X-ray study of accretion flow in narrow-line Seyfert 1 galaxies

We present the results of a systematic study of narrow-line Seyfert 1 galaxies (NLS1s) observed with XMM-Newton. The 2–12 keV X-ray spectra of NLS1s are well represented by a single power law with a photon index Γ ∼ 2. When this hard power law continuum is extrapolated into the low energy band, we found that all objects in our sample show prominent soft excess emission. This excess emission is well parameterized by the thermal emission expected from an optically thick accretion disk, and we found the following three peculiar features: (1) The derived disk temperatures are significantly higher than the expectation from a standard Shakura-Sunyaev accretion disk, if we assume a central mass of a black hole to be 10^6–8M_⊙. (2) The temperatures are distributed within narrow range (ΔkT ∼ 0.08 keV) with an average temperature of 0.18 keV in spite of the range of four orders of magnitude in luminosity (10^41–45 erg s⁻¹). (3) We found a peculiar temperature–luminosity relation, where the luminosity seems to be almost saturated in spite of the significant change in temperature, during the observations of the most luminous NLS1 PKS 0558-504. These results strongly suggest that the standard accretion disk picture is no longer appropriate in the nuclei of NLS1s. We discuss a possible origin for the soft excess component, and suggest that a slim disk may be able to explain the observational results, if the photon trapping effect is properly taken into account.     

ROSAT PSPC spectra of six Seyfert 1 galaxies

We present six ROSAT PSPC observations of Seyfert 1 galaxies chosen to have low Galactic line-of-sight absorption (N_H 10²⁰ cm⁻²). As expected, it is found that all of these sources possess significantly steeper spectra below 1 keV, than that observed at higher X-ray energies. In addition we find evidence for soft X-ray spectral features, which are best parameterized as line emission at 0.63 keV in NGC7469 and 0.75 keV in ESO198-G24. We examine these results in the light of the accuracy of the PSPC spectral calibration.     

Blazar nuclei in radio-loud narrow-line Seyfert 1?

It has been suggested that some radio-loud narrow-line Seyfert 1 contain relativistic jets, on the basis of their flat-spectrum radio nuclei and studies on variability. We present preliminary results of an ongoing investigation of the X-ray and multiwavelength properties of 5 radio-loud NLS1 based on archival data from Swift and XMM-Newton. Some sources present interesting characteristics, very uncharacteristic for a radio-quiet narrow-line Seyfert 1, such as very hard X-ray spectra, and correlated optical and ultraviolet variability. However, none of the studied sources show conclusive evidence for relativistic jets. γ-Ray observations with Fermi are strongly recommended to definitely decide on the presence or not of relativistic jets.     

Spectra and time variability of black-hole binaries in the low/hard state

We propose a jet model for the low/hard state of galactic black-hole X-ray sources which explains the energy spectra from radio to X-rays and a number of timing properties in the X-ray domain such as the time lag spectra, the hardening of the power density spectra and the narrowing of the autocorrelation function with increasing photon energy. The model assumes that (i) there is a magnetic field along the axis of the jet, (ii) the electron density in the jet drops inversely proportional to distance, (iii) the jet is “hotter” near its center than at its periphery, and (iv) the electrons in the jet follow a power-law distribution function. We have performed Monte Carlo simulations of Compton upscattering of soft photons from the accretion disk and have found power-law high-energy spectra with photon-number index in the range 1.5–2 and cutoff at a few hundred keV, power-law time lags versus Fourier frequency with index 0.8, and an increase of the rms amplitude of variability and a narrowing of the autocorrelation function with increasing photon energy as they have been observed in Cygnus X-1. The spectrum at long wavelengths (radio, infrared, optical) is modeled to come from synchrotron radiation of the energetic electrons in the jet. We find flat to inverted radio spectra that extend from the radio up to about the optical band. For magnetic field strengths of the order 10⁵–10⁶ G at the base of the jet, the calculated spectra agree well in slope and flux with the observations.     

Hard X-ray focusing optics up to 80 keV for the future missions

X-ray telescopes have been providing high sensitivity X-ray observations in numerous missions. For X-ray telescopes in the future, one of the key technologies is to expand the energy band beyond 10 keV. We designed depth-graded multilayer, so-called supermirrors, for a hard X-ray telescope in the energy band up to 40 keV using lightweight thin-foil optics. They were successfully flown in a balloon flight and obtained a hard X-ray image of Cyg X-1 in the 20–40 keV band. Now supermirrors are promising to realize a hard X-ray telescope. We have estimated the performance of a hard X-ray telescope using a platinum–carbon supermirror for future satellite missions, such as NeXT (Japan) and XEUS (Europe). According to calculations, they will have a significant effective area up to 80 keV, and their effective areas will be more than 280 cm² even at 60 keV. Limiting sensitivity will be down to 1.7 × 10⁻¹³ erg cm⁻² s⁻¹ in the 10–80 keV band at a 100 ks observation. In this paper, we present the results of the balloon experiment with the first supermirror flown and projected effective areas of hard X-ray telescopes and action items for future missions.     

