The Variabilities of the Soft and Hard X-ray Components of NGC 7314 and NGC 7582 and the Distribution of Absorbing Matter in Type II AGNs

ASCA observations of the two Type II AGNs, NGC 7314 and NGC 7582, show clear variations in the broad X-ray band (0.4-10keV) on short timescales - 104s. Spectral analysis indicates that they both have an absorbed hard X-ray component and an unabsorbed soft "excess" component. To clarify the origin of the latter, we made a cross-correlation analysis of the two components. The results show that, for NGC 7314, the soft X-ray variability is proportional to that of the hard X-ray component. This indicates that the active nucleus of NGC 7314 must be partially covered and so the soft emission is a "leaking" of the variable hard component. For NGC 7582, there is no detectable variability in the soft component, although there is a definite one in the hard component. This indicates that the variable nucleus of NGC 7582 must be fully blocked by absorbing matter, and the soft emission is most likely the scattered component predicted by the AGN unified model.     

Chandra Observations of the Flat Spectrum Seyfert-2 Galaxies NGC 2110 and NGC 7582

Chandra observations of the Seyfert-2 galaxies NGC 2110 and NGC 7582 are presented. With the superb spatial resolution of Chandra we found that in NGC 7582 the soft (≤2 keV) and hard (2-10 keV) X-rays are emitted in different regions, consistent with the report by Xue et al. By comparing the present X-ray data with the previous infrared data, we determined that the soft X-ray region is the site of starburst activities. We found no significant temporal variations during our observations. We confirm the previous finding that NGC 2110 and NGC 7582 are flat-spectrum sources. We argue that the flat spectra may result from a cold absorbing material such as envisaged in the “dual absorbed” model. Strong Fe Kα emission feature is detected in 6-7keV. Its equivalent width is so large that it cannot be reproduced by using the Galactic column density of - 10^22 cm^-2.     

A Study of the Star-forming Dwarf Galaxy NGC 855 with Spitzer

We present a study of the dwarf elliptical galaxy NGC 855 using the narrow-band Hα and Spitzer data. Both the Hα and Spitzer IRAC images confirm star-forming activity in the center of NGC 855. We obtained a star formation rate (SFR) of 0.022 and 0.025M◎yr-1,respectively,from the Spitzer IRAC 8.0μm and MIPS 24μm emission data. The HI observa-tion suggests that the star-forming activity might be triggered by a minor merger. We also find that there is a distinct IR emission region in 5.8 and 8.0μm bands,located at about 10 away from the nucleus of NGC 855. Given the strong 8.0μm but faint Hα emission,we expect that it is a heavily obscured star-forming region,which needs to be confirmed by further optical spectroscopic observations.     

A Study of the Star-forming Dwarf Galaxy NGC 855 with Spitzer

We present a study of the dwarf elliptical galaxy NGC 855 using the narrow-band Hα and Spitzer data. Both the Hα and Spitzer IRAC images confirm star-forming activity in the center of NGC 855. We obtained a star formation rate （SFR） of 0.022 and 0.025Mo yr^-1, respectively, from the Spitzer IRAC 8.0 μm and MIPS 24 #m emission data. The HI observation suggests that the star-forming activity might be triggered by a minor merger. We also find that there is a distinct IR emission region in 5.8 and 8.0 μm bands, located at about 10＂ away from the nucleus of NGC 855. Given the strong 8.0 μm but faint Hα emission, we expect that it is a heavily obscured star-forming region, which needs to be confirmed by further optical spectroscopic observations.     

