J. Kataoka

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.

The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images

The Monitor of All-sky X-ray Image (MAXI) mission is the first astronomical payload to be installed on the Japanese Experiment Module — Exposed Facility (JEM-EF or Kibo-EF) on the International Space Station. It has two types of X-ray slit cameras with wide FOVs and two kinds of X-ray detectors consisting of gas proportional counters covering the energy range of 2 to 30 keV and X-ray CCDs covering the energy range of 0.5 to 12 keV. MAXI will be more powerful than any previous X-ray All Sky Monitor payloads, being able to monitor hundreds of Active Galactic Nuclei. A realistic simulation under optimal observation conditions suggests that MAXI will provide all-sky images of X-ray sources of � 20 mCrab (� 7 � 10 � 10 erg cm � 2 s � 1 in the energy band of 2–30 keV) from observations during one ISS orbit (90 min), � 4.5 mCrab for one day, and � 2 mCrab for one week. The final detectability of MAXI could be � 0.2 mCrab for two years, which is comparable to the source confusion limit of the MAXI field of view (FOV). The MAXI objectives are: (1) to alert the community to X-ray novae and transient X-ray sources, (2) to monitor long-term variabilities of X-ray sources, (3) to stimulate multi-wavelength observations of variable objects, (4) to create unbiased X-ray source cataloges, and (5) to observe diffuse cosmic X-ray emissions, especially with better energy resolution for soft X-rays down to 0.5 keV.

The acceleration of cosmic-ray protons in the supernova remnant RX J1713.7-3946.

Protons with energies up to ∼1015 eV are the main component of cosmic rays, but evidence for the specific locations where they could have been accelerated to these energies has been lacking. Electrons are known to be accelerated to cosmic-ray energies in supernova remnants, and the shock waves associated with such remnants, when they hit the surrounding interstellar medium, could also provide the energy to accelerate protons. The signature of such a process would be the decay of pions (π0), which are generated when the protons collide with atoms and molecules in an interstellar cloud: pion decay results in γ-rays with a particular spectral-energy distribution. Here we report the observation of cascade showers of optical photons resulting from γ-rays at energies of ∼1012 eV hitting Earths upper atmosphere, in the direction of the supernova remnant RX J1713.7–3946. The spectrum is a good match to that predicted by pion decay, and cannot be explained by other mechanisms.

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.

X-Ray Emission Properties of Large-Scale Jets, Hot Spots, and Lobes in Active Galactic Nuclei

We examine a systematic comparison of jet knots, hot spots, and radio lobes recently observed with Chandra and ASCA. This report discusses the origin of their X-ray emissions and investigates the dynamics of the jets. The data were compiled at well-sampled radio (5 GHz) and X-ray (1 keV) frequencies for more than 40 radio galaxies. We examine three models for the X-ray production: synchrotron (SYN), synchrotron self-Compton (SSC), and external Compton (EC) on cosmic microwave background (CMB) photons. For the SYN sources?mostly jet knots in nearby low-luminosity radio galaxies?X-ray photons are produced by ultrarelativistic electrons with energies 10-100 TeV that must be accelerated in situ. For the other objects, conservatively classified as SSC or EC sources, a simple formulation of calculating the expected X-ray fluxes under an equipartition hypothesis is presented. We confirm that the observed X-ray fluxes are close to the expected ones for nonrelativistic emitting plasma velocities in the case of radio lobes and the majority of hot spots, whereas a considerable fraction of jet knots are too bright in X-rays to be explained in this way. We examine two possibilities to account for the discrepancy in a framework of the inverse Compton model: (1) the magnetic field is much smaller than the equipartition value, and (2) the jets are highly relativistic on kiloparsec and megaparsec scales. We conclude that if the inverse Compton model is the case, the X-ray-bright jet knots are most likely far from the minimum-power condition. We also briefly discuss the other possibility, namely, that the observed X-ray emission from all the jet knots is synchrotron in origin.

HIGH-ENERGY EMISSION FROM THE TeV BLAZAR MARKARIAN 501 DURING MULTIWAVELENGTH OBSERVATIONS IN 1996

We present the results of a multiwavelength campaign for Mrk 501 performed in 1996 March with ASCA, EGRET, Whipple, and optical telescopes. The X-ray —ux observed with ASCA was 5 times higher than the quiescent level and gradually decreased by a factor of 2 during the observation in 1996 March. In the X-ray band, a spectral break was observed around 2 keV. We report here for the —rst time the detection of high-energy c-ray —ux from Mrk 501 with EGRET with 3.5 p signi—cance (E ( 100 MeV). Higher —ux was also observed in 1996 AprilMay, with 4.0 p signi—cance for E ( 100 MeV and 5.2 p signi—cance for E ( 500 MeV. The c-ray spectrum was measured to be —atter than most of the c-ray blazars. We —nd that the multiband spectrum in 1996 is consistent with that calculated from a one-zone synchrotron self-Compton (SSC) model in which X-rays are produced via synchrotron emission and c-rays are produced via inverse Compton scattering of synchrotron photons in a homogeneous region. The —ux of TeV c-rays is consistent with the predictions of the model if the decrease of the Compton scattering cross section in the Klein-Nishina regime is considered. In the context of this model, we investigate the values of the magnetic —eld strength and the beaming factor allowed by the obser- vational results. We compare the 1996 March multiwavelength spectrum with that in the —are state in 1997 April. Between these two epochs, the TeV —ux increase is well correlated with that observed in keV range. The keV and TeV amplitudes during the 1997 April —are are accurately reproduced by a one-zone SSC model, assuming that the population of synchrotron photons in 1996 are scattered by newly injected relativistic electrons having maximum energies of However, the TeV spectrum c max D 6 ) 106. observed during the 1996 March campaign is —atter than predicted by our models. We —nd that this cannot be explained by either higher order Comptonization or the contribution of the ii seed ˇˇ IR photons from the host galaxy for the —rst-order external radiation Comptonization, but we cannot exclude possible eUects of the IR photons that may arise in the parsec-size tori postulated to exist in active galactic nuclei. Subject headings: BL Lacertae objects: individual (Markarian 501) ¨ gamma rays: observations ¨ radiation mechanisms: nonthermalX-rays: galaxies