MIRAX: a Brazilian X-ray astronomy satellite mission

We describe the “Monitor e Imageador de Raios-X” (MIRAX), an X-ray astronomy satellite mission proposed by the high-energy astrophysics group at the National Institute for Space Research (INPE) in Brazil to the Brazilian Space Agency. MIRAX is an international collaboration that includes, besides INPE, the University of California San Diego, the University of Tübingen in Germany, the Massachusetts Institute of Technology and the Space Research Organization Netherlands. The payload of MIRAX will consist of two identical hard X-ray cameras (10–200 keV) and one soft X-ray camera (2–28 keV), both with angular resolution of 5–7^′. The basic objective of MIRAX is to carry out continuous broadband imaging spectroscopy observations of a large source sample (9 months/yr) in the central Galactic plane region. This will allow the detection, localization, possible identification, and spectral/temporal study of the entire history of transient phenomena to be carried out in one single mission. MIRAX will have sensitivities of 5 mCrab/day in the 2–10 keV band (2 times better than the All Sky Monitor on Rossi X-ray Timing Explorer) and 2.6 mCrab/day in the 10–100 keV band (40 times better than the Earth Occultation technique of the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory). The MIRAX spacecraft will weigh about 200 kg and is expected to be launched in a low-altitude (600 km) circular equatorial orbit around 2007/2008.     

The dark matter halo structure of “fossil” groups candidates

We investigated properties of four isolated giant elliptical galaxies with extended X-ray halo using ASCA data. The derived size of X-ray halo, X-ray luminosity, and gravitational mass of the dark halo are unusually large those of X-ray halo of a single galaxy, but are typical for X-ray halos of groups and poor clusters of galaxies. The measured temperatures and abundances of the X-ray halo gas in these galaxies are also similar to those of the groups and poor clusters. Based on these results we identified these galaxies as “isolated X-ray overluminous elliptical galaxy” (IOLEG). The radial profiles of dark halo in these objects were derived from X-ray data. It is found that some are similar to those of compact groups while others are the same as those of normal ellipticals. The dark halos of lOLEGs are thus indistinguishable from those of groups (and poor clusters), which appears to be consistent with a widely believed idea that lOLEGs are a product of dynamical evolution of a compact group. However, mass-to-light ratios of IOLEGs (M₂₀₀/L_B  100–1000) are far greater than those of Hickson compact groups M₂₀₀/L_B  40–60). Since it is hard to consider that total optical luminosity of a compact group decreases by an order of magnitude in the course of dynamical evolution, such difference in the observed mass-to-light ratio between IOLEGs and Hickson compact groups strongly suggests that most IOLEGs have not evolved from compact groups which are observed at present.     

Ginga observations of Seyfert galaxies

We observed twenty-eight Seyfert 2 galaxies with the Japanese X-ray satellite, Ginga, and found Seyfert 2 galaxies, in general, have the X-ray spectral characteristics of obscured Seyfert 1 nuclei. This result agrees with the predictions from Unified Seyfert model proposed by Antonucci and Miller /1/. However, among the observed Seyfert 2 galaxies, there are a few galaxies with no evidence of an obscuration, contrary to the general predictions of the unified model. We note that type 2 AGN will contribute to the Cosmic Diffuse X-ray Background, if the unified Seyfert model can be extended to the far distant AGN such as quasars.     

The optical to X-ray spectrum of the broad-line ultrasoft AGN RE J2248-511

We present a series of monitoring observations of the ultrasoft broad-line Seyfert galaxy RE J2248-511 with XMM-Newton. Previous X-ray observations showed a transition from a very soft state to a harder state five years later. We find that the ultrasoft X-ray excess has re-emerged, yet there is no change in the hard power-law. Reflection models with a reflection fraction of 15, and Comptonisation models with two components of different temperatures and optical depths (kT₁ = 83 keV, T₁ = 30 eV, τ₁ = 0.8; KT₂ = 3.5 keV, T₂ = 60 eV, τ₂ = 2.8) can be fit to the spectrum, but cannot be constrained. The best representation of the spectrum is a model consisting of two blackbodies (kT₁ = 0.09 ± 0.01 keV, kT₂ = 0.21 ± 0.03 keV) plus a power-law (Γ = 1.8 ± 0.08). We also present simultaneous optical and infrared data showing that the optical spectral slope also changes dramatically on timescales of years. If the optical to X-ray flux comes primarily from a Comptonised accretion disk we obtain estimates for the black hole mass , accretion rate and inclination cos(i)  0.8 of the disk.     

