type emission with a photon index of about 2.3, when fitted with a single power-law function modified with absorption. An additional blackbody component of about 0.14 keV increases the quality of the spectral fit. The observed X-ray flux is 2.1e-13 ergs/s/cm2 in the 1-10 keV band. We find that many characteristics of PSR J0631+1036 are similar to those of middle-aged gamma-ray pulsars such as PSR B1055-52, PSR B0633+17 (Geminga), and PSR B0656+14.

astro-ph↗

arXiv2000

Observation of X-ray lines from a Gamma-Ray Burst (GRB991216): Evidence of Moving Ejecta from the Progenitor

We report on the discovery of two emission features observed in the X-ray spectrum of the afterglow of the gamma-ray burst (GRB) of 16 Dec. 1999 by the Chandra X-Ray Observatory. These features are identified with the Ly$_α$ line and the narrow recombination continuum by hydrogenic ions of iron at a redshift $z=1.00\pm0.02$, providing an unambiguous measurement of the distance of a GRB. Line width and intensity imply that the progenitor of the GRB was a massive star system that ejected, before the GRB event, $\approx 0.01 \Ms$ of iron at a velocity $\approx 0.1 c$, probably by a supernova explosion.

astro-ph↗

arXiv2000

Discovery of a Luminous, Variable, Off-Center Source in the Nucleus of M82 with the Chandra HRC

We present results from observations of the most famous starburst galaxy M82 with the High-Resolution Camera onboard the Chandra X-Ray Observatory. We found nine sources in the central 1' x 1' region, but no source was detected at the galactic center. Comparing the observations on 1999 October 28 and on 2000 January 20, we found four of the nine sources showed significant time variability. In particular, CXOM82J095550.2+694047, which is 9" away from the galactic center, showed extremely large time variability. We conclude that this source is the origin of the hard X-ray time variability of M82 detected with ASCA. Assuming a spectral shape obtained by the ASCA observation, its luminosity in the 0.5 -- 10 keV band changed from 1.2E40 erg/s on 1999 October 28 to 8.7E40 erg/s on 2000 January 20.

astro-ph↗

arXiv2000

ASCA Observations of the Jet Source XTE J1748-288

XTE J1748-288 is a new X-ray transient with a one-sided radio jet. It was observed with ASCA on 1998/09/06 and 1998/09/26, 100 days after the onset of the radio-X-ray outburst. The spectra were fitted with an attenuated power-law model, and the 2-6-keV flux was 4.6 * 10^{-11} erg s^{-1} cm^{-2} and 2.2 * 10^{-12} on 09/06 and 09/26, respectively. The light curve showed that the steady exponential decay with an e-folding time of 14 days lasted over 100 days and 4 orders of magnitude from the peak of the outburst. The celestial region including the source had been observed with ASCA on 1993/10/01 and 1994/09/22, years before the discovery. In those period, the flux was < 10^{-13} erg s^{-1} cm^{-2}, below ASCA's detection limit. The jet blob colliding to the environmental matter was supposedly not the X-ray source, although the emission mechanism has not been determined. A possible detection of a K line from highly ionized iron is discussed.

astro-ph↗

arXiv2000

Rapid Synchrotron Flares from BL Lacertae Detected by ASCA and RXTE

We report the variable X-ray emission from BL Lacertae detected in the ASCA ToO observation conducted during the EGRET and RXTE pointings, coincident with the 1997 July outburst. The source showed a historically high state of X-ray, optical, and gamma-ray emission, with its 2-10 keV flux peaking at ~3.3 E-11 erg/cm2/s. This is more than 3 times higher than the value measured by ASCA in 1995. We detected two rapid flares which occured only in the soft X-ray band, while the hard X-ray flux also increased, but decayed with a much longer time scale. Together with the requirement of a very steep and varying power law dominating the soft X-ray band in addition to the hard power law, we suggest that both the high energy end of the synchrotron spectrum and the hard inverse Compton spectrum were visible in this source during the outburst. We discuss the possible origins of the observed variability time scales, and interpret the short time scales of the soft X-ray variability as reflecting the size of the emission region.

astro-ph↗

arXiv1999

The putative pulsar-wind nebulae of the three Musketeers PSR B1055-52, B0656+14 and Geminga revisited

We report on the analysis of archival ASCA, ROSAT and BeppoSAX data of PSR B1055-52, PSR B0656+14 and Geminga, performed in order to investigate the reality of the putative ~10-20 arcmin wide X-ray pulsar-wind nebulae recently proposed to exist around these pulsars. In all these cases, the ROSAT and/or BeppoSAX data show that the diffuse and clumpy X-ray nebulae found by ASCA can be resolved in the contribution of unidentified point sources, unlikely related to the pulsars.

astro-ph↗

arXiv1999

Spatial and spectral X-ray properties of the powerful radio galaxy Hercules A: environment and jet/ICM interaction

