The X-ray properties of normal galaxies

An extensive program to study nearby normal galaxies was carried out by various observers using the imaging instruments on the $\text{Einstein}$ Observatory; more than 50 such galaxies were detected with 0.5 – 3.0 keV luminosities ranging from 2 × 10³⁸ ergs s^?1 to 3 × 10⁴¹ergs s^?1. The X-ray luminosity of normal galaxies is ~2 × 10^?4 of the optical luminosity and shows no strong correlation with morphological type. For the nearest galaxies, (the Large and Small Magellanic Clouds, M31 and M33,) studies, performed with the Observatory, were comparable to the $\text{Uhuru}$ survey of the Galaxy. Approximately 30 new SNR were recognized in the Magellanic Clouds as a result. Over 90 sources were detected in M31 of which at least 20 are identified with globular cluster. The numbers of luminous (>10³⁷ ergs s^?1) sources detected in the nearest galaxies per unit mass are similar to that found in our own galaxy. Individual X-ray sources in the arms of nearby spirals can be very luminous; seven with luminosities in excess of 10³⁹ergs s^?1 have been discovered. The nuclei of some, but not all, normal galaxies are luminous X-ray sources; X-ray activity is not presently predictable from the radio or optical properties of the nucleus.     

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.     

X-ray spectra and time variability of active galactic nuclei

The X-ray spectra of broad line active galactic nuclei of all types (Seyfert I's, NELG's, Broadline radio galaxies) is well fit by a power law in the .5–100 keV band of mean energy slope α = .68±.15. There is, as yet, no strong evidence for time variability of this slope in a given object. The constraints that this places on simple models of the central energy source are discussed. BL Lac objects have quite different X-ray spectral properties and show pronounced X-ray spectral variability.On time scales longer than 12 hours most radio quiet AGN do not show strong, ΔI/I > .5, variability. The probability of variability of these AGN seems to be inversely related to their luminosity. However characteristic timescales for variability have not been measured for many objects. This general lack of variability may imply that most AGN are well below the Eddington limit. Radio bright AGN tend to be more variable on long, τ 6 month, timescales.     

Some recent results on extragalactic X-ray sources from the Einstein observatory

Extragalactic research studies by the Harvard/Smithsonian group with the Einstein Observatory have emphasized quasars and clusters of galaxies. More than 100 QSO's have been detected, including 20 serendipitous discoveries. The ratio of L_x/L_o for radio loud quasars is on the average 3 times that of radio quiet ones. QSO's with a large intrinsic optical luminosity have a smaller ratio of L_x/L_o. X-ray images of clusters of galaxies reveal a variety of morphological types which may correspond to different stages in their evolution. Several examples of bi-modal clusters have been discovered. An X-ray plume associated with M86 is apparently gas being stripped. From X-ray studies, a mass between 1.7 × 10¹³M_θ and 4.0 × 10¹³ M_θ has been derived for M87.     

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.     

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。      

“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.     

Bolometric luminosities and brightness temperatures of BL Lac Objects and FR I radio galaxies

To understand the connection among the subclasses of BL Lac Objects, FR I radio galaxies and Flat spectrum radio quasars (FSRQs), here the correlations of the bolometric luminosities with redshifts and brightness temperatures of these objects are studied. The bolometric luminosities vary linearly with redshifts, but few objects are scattered at high redshift. The bolometric luminosity versus brightness temperature distribution shows a correlation between these two components, except a few scattered objects, mostly RBLs. The bolometric luminosities and brightness temperatures of FR I radio galaxies with low redshift (<0.1) and low spectral index (α_rx < 0.75) are comparable to those of XBLs and those characteristics of FR I radio galaxies, with relatively high redshift (>0.2) and high spectral index, can be comparable with RBLs with low redshift (z < 0.5) and low bolometric luminosity. Those scattered RBLs with high redshifts (z > 0.5) are believed to be in complex environment with companion galaxies, most of these RBLs are still unresolved. The bolometric luminosity and brightness temperature of these scattered RBLs are comparable to those of quasars. The FSRQs are at high redshifts and bolometric luminosities and the brightness temperatures are also high relative to BL Lac Objects. These results support the FRI/BL Lac unification scheme. It suggests that, the FR I radio galaxies may be the parent populations of the BL Lac Objects, but it needs more investigation to confirm the unification of FR I radio galaxies, XBLs and RBLs.     

Spherical accretion of massive black holes: A model for galactic nuclei

The possibility of explaining the continuous emission of active galactic nuclei in the frame of a model of spherical accretion onto a massive black hole is discussed. Cool inhomogeneities (T 10⁴°K) within the accretion flow could be responsible for the broad line emission if half of the accreting matter is in the dense phase. A crucial test of this hypothesis is the expected correlation between the ratio of the luminosity in lines to the total luminosity and the hardness of the continuous spectrum.     