Extreme starburst or central activity in X-ray luminous IRAS galaxies

ROSAT All Sky Survey observations of IRAS galaxies have revealed up to now a number of 10 optically non-Seyfert galaxies with X-ray (0.1–2.4) luminosities up to a few 10⁴³erg · s⁻¹ (Boller et al. 1992). The sources are brighter than previous detection limits of a few 10⁴¹erg · s⁻¹ as found by Stocke et al. (1991) or Green, Anderson and Ward (1992) for Einstein sources. The optical classification is based on follow-up observations which indicate clearly the non-Seyfert (LINER and HII region-like galaxies) nature. Our investigations reveal that galaxies classified as non-Seyferts on the basis of optical spectroscopy may reach exceptionally high X-ray luminosities which are similar to that of Seyfert galaxies. On the basis of the present observational material we suppose a hidden low luminosity AGN in the centre of these objects as the source of energy production. The objects are of interest when evaluating starburst versus central activity.     

Compact X-ray sources in nearby galaxy nuclei

We have found compact, near-nuclear X-ray sources in 21 (54%) of a complete sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT HRI data. ROSAT X-ray luminosities (0.2 – 2.4 keV) of these compact X-ray sources are ∼10³⁷ – 10⁴⁰ erg s⁻¹. The mean displacement between the location of the compact X-ray source and the optical photometric center of the galaxy is ∼390 pc. ASCA spectra of six of the 21 galaxies show the presence of a hard component with relatively steep (Γ ≈ 2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the data from the spiral galaxies well, suggesting that the X-ray objects in these galaxies may be similar to a black hole candidate (BHC) in its soft (high) state. ASCA data from the elliptical galaxies indicate that hot (kT ≈ 0.7 keV) gas dominates the emission. The fact that the spectral slope of the spiral galaxy sources is steeper than in normal type 1 active galactic nuclei (AGNs) and that relatively low absorbing columns (N_H ≈ 10²¹ cm⁻²) were found to the power-law component indicates that these objects are somehow geometrically and/or physically different from AGNs in normal active galaxies. The X-ray sources in the spiral galaxies may be BHCs, low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs or AGNs) to be ∼10²–10³ M_⊙. The X-ray sources in the elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.     

Hard X-ray observation of galactic X-ray sources

A large (1455 cm²) hard X-ray telescope was successfully launched aboard a stratospheric balloon on October 4, 1980. During this flight four galactic X-ray sources were observed, namely the transient recurrent X-ray pulsar A0535+26, the Crab Nebula, Cygnus X-1 and X Persei. Here we report the results on the latter two sources. From Cygnus X-1 we measured a photon flux in the band 30 to 200 keV, of 3.5 × 10^?2 photons cm^?2 which is 6.5 times lower than that recieved from the source in a “low” intensity state in the same energy band. In addition, the photon spectrum in the same energy band was very soft and consistent with a power law with photon index α = 2.71 ± 0.14. Even if a simultaneous observation of the source at lower energies was not available, our data strongly suggest that we observed the source during a “high” intensity state. We report also positive detection in the band 30 to 200 keV of the low luminosity X-ray pulsar X Persei. In its spectrum we confirm the presence of a hard X-ray tail consistent with a power law (photon index α = 2.17 ± 0.42).     

ROSAT PSPC observations of two starburst galaxies

We present results from ROSAT observations of NGC 1808 and NGC 2903. Exposures of 10 ksec each with the PSPC detector show X-ray sources at the central positions of both galaxies which are classified as nuclear starburst galaxies. Both targets, NGC 1808 and NGC 2903 appear slightly extended in X-ray maps in the energy band 0.1–2.4 keV. The X-ray spectrum of NGC 1808 shows almost complete absorption below 0.5 keV, indicating an extremely high hydrogen column density towards that source (N_H ≈ 8 × 10²¹cm⁻² resulting from model fits on the PSPC spectrum). In case of NGC 2903, the number of counts in the ROSAT band is significantly lower than expected from a previous EINSTEIN investigation of the source.     

Hard X-ray spectral properties and discovery of narrow annihilation line in the spectrum of Nova Muscae

The observations of X-ray Nova in Musca (GRS1124-684) by two coded mask telescopes on board GRANAT observatory provided spectral data in broad 3 – 1300 keV band. During these observations, spanned over a year, the Nova was detected in a three apparently different spectral states, corresponding to different epochs of the soft X-ray light curve: (1) A spectrum with two distinct components (soft, below 8 keV and hard power law tail with slope 2.5, detected up to 300 keV). The soft emission changed gradually with characteristic decay time around 30 days, while power law component exhibited strong variability on the time scales of several hours and decreased much more slowly. (2) A soft spectrum (without hard power law tail), observed during the “kick” of the soft X-ray light curve. (3) A hard power law spectrum with slope 2.2. Thus, while the 3 – 300 keV luminosity decreased by more than order of magnitude, the source passed through all spectral states known for galactic black hole candidates (Cyg X-1, GX339-4, 1E1740.7-2942, GRS1758-258 etc.).