X-ray Emission Lines in GRB Afterglows： Evidence for a Two-component Jet Model

X-ray emission lines have been observed in X-ray afterglows of several γ-ray bursts (GRBs). It is a major breakthrough for understanding the nature of the progenitors. It has been proposed that the X-ray emission lines can be well explained by the Geometry-Dominated models, but in these models the illuminating angle is much larger than that of the collimated jet of the GRB. For GRB 011211, we have obtained an illuminating angle of about θ- 45°, while the angle of the GRB jet is only 3.6°. So we propose that the outflow of GRBs with emission lines should have two distinct components: a wide component that illuminates the reprocessing material and produces the emission lines and a narrow one that produces the GRB. Observations show the energy for producing the emission lines is higher than that of the GRB. In this case, when the wide component dominates the afterglows, a bump should appear in the GRB afterglow. For GRB 011211, the bump should occur within 0.05 days of the GRB, which is obviously too early for the observation to catch it. Alongside the X-ray emission lines there should also be a bright emission component between the UV and the soft X-rays. These features can be tested by the Swift satellite in the near future.     

Comptonization and Reprocessing Processes in Accretion Disks： Applications to the Seyfert 1 Galaxies NGC 5548 and NGC 4051

Simultaneous multi-wavelength observations have revealed complex variability in AGNs. To explain the variability we considered a theoretical model consisting of an inner hot comptonizing corona and an outer thin accretion disk, with interactions between the two components in the form of comptonization and reprocessing. We found that the variability of AGNs is strongly affected by the parameters of the model, namely, the truncated disk radius rmin, the corona radius rs, the temperature KTe and the optical depth τ0 of the corona. We applied this model to the two best observed Seyfert 1 galaxies, NGC 5548 and NGC 4051. Our model can reproduce satisfactory the observed SEDs. Our fits indicate that NGC 5548 may have experienced dramatic changes in physical parameters between 1989-1990 and 1998, and that NGC 4051 has a much larger truncated disk radius (700 Schwarzschild radii) than NGC 5548 (several tens of Schwarzschild radii). Since we adopted a more refined treatment of the comptonization process rather than simply assuming a cut-off power law, our results should be more reasonable than the previous ones.     

The kilosecond variability of X-ray sources in nearby galaxies

Chandra observations of 17 nearby galaxies were analyzed and 166 bright sources with X-ray counts 〉 100 were chosen for temporal analysis. Fractional root mean square （rms） variability amplitudes were estimated for light curves, binned at 4 kilosecond （ks）, with length 〈 40 ks. While there are nine ultra-luminous X- ray sources （ULXs） with unabsorbed luminosity （in the 0.3-8.0 keV band） L 〉 1039 erg s-1 in the sample for which the fractional rms variability is constrained to be 〈 10%, only two of them show variability. One of the variable ULXs exhibits a secular transition and has an ultra-soft spectrum with temperature - 0.3 keV while the other is a rapidly varying source in NGC 0628, which has previously been com- pared to the Galactic microquasar GRS 1915＋105. These results seem to indicate that ULXs are typically not highly variable on ks timescales, except for some ultra-soft ones. Among the relatively low luminosity sources （L - 10^38 erg s-1）, we find five of them to be variable. Apart from an earlier known source in NGC 1569, we identify a source in NGC 2403 that exhibits persistent high amplitude fluctuations. In general, the variability of the sources does not seem to be correlated with hardness, which indicates that they may not be due to variations in any absorbing material, but instead could reflect instabilities in the inner accretion disk.     

The Origin of Infrared Emission from the Infrared Luminous Galaxy NGC 4418

We present a study of the origin of infrared (IR) emission in the optically normal, infrared luminous galaxy NGC 4418. By decomposing the stellar absorption features and continua in the range of 3600-8000 A from the Sloan Digital Sky Survey into a set of simple stellar populations, we derive the stellar properties for the nuclear region of NGC 4418. We compare the observed infrared luminosity with the one derived from the starburst model, and find that star-forming activity contributes only 7% to the total IR emission, that as the IR emission region is spatially very compact, the most possible source for the greater part of the IR emission is a deeply embedded AGN, though an AGN component is found to be unnecessary for fitting the optical spectrum.     