We investigate the X-ray properties of the powerful radio galaxy Hercules A (3C 348) using ROSAT HRI, PSPC and ASCA observations. The ASCA data are well fit by a thermal plasma model with a temperature of about 4.3 keV and abundances 0.4 solar. The HRI observation clearly reveals extended and elongated X-ray emission. For radii greater than 10 arcsec, the surface brightness profile perpendicular to the elongation is well fit by an isothermal beta--model with beta = 0.63 and a core radius of about 120 kpc. We derive a total mass of the putative cluster of 8.4e+13 M_sun and a gas mass fraction of about 18 per cent. The inner part of the surface brightness profile clealry reveals the presence of a point source, which contributes about 8 per cent to the total flux. The 0.1-2.4 keV luminosity of the point-like and the extended emission is 3.4e+43 and 4.3e+44 erg/s, respectively. After subtracting the cluster X-ray emission from the HRI image, residual structures are visible, which partly coincide with the radio jet and lobes. This indicates an interaction of the radio jet with the intracluster medium.

astro-ph↗

arXiv1999

A Dark Cloud Associated an Unidentified EGRET Source

CO J=1-0 mapping observations of the dark nebula Lynds 227, which is possibly associated with the unidentified EGRET source 2EG J1811-2339, are presented. We detected a large amount of molecular gas along Lynds 227 with a total mass of (1-2)E4 Msun surrounding an X-ray synchrotron nebula, which was detected by the ASCA satellite within the error circle of 2EG J1811-2339. Molecular gas along Lynds 227 shows spatial anti-correlation with the X-ray synchrotron nebula, suggesting that the synchrotron nebula is interacting with the Lynds 227 cloud. We propose a $γ$-ray emission mechanism for 2EG J1811-2339: High energy electrons are injected from a rotation-powered pulsar and accelerated further in shock waves generated by the interaction with ambient matter. The high energy electrons move into the molecular cloud at the Lynds 227 and collide with dense interstellar matter to yield high energy $γ$-ray photons, mainly through relativistic bremsstrahlung.

astro-ph↗

arXiv1999

X-Ray/Gamma-Ray Observations of the PSR B1259-63/SS 2883 system near Apastron

We report the results from X-ray and hard X-ray observations of the PSR B1259-63/SS 2883 system with the ASCA and CGRO satellites, performed between 1995 February and 1996 January when the pulsar was near apastron. The system was clearly detected in each of the two ASCA observations with luminosity in the 1-10 keV band of Lx = (9 +- 3) x10^32 (d/2 kpc)^2 erg/s, while CGRO/OSSE did not detect significant hard X-rays from the system. X-ray spectra obtained with ASCA are well-fit by a single power-law spectrum with a photon index of 1.6 +- 0.3. No pulsations were detected in either the ASCA or the OSSE data. We combine all existing X-ray and hard X-ray observations and present the orbital modulation in the luminosity and photon index for the entire orbit. The results are in agreement with predictions based on a synchrotron emission model from relativistic particles in a shocked pulsar wind interacting with the gaseous outflow from the Be star.

astro-ph↗

arXiv1999

The X-ray CCD camera of the MAXI Experiment on the ISS/JEM

MAXI, Monitor of All-sky X-ray Image, is the X-ray observatory on the Japanese experimental module (JEM) Exposed Facility (EF) on the International Space Station (ISS). MAXI is a slit scanning camera which consists of two kinds of X-ray detectors: one is a one-dimensional position-sensitive proportional counter with a total area of $\sim 5000 cm^2$, the Gas Slit Camera (GSC), and the other is an X-ray CCD array with a total area $\sim 200 cm^2$, the Solid-state Slit Camera (SSC). The GSC subtends a field of view with an angular dimension of 1$^\circ\times 180^\circ$ while the SSC subtends a field of view with an angular dimension of 1$^\circ$ times a little less than 180$^\circ$. In the course of one station orbit, MAXI can scan almost the entire sky with a precision of 1$^\circ$ and with an X-ray energy range of 0.5-30 keV. We have developed the engineering model of CCD chips and the analogue electronics for the SSC. The energy resolution of EM CCD for Mn K$α$ has a full-width at half maximum of $\simeq$ 182 eV. Readout noise is $\simeq$ 11 e^- rms.

physics.ins-det↗

arXiv1998

RXTE Absolute Timing Results for the Pulsars B1821-24 and B1509-58

Observations with the Rossi X-ray Timing Explorer and the Jodrell Bank, Parkes, and Green Bank telescopes have enabled us to determine the time delay between radio and X-ray pulses in the two isolated pulsars B1821-24 and B1509-58. For the former we find that the narrow X-ray and radio pulse components are close to being coincident in time, with the radio peak leading by 0.02 period (60 +/- 20 microsec), while the wide X-ray pulse component lags the last of the two wider radio components by about 0.08 period. For the latter pulsar we find, using the standard value for the dispersion measure, that the X-ray pulse lags the radio by about 0.27 period, with no evidence for any energy-dependence in the range 2-100 keV. However, uncertainties in the history of the dispersion measure for this pulsar make a comparison with previous results difficult. It is clear that there are no perceptable variations in either the lag or the dispersion measure at time scales of a year or less.

astro-ph↗