Makoto Tashiro

The nature of ultraluminous compact X-ray sources in nearby spiral galaxies

Studies were made of ASCA spectra of seven ultraluminous compact X-ray sources in nearby spiral galaxies: M33 X-8, M81 X-6, IC 342 source 1, Dwingeloo 1 X-1, NGC 1313 source B, and two sources in NGC 4565. With the 0.5-10 keV luminosities in the range 1039-1040 ergs s-1, they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all seven sources have been described successfully with so-called multicolor disk blackbody emission arising from optically thick standard accretion disks around black holes. Except for the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ~100 M☉. However, the observed innermost disk temperatures of these objects, Tin = 1.1-1.8 keV, are too high to be compatible with the required high black hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole and hence hotter.

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

Hard X-ray Detector (HXD) on board Suzaku

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit back ground based on a combination of new techniques, including the concept of well-type active shield counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV, the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (~5 degree). The on-board signal-processing system and the data transmitted to the ground are also described.

In-orbit performance of the hard X-ray detector on board Suzaku

The in-orbit performance and calibration of the Hard X-ray Detector (HXD) on board the X-ray astronomy satellite Suzaku are described. Its basic performances, including a wide energy bandpass of 10–600keV, energy resolutions of ∼ 4keV (FWHM) at 40keV and ∼ 11% at 511keV, and a high background rejection efficiency, have been confirmed by extensive in-orbit calibrations. The long-term gains of PIN-Si diodes have been stable within 1% for half a year, and those of scintillators have decreased by 5–20%. The residual non-X-ray background of the HXD is the lowest among past non-imaging hard X-ray instruments in energy ranges of 15–70 and 150–500keV. We provide accurate calibrations of energy responses, angular responses, timing accuracy of the HXD, and relative normalizations to the X-ray CCD cameras using multiple observations of the Crab Nebula.

ASCA Observation of an X-Ray/TeV Flare from the BL Lacertae Object Markarian 421

We observed the BL Lac object Mrk 421 with the X-ray satellite ASCA in 1994 as part of a multifrequency observation. The 24 hr observation was conducted 1 day after the onset of a TeV flare detected by the Whipple Observatory and detected an X-ray flare, with no apparent variability in the optical, UV, and EGRET GeV flux. The ASCA 2-10 keV flux peaked at 3.7 ? 10-10 ergs cm-2 s-1 and then decreased to 1.8 ? 10-10 ergs cm-2 s-1 with a doubling timescale of ~12 hr. The shape of the X-ray spectrum varied during the observation, such that the hard X-rays always led the soft X-rays, both in brightening and dimming of the source, with a lag of the 0.5-1 keV photons versus those in the 2-7.5 keV band of ~1 hr. The rapid TeV variability indicates a compact TeV-producing region, suggesting relativistic beaming with a Doppler factor ? ? 5. The correlation of the flux in the X-ray and the TeV bands indicates that a high-energy tail of a single electron population is responsible for both X-rays and TeV ?-rays, with radio, IR, UV and X-rays produced via the synchrotron process and GeV and TeV ?-rays produced via Comptonization. Under the assumption that the soft lag observed in the X-ray band is due to the synchrotron-lifetime effects, with ? = 5, we calculate the magnetic field for the X-ray-producing region to be ~0.2 G. The Lorentz factors ?el of the electrons responsible for the emission in the keV and TeV bands are ~106, consistent with the values implied by the Klein-Nishina limit.

ASCA Observations of Blazars and Multiband Analysis

We present data for 18 blazars observed with the X-ray satellite ASCA, half of which were also observed contemporaneously with the EGRET instrument onboard Compton Gamma Ray Observatory as parts of multiwavelength campaigns. The observations show a clear difference in the spectra between three subclasses of blazars, namely, high-energy peaked BL Lacertae objects (HBLs), low-energy peaked BL Lac objects (LBLs), and quasar-hosted blazars (QHBs). The ASCA X-ray spectra of HBLs are the softest, with the power-law energy index α ~ 1-2, and they form the highest observable energy tail of the low-energy (LE, synchrotron) component. The X-ray spectra of the QHBs are the hardest (α ~ 0.6) and are consistent with the lowest observable energy end of the high-energy (HE, Compton) component. For LBLs, the X-ray spectra are intermediate. We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Doppler-boosted synchrotron self-Compton (SSC) emission, with the Doppler factor δ consistent with the VLBI and/or γ-ray variability data. For many QHBs, on the other hand, the γ-rays cannot be solely a result of the SSC mechanism unless δ is significantly in excess of that inferred from VLBI data. We consider an alternative scenario consistent with the measured values of δ, where the SSC component is still present in QHBs and dominates in the X-ray band but is below the observed γ-ray spectrum. With an assumption that the peak of the SSC emission is on the extrapolation of the X-ray spectrum, and adopting a value of 10 for δ, we infer the magnetic field B to be 0.1-1 G and Lorentz factors γb of electrons radiating at the peak of the νF(ν) spectrum of ~103 for QHBs; this is much lower than γb ~ 105 for HBLs, even though the values of B are comparable in the two subclasses. This difference of γb is most likely due to the large photon density expected in QHBs (e.g., from thermal components visible in these objects) as compared with that of HBLs; Compton upscattering of these photons may well provide the observed GeV flux.