On January 20–21 1991, the SIGMA telescope aboard GRANAT detected a relatively narrow variable emission line near 500 keV (Fig.1,2) with net flux ≈ 6 · 10⁻³ phot/s/cm², most probably related with electron-positron annihilation processes, occurring in the source /1–4/. Additional excess above power law continuum, centered around 200 keV, was found during this observation.     

LINERs as low-luminosity active galactic nuclei

Many nearby galaxies contain optical signatures of nuclear activity in the form of LINER nuclei. LINERs may be the weakest and most common manifestation of the quasar phenomenon. The physical origin of this class of objects, however, has been ambiguous. I draw upon a number of recent observations to argue that a significant fraction of LINERs are low-luminosity active galactic nuclei. The evidence in favor of this interpretation includes the similarity in the properties of the host galaxies of LINERs and Seyferts, the detection of broad-line regions, the detection of black holes in a handful of nearby galaxies that are spectroscopically recognized as LINERs, the presence of a compact nuclear source seen in the radio, ultraviolet, and X-rays, and the detection of nonthermal spectra in the hard X-ray band. The spectral energy distributions of LINERs differ dramatically from those of luminous AGNs, most notably in the absence of the “big blue bump.” This and other characteristics suggest that the central engine in LINERs is fed by a very low accretion rate.     

“Microquasars” and the X-ray background

The results of 1–20 μm infrared photometry of seven 3CR radio galaxies are discussed. The broad line galaxies all show steep infrared spectra with a power law index α −2.4 (F ∝ ν^+α) in direct continuation of the optical spectra. These spectra are far steeper than those observed in Seyfert 1 galaxies, the radio quiet counterparts of broad line radio galaxies. No infrared excesses were observed in narrow line radio galaxies. However, more sensitive observations are needed before any resemblance in the infrared between narrow line radio galaxies and Seyfert 2 galaxies can be excluded.     

Cyclotron features in X-ray spectra of accreting pulsars

The hard X-ray spectra of small subset of accreting pulsars show absorption-like line features in the range 10–100 keV. These lines, referred to as cyclotron lines or cyclotron resonance scattering features, are due to photons scattered out of the line of sight by electrons trapped in the 10¹² G pulsar polar cap magnetic field. In this paper we present a review of observations, from the discovery of a cyclotron line in Hercules X-1 to recent results with RXTE and INTEGRAL.     

Study of combined soft and hard X-ray images of solar flares

We have studied soft and hard X-ray images of 13 solar flares from six active regions observed by the Hard X-ray Imaging Spectrometer (HXIS). Our results indicate the presence of pre-hard X-ray burst excesses in the 11.5–30.0 keV range, indicating a slow buildup of the acceleration process or a strong preheating. During the impulsive phase, all of the events show the simultaneous energization of neighboring field structures, which, in the case we show in some detail, share about equal amounts of the released energy. This association seems to be indicative of strong acceleration and energy release triggered by the interaction between magnetic loops.     

类星体X射线辐射的性质

本文对247个类星体的X射线性质进行了统计分析,这些X射线资料是由爱因斯坦卫星在0.5—4.5keV的能带内观测到的。主要结果是:在各种视亮度和红移(或距离)的已知1619个类星体中,都有百分比相近的部分为X射线源。Ⅰ型Seyferc星系的发射线H_β与[OⅢ]的等值宽度比R_EW同X射线光度L_x强相关,这可作为Ⅰ型Seyfert星系的X射线光度的一个新指标。光学辐射以及2.5GHz以上的高频射电辐射都同X射线辐射相关,说明这三种辐射的区域依次相邻而且机制也相似。色指数U-B和B-V都同X射线辐射不相关,但U-B同B-V弱相关。      

Positron annihilaton radiation from the nuclei of Seyfert galaxies

Recent gamma-ray observations of two Seyfert Galaxies are interpreted in terms of electron-positron pair annihilation radiation. A simplified scenario is envisaged in which a massive black hole is accreting material from an optically thin disk characterized by a hot (T > 10⁹ °K) e^± plasma. At these very high temperatures the 511 keV line emission loses its characteristic features to become both broadened and blue shifted. Observational X and gamma-ray data are used to investigate the possibility that the “bump” in the spectral emission at photon energies E ～ 1 MeV observed in Seyfert galaxies may be due to this annihilation feature. In particular the self consistency of the parameters estimated from the gamma-ray data is explored. Furthermore we investigate the possibility that this annihilation feature may be mirrored in the cosmic diffuse background and, under this assumption, we calculate the maximum temperature of the annihilation region and the average annihilation rate for Seyfert galaxies.