Separation of Different Contributions to the Total X-ray Luminosity in Gamma-ray Loud Blazars

The relativistic beaming model has been successfully used to explain many of the observational properties of active galactic nuclei. In this model the total emission is formed by two components, one beamed, one unbeamed. However, the exact contribution from each component in unresolved sources is still not clear. In the radio band, the core and extended emissions are clearly separated. We adopt the method proposed by Kembhavi to separate the two contributions in the X-ray emissions in a sample of 19 gamma-ray loud blazars. It is clearly shown that the beamed emission dominates the X-ray flux and the unbeamed X-ray emission is correlated with the extended radio emission of the considered objects. We also find that the ratio of the beamed to the unbeamed X-ray luminosity is correlated with the X-ray spectral index, an effect that should be a consequence of the underlying X-ray emission mechanism.     

Long term optical variability of bright X-ray point sources in elliptical galaxies

We present long term optical variability studies of bright X-ray sources in four nearby elliptical galaxies with the Chandra Advanced CCD Imaging Spectrometer array(ACIS-S) and observations from the Hubble Space Telescope(HST) Advanced Camera for Surveys.Out of the 46 bright(X-ray counts 60)sources that are in the common field of view of the Chandra and HST observations,34 of them have potential optical counterparts,while the rest of them are optically dark.After taking into account systematic errors,estimated using optical sources in the field as a reference,we find that four of the X-ray sources(three in NGC 1399 and one in NGC 1427) have variable optical counterparts at a high level of significance.The X-ray luminosities of these sources are ~ 10~(38) erg s~(-1) and are also variable on similar time scales.The optical variability implies that the optical emission is associated with the X-ray source itself rather than being the integrated light from a host globular cluster.For one source,the change in optical magnitude is 0.3,which is one of the highest reported for this class of X-ray sources and this suggests that the optical variability is induced by the X-ray activity.However,the optically variable sources in NGC 1399 have been reported to have blue colors(g- z 1).All four sources have been detected in the infrared(IR) by Spitzer as point sources,and their ratios of 5.8 to 3.6 μm flux are 0.63,indicating that their IR spectra are like those of Active Galactic Nuclei(AGNs).While spectroscopic confirmation is required,it is likely that all four sources are background AGNs.We find none of the X-ray sources having optical/IR colors different from AGNs to be optically variable.     

Timing in the Time Domain： Cygnus X—1

1 INTRODUCTIONStudying short time scale variability of the X-ray emission of blaCk-hole systems and lowmass X-ray binaries is an importal approach to understanding the emitting region and elinssionmechanism of high-energy photons. The time-averaged spectra of hard X-rays from Cyg X-1and other black-hole candidates are relatively well explained in terms of a simple model: hardX-rays result from the Comptonization of soft photons in a hot electron cloud of constattemperature and optical d…     

The Elusive Dust Torus of NGC 1068 Unveiled by Adaptive Optics Observations from 2.2 µm TO 4.8 µm

Near-infrared, adaptive optics observations of AGN achieve a spatial resolution comparable to that of the Hubble Space Telescope at visible and ultraviolet wavelengths. Using the ESO adaptive optics system, we have mapped the nuclear region of NGC 1068 in the near-infrared at 2.2, 3.5 and 4.8 μm. These data show the presence of strong near-infrared emission within the central 100 pc around the nucleus. At the K, L and M bands, the dominant emission peak is unresolved at resolutions of 35 pc (K), 15 pc (L) and 23 pc (M). We find that the emission peaks at 2.2, 3.5 and 4.8 μm are offset by 0.3±0.1" south of the HST optical continuum peak. The infrared peaks are coincident with the HST imaging polarimetry center thought to mark the hidden nucleus. We conclude that the infrared emission peaks are also directly related with the hidden nucleus and that they outline the dusty torus around the central engine, as expected from current AGN models. From our first estimate of the fluxes in L and M, we derive a dust temperature of 750 K for the unresolved component. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chandra Observation of Point Sources in the X-Ray Elliptical Galaxy NGC 1407