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

Study of Nonthermal Emission from SNR RX J1713.7–3946 with Suzaku

We present results obtained from a series of observations of the supernova remnant RX J1713.7–3946 by Suzaku. Hard X-rays have been detected up to ~40 keV. The hard X-ray spectra are described by a power law with photon indices of ~3.0, which is larger than those below 10 keV. The combination of the spatially integrated XIS and HXD spectra clearly reveals a spectral cutoff which is linked to the maximum energy of accelerated electrons. The broadband coverage of Suzaku allows us to derive, for the first time, the energy spectrum of parent electrons in the cutoff region. The cutoff energy in the X-ray spectrum indicates that the electron acceleration in the remnant proceeds close to the Bohm diffusion limit. We discuss the implications of the spectral and morphological properties of the Suzaku data in the context of the origin of nonthermal emission. The Suzaku X-ray and H.E.S.S. gamma-ray data together hardly can be explained within a pure leptonic scenario. Moreover, the leptonic models require a weak magnetic field, which is inconsistent with the recently discovered X-ray filamentary structures and their short-term variability. The hadronic models with strong magnetic fields provide reasonable fits to the observed spectra, but require special arrangements of parameters to explain the lack of thermal X-ray emission. For morphology studies, we compare the X-ray and TeV gamma-ray surface brightness. We confirm the previously reported strong correlation between X-rays and TeV gamma rays. At the same time, the Suzaku data reveal a deviation from the general tendency, namely, the X-ray emission in the western rims appears brighter than expected from the average X-ray to gamma-ray ratio.

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 γ-ray sources: Mrk 421, Mrk 501, and PKS 2155-304. Analyzing the X-ray light curves obtained from ASCA and/or Rossi X-Ray Timing Explorer observations between 1993 and 1998, we have investigated the variability in the time domain from 103 to 108 s. For all three sources, both the power spectrum density (PSD) and the structure function (SF) show a rollover with a timescale of the order of 1 day or longer, which may be interpreted as the typical timescale of successive flare events. Although the exact shape of turnover is not well constrained and the low-frequency (long timescale) behavior is still unclear, the high-frequency (short timescale) behavior is clearly resolved. We found that, on timescales 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-timescale variability is well characterized by a fractal, flickering-noise PSD (f-1~-2). The steep PSD index and the characteristic timescale of flares imply that the X-ray-emitting site in the jet is of limited spatial extent: D ≥ 1017 cm distant from the base of the jet, which corresponds to ≥102 Schwarzschild radii for 107-10 M☉ black hole systems.

MULTIWAVELENGTH OBSERVATIONS OF THE POWERFUL GAMMA-RAY QUASAR PKS 1510 089: CLUES ON THE JET COMPOSITION

We present the results from a multiwavelength campaign conducted in August 2006 of the powerful {gamma}-ray quasar PKS 1510--089 (z = 0.361). This campaign commenced with a deep Suzaku observation lasting three days for a total exposure time of 120 ks, and continued with Swift monitoring over 18 days. Besides Swift observations, which sampled the optical/UV flux in all 6 UVOT filters as well as the X-ray spectrum in the 0.3--10 keV energy range, the campaign included ground-based optical and radio data, and yielded a quasi-simultaneous broad-band spectral energy distribution from 109 Hz to 1019 Hz. Thanks to its low instrumental background, the Suzaku observation provided a high S/N X-ray spectrum, which is well represented by an extremely hard power-law with photon index {Gamma}{approx_equal}1.2, augmented by a soft component apparent below 1 keV, which is well described by a black-body model with temperature kT {approx_equal}0.2 keV. Monitoring by Suzaku revealed temporal variability which is different between the low and high energy bands, again suggesting the presence of a second, variable component in addition to the primary power-law emission. We model the broadband spectrum of PKS 1510--089 assuming that the high energy spectral component results from Comptonization of infrared radiation produced by hot dust located in the surrounding molecular torus. In the adopted internal shock scenario, the derived model parameters imply that the power of the jet is dominated by protons but with a number of electrons/positrons exceeding a number of protons by a factor {approx} 10. We also find that inhomogeneities responsible for the shock formation, prior to the collision may produce bulk-Compton radiation which can explain the observed soft X-ray excess and possible excess at {approx} 18 keV. We note, however, that the bulk-Compton interpretation is not unique, and the observed soft excess could arise as well via some other processes discussed briefly in the text.