Variability Pattern and the Spectral Evolution of the BL Lacertae Object PKS 2155–304

The TeV blazar PKS 2155(304 was monitored with the X-ray satellite ASCA in 1994 May as part of a multiwavelength campaign from the radio to X-ray bands. At the beginning of the two-day continuous observation, we detected a large —are, in which the 2¨10 keV —ux changed by a factor of 2 on a time- scale of 3 ) 104 s. During the —are, the increase in the hard X-ray —ux clearly preceded that observed in the soft X-rays, with the spectral evolution tracking a ii clockwise loop ˇˇ in the —ux versus photon index plane. Ascribing the energy-dependent variability to diUerential synchrotron cooling of relativistic elec- trons, we estimate the magnetic —eld B in the emission region. We tested two diUerent methods of com- paring the time series in various X-ray bands: (1) —tting the light curves to a Gaussian function and searching for the time shift of the peak of the —are, and (2) calculating the discrete correlation function. Both methods yielded a consistent solution of B D 0.1 G. We also found that the —are amplitude becomes larger as the photon energy increases, while the duration of the —are stays roughly constant throughout the ASCA energy band (0.7¨7.5 keV). In the framework of the time-dependent synchrotron self-Compton model in a homogeneous region, we consider a —are where the maximum Lorentz factor of the injected electrons increases uniformly throughout the emission volume. The temporal evolu- (c max ) tion of spectra as well as the light curves were reproduced with the physical parameters self-consistently determined from seven observables. We obtained B D 0.1¨0.2 G and a region size R D 10~2 pc for rela- tivistic beaming with a Doppler factor of d D 20¨30. We discuss the signi—cance of light-travel time eUects. Subject headings: BL Lacertae objects: individual (PKS 2155(304) ¨ gamma rays: theory ¨ radiation mechanisms: nonthermalX-rays: galaxies

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155?304 in 1994 May. III. Probing the Inner Jet through Multiwavelength Correlations

In 1994 May, the BL Lac object PKS 2155-304 was observed continuously for ~10 days with the International Ultraviolet Explorer and the Extreme Ultraviolet Explorer and for 2 days with ASCA, as well as with ROSAT and with ground-based radio, infrared, and optical telescopes. The light curves show a well-defined X-ray flare followed by a broader, lower amplitude extreme-ultraviolet flare ~1 day later and a broad, low-amplitude UV flare ~2 days later. X-ray fluxes obtained at three well-separated times the preceding week indicate at least one previous flare of comparable amplitude or perhaps ongoing stochastic X-ray variations, and additional rapid variability was seen at the beginning of the IUE observation, when extremely sharp changes in UV flux occurred. The X-ray flux observed with ASCA flared by a factor of ~2 in about half a day and decayed roughly as fast. In contrast, the subsequent UV flare had an amplitude of only ~35% and lasted longer than 2 days. Assuming that the X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous, relativistic jet. Because of the high quality of the data, we can rule out that the observed flares were caused by either a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t hours) injection of high-energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions. This kind of situation is expected to occur behind a shock front and/or in an inhomogeneous jet. The present light curves are in sharp contrast to the multiwavelength variability observed in 1991 November, when the amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hr. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes.

Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare

A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at GeV gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission.

Discovery of Spectral Transitions from Two Ultraluminous Compact X-Ray Sources in IC 342

Two ASCA observations were made of two ultraluminous compact X-ray sources (ULXs) in the spiral galaxy IC 342. In the 1993 observation, source 2 showed a 0.5-10 keV luminosity of 6 × 1039 ergs s-1 (assuming a distance of 4.0 Mpc) and a hard power-law spectrum of photon index ~1.4. As already reported, source 1 was ~3 times brighter on that occasion and exhibited a soft spectrum represented by a multicolor disk model with an inner-disk temperature of ~1.8 keV. The second observation, made in 2000 February, revealed that source 1 had made a transition into a hard spectral state, while source 2 made a transition into a soft spectral state. The ULXs are therefore inferred to exhibit two distinct spectral states, and they sometimes make transitions between them. These results significantly reinforce the scenario that describes ULXs as mass-accreting black holes.