With the Chandra pointing observation of about 50 ks, we have detected41 point sources in the X-ray relatively faint elliptical galaxy NGC 1407, most ofwhich appear to be low mass X-ray binaries. In luminosity flux units, these resolvedpoint sources contribute about 17.8% of the total emission of the galaxy in 0.3-10 keV. Of the remaining diffuse emission, about 53.4% (or 43.9% of total) may arisefrom unresolved point sources. We find six ultraluminous X-ray sources with lumi-nosities above 10~(39) erg s~(-1). This number is less than has been found in NGC 720,but more than in NGC 4697 and NGC 1553, so suggesting a possible correlationbetween the number of ULXs and the total X-ray luminosity of early-type galaxies.A central point-like source is detected whose luminosity (2.36±0.14×10~(39)erg s~(-1))is the highest among all resolved sources. However, because of its relatively lowhardness ratio, we speculate that it is not likely to be a low luminosity AGN.     

X-ray Spectroscopy of Dips of Cir X-1

We present X-ray spectral analyses of the low-mass X-ray binary Cir X-1 dur- ing X-ray dips,using the Rossi X-ray Timing Explorer(RXTE)data.Each dip was divided into several segments,and the spectrum of each segment was fitted with a three-component blackbody model,in which the first two components are affected by partial covering and the third one is unaffected.A Gaussian emission line is also included in the spectral model to represent the Fe Kαline at～6.4 keV.The fitted temperatures of the two partially covered components are about 2 keV and 1 keV,while the uncovered component has a temperature of～0.5-0.6 keV.The equivalent blackbody emission radius of the hottest component is the smallest and that of the coolest component is the largest.During the dips the fluxes of the two hot components are linearly correlated,while that of the third component does not show any significant variation.The Fe line flux remains constant,within the errors,during the short dips.However,during the long dips the line flux varies significantly and is positively correlated with the fluxes of the two hot components.These results suggest:(1)that the tem- perature of the X-ray emitting region decreases with radius,(2)that the Fe Kαline emitting region is close to the hot continuum emitting region,and(3)that the size of the Fe line emit- ting region is larger than that of the obscuring matter causing the short dips but smaller than the region of that causing the long dips.     

A synchrotron self-Compton scenario for the very high energy γ-ray emission of the intermediate BL Lacertae object W Comae

W Comae has significant variability in multi-wavelengthes, from radio to gamma-ray bands. A bright outburst in optical and X-ray bands was observed in 1998, and most recently, a strong TeV flare was detected by VERITAS in 2008. It is the first TeV intermediate-frequency-peaked BL Lacertae source. I find that both the broadband spectral energy distributions （SEDs） which were quasi-simultaneously obtained during the TeV flare and during the optical/X-ray outburst are well fit by using a single-zone synchrotron ＋ synchrotron-self-Compton model. The satisfactory fitting requires a large beaming factor, i.e., δ- 25 and δ- 20 for the TeV flare and the optical/X-ray outburst, respectively, suggesting that both the optical/X-ray outburst and the TeV flare are from a relativistic jet. The size of the emission region of the TeV flare is three times larger than that of the optical/X-ray outburst, and the strength of the magnetic field for the TeV flare is - 14 times smaller than that of the X-ray/optical outburst, likely indicating that the region of the TeV flare is more distant from the core than that of the X-ray/optical outburst. The inverse Compton component of the TeV flare peaks around 1.3 GeV, but it is around 20 MeV for the X-ray/optical outburst, lower than that for the TeV flare by two orders of magnitude. The model predicts that the optical/X-ray outburst might be accompanied by a strong MeV/GeV emission, but the TeV flare may be not associated with the X-ray/optical outburst. The GeV emission is critical for characterizing the SEDs of the optical/X-ray outburst and the TeV flare. The predicted GeV flux is above the sensitivity of Fermi/LAT, and it could be verified with the observations by Fermi/LAT in the near future.     

On the reality of broad iron L lines from the narrow line Seyfert 1 galaxies 1H0707–495 and IRAS 13224–3809

We performed time resolved spectroscopy of 1H0707–495 and IRAS 13224–3809 using long XMM-Newton observations. These are strongly variable narrow line Seyfert 1 galaxies and show broad features around 1 ke V that have been interpreted as relativistically broad Fe Lα lines. Such features are not clearly observed in other active galactic nuclei despite sometimes having high iron abundance required by the best fitted blurred reflection models. Given the importance of these lines, we explore whether the rapid variability of spectral parameters may introduce broad bumps/dips artificially in the time averaged spectrum, which may then be mistaken as broadened lines. We tested this hypothesis by performing time resolved spectroscopy using long(100 ks) XMM-Newton observations and by dividing them into segments with typical exposures of a few ks. We extracted spectra from each such segment and modeled them using a two component phenomenological model consisting of a power law to represent the hard component and a black body to represent the soft emission. As expected, both the sources showed variations in the spectral parameters. Using these variation trends, we simulated model spectra for each segment and then co-added to get a combined simulated spectrum. In the simulated spectra, we found no broad features below 1 ke V and in particular no deviation near 0.9 ke V as seen in the real averaged spectra. This implies that the broad Fe Lα line that is seen in the spectra of these sources is not an artifact of the variation of spectral components and, hence, provides evidence that the line is indeed genuine.     

Analysis of the X-ray emission from OB stars Ⅲ: low-resolution spectra of OB stars

This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars.In our two previous papers,we study the high-resolution X-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters.We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field,mass loss rate and terminal wind velocity.Our previous analysis of high-resolution spectra showed that the correlations are weak or even absent.In the present paper,we analyzed low-resolution X-ray spectra,using model-independent X-ray hardness values for checking the above mentioned dependencies.We establish that X-ray luminosities L_X weakly depend on the stellar magnetic field.At the same time,Lx ∝ M~(0.5) and L_X ∝ E_(kin)~(0.5),where M is the mass loss rate and E_(kin) is the kinetic energy of the wind.The X-ray luminosities decrease with growing magnetic confinement parameter η.We also argue that there is an additional(probably non-thermal) component contributed to the stellar X-ray emission.     

Prompt optical emission from gamma-ray bursts with multiple timescale variability of central engine activities

Complete high-resolution light curves of GRB 080319B observed by Swift present an opportunity for detailed temporal analysis of prompt optical emission. With a two-component distribution of initial Lorentz factors, we simulate the dynamical process of shells being ejected from the central engine in the framework of the in- ternal shock model. The emitted radiations are decomposed into different frequency ranges for a temporal correlation analysis between the light curves in different energy bands. The resulting prompt optical and gamma-ray emissions show similar tempo- ral profiles, with both showing a superposition of a component with slow variability and a component with fast variability, except that the gamma-ray light curve is much more variable than its optical counterpart. The variability in the simulated light curves and the strong correlation with a time lag between the optical and gamma-ray emis- sions are in good agreement with observations of GRB 080319B. Our simulations suggest that the variations seen in the light curves stem from the temporal structure of the shells injected from the central engine of gamma-ray bursts. Future observations with high temporal resolution of prompt optical emission from GRBs, e.g., by UFFO- Pathfinder and SVOM-GWAC, will provide a useful tool for investigating the central engine activity.     

Temporal variations and spectral properties of the Be/X-ray pulsar GRO J1008–57 studied by INTEGRAL

The spin period variations and hard X-ray spectral properties of the Be/Xray pulsar GRO J1008–57 are studied with INTEGRAL observations during two outbursts in 2004 June and 2009 March.The pulsation periods of～93.66 s in 2004and～93.73 s in 2009 are determined.Pulse profiles of GRO J1008–57 during outbursts are strongly energy dependent with a double-peaked profile from 3–7 keV and a single-peaked profile in hard X-rays above 7 keV.Combined with previous measurements,we find that GRO J1008–57 has undergone a spin-down trend from 1993–2009 with a rate of～4.1×10-5s d-1,and could have changed into a spin-up trend after 2009.We find a relatively soft spectrum in the early phase of the 2009 outburst with cutoff energy～13 keV.Above a hard X-ray flux of～10-9erg cm-2s-1,the spectra of GRO J1008–57 during outbursts need an enhanced hydrogen absorption with column density～6×1022cm-2.The observed dip-like pulse profile of GRO J1008–57 in soft X-ray bands could be caused by this intrinsic absorption.Around the outburst peaks,a possible cyclotron resonance scattering feature at～74 keV is detected in the spectra of GRO J1008–57 which is consistent with the feature that was reported in MAXI/GSC observations,making the source a neutron star with the highest known magnetic field(～6.6×1012G)among accreting X-ray pulsars.This marginal feature is supported by the present detections in GRO J1008–57 following the correlation between the fundamental line energies and cutoff energies in accreting X-ray pulsars.Finally we discovered two modulation periods at～124.38 d and～248.78 d using RXTE/ASM light curves of GRO J1008–57.Two flare peaks appearing in the folded light curve had different spectral properties.The normal outburst lasting 0.1 of an orbital phase had a hard spectrum and could not be significantly detected below 3 keV.The second flare lasting ten days showed a very soft spectrum without significant detections above 5 keV.GRO J1008–57 is a good candidate of an accreting system with an equatorial circumstellar disk around the companion star.The neutron star passing the disk of the Be star near periastron and apastron produces two X-ray flares.The soft spectral properties in the secondary flares still need further detailed studies with soft X-ray spectroscopy.     

Infrared variability of the nucleus of the Seyfert galaxy NGC 1068 in 1998–2004

Our 8-year-long JHKLM photometry of the Seyfert galaxy NGC 1068 has confirmed its IR variability. The amplitudes of the brightness variations in the J (1.25 μm) and K (2.2 μm) bands are within 0 _. ^m 15 and 0 _. ^m 3, respectively, and exceed the observational errors by more than a factor of 5. The nucleus of NGC 1068 is a variable source and can be at different phases of activity. The brightness of the galaxy in all bands except J decreased from 1998 until 2004. In this period, there was a tendency for the J brightness to increase. The variable source in NGC 1068 is a complex structured object. At least two sources radiate in the wavelength range 1.25–5 μm: a hot source whose radiation shows up in the range 1.25–1.65 μm and a cold source radiating at long wavelengths (2.2–5 μm). The color temperature of the hot source increased from 2300 K (the beginning of our observations) to ∼2700 K (the end of our observations). In contrast, the temperature of the cold source decreased by several tens of degrees (in the temperature range 800–900 K). The IR brightness and color variations observed in 1998–2004 are attributable to the dispersal of the dust envelope that formed around the galactic nucleus some 30 years ago and reached its maximum density in 1994–1995. Our analysis of the spectral energy distributions for the galaxy has shown that the observed radiation in the range 1.25–5 μm can be represented as the sum of radiations from two blackbody sources. For the first period of our observations (JD 2451400), the temperatures of the hot and cold sources are ∼3100 and 760 K, respectively. For the second period (JD 2453230), they are ∼3200 and 720 K, respectively. The hot source is relatively compact; it is smaller in size than the cold source by several tens of times. The mean sizes of the hot and cold sources are ∼2.35 × 10¹⁶ and ∼7.8 × 10¹⁷ cm, respectively. The total mean luminosity of the two sources did not change between the beginning and the end of our observations. The optical depth of the dust envelope averaged over the spectrum of the hot source is τ ∼ 1.5. In 2004, the state of the dust envelope almost returned to its 1974 level, i.e., the dust envelope formation and dispersal cycle was ∼11 000 days (∼30 yr). Original Russian Text ? O.G. Taranova, V.I. Shenavrin, 2006, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2006, Vol. 32, No. 7, pp. 489–496.