Galaxies at the detection limits of deep X-ray surveys

The great sensitivities of the Chandra X-ray Observatory and XMM-Newton have allowed us to begin to explore the X-ray emission from galaxies at moderate to high redshift. By using the stacking method, we show that we can detect the ensemble emission from normal elliptical, spiral and irregular galaxies out to redshifts approaching unity. The average X-ray luminosity of these galaxy types can then be compared with the results of models of the evolution in the numbers of low-mass and high-mass X-ray binaries and can possibly be used to constrain models of star formation.     

Asymmetric precipitation in a coronal loop as explanation of a singular observed spectrum

Almost 10 years of solar submillimeter observations have shown new aspects of solar activity, such as the presence of rapid solar spikes associated with the launch of coronal mass ejections and an increasing submillimeter spectral component in flares. We analyse the singular microwave–submillimeter spectrum of an M class solar flare on 20 December, 2002. Flux density observations measured by Sun patrol telescopes and the Solar Submillimeter Telescope are used to build the radio spectrum, which is fitted using Ramaty’s code. At submillimeter frequencies the spectrum shows a component different from the microwave classical burst. The fitting is achieved proposing two homogeneous sources of emission. This theoretical fitting is in agreement with differential precipitation through a magnetically asymmetric loop or set of loops. From a coronal magnetic field model we infer an asymmetric magnetic structure at the flare location. The model proposed to quantify the differential precipitation rates due to the asymmetry results in a total precipitation ratio Q₂/Q₁≈10⁴–10⁵, where Q₁(Q₂) represents the total precipitation in the loop foot with the high (low) magnetic field intensity. This ratio agrees with the electron total number ratio of the two sources proposed to fit the radio spectrum.     

Some implications of high luminosity and fast variability in active galactic nuclei

An analysis of the variability timescale against bolometric luminosity for Active Galactic Nuclei shows that a number of sources violate the Eddington limit. The average ratio (L/L_E) is found to change according to group classification. Whilst Seyfert Galaxies have luminosites well within the Eddington limit, Quasars and BL Lac object tend to approach and exceed this limit. Furthermore, BL Lac objects may be further subdivided on the basis of their (L/L_E) ratio. The data on luminosity and variability timescale indicate the existence of two types of active galaxies, one having highly anisotropic emission, probably collimated into jets with pointing angles within few degrees to the line of sight, and the other relating to isotropic emission of photons from the nuclear region. The results are discussed in the light of the high γ-ray luminosity suggested by recent observations of active galaxies.     

The optical emission lines of type 1 X-ray bright Active Galactic Nuclei

A strong X-ray emission is one of the defining signatures of nuclear activity in galaxies. According to the Unified Model for Active Galactic Nuclei (AGN), both the X-ray radiation and the prominent broad emission lines, characterizing the optical and UV spectra of Type 1 AGNs, are originated in the innermost regions of the sources, close to the Super Massive Black Holes (SMBH), which power the central engine. Since the emission is concentrated in a very compact region (with typical size r?0.1$r?0.1$ pc) and it is not possible to obtain resolved images of the source, spectroscopic studies of this radiation represent the only valuable key to constrain the physical properties of matter and its structure in the center of active galaxies. Based on previous studies on the physics of the Broad Line Region (BLR) and on the X-ray spectra of broad (FWHM_Hβ ? 2000 km s⁻¹) and narrow line (1000 km s⁻¹ ?FWHM_Hβ ? 2000 km s⁻¹) emitting objects, it has been observed that the kinematic and ionization properties of matter close to the SMBHs are related together, and, in particular, that ionization is higher in narrow line sources. Here we report on the study of the optical and X-ray spectra of a sample of Type 1 AGNs, selected from the Sloan Digital Sky Survey (SDSS) database, within an upper redshift limit of z=0.35$z=0.35$, and detected at X-ray energies. We present analysis of the broad emission line fluxes and profiles, as well as the properties of the X-ray continuum and Fe Kα emission and we use these parameters to assess the consistency of our current AGN understanding.     

ROSAT observations of nearby galaxies

First results of pointed and All Sky Survey observations of galaxies with the X-ray observatory satellite ROSAT are reported. During observations of the Magellanic Clouds and the Andromeda galaxy new super-soft X-ray sources have been detected. This new class of luminous X-ray sources may help to solve the millisecond pulsar progenitor problem. Due to the improved sensitivity and longer observation times of ROSAT new X-ray point sources have been resolved in several nearby galaxies. The diffuse emission of the LMC that was already reported by EINSTEIN has been mapped in detail. It shows a lot of fine structure and temperatures around 5 × 10⁶ K. The improved low energy response of ROSAT led to the discovery of 10⁶ K gas from the spiral galaxy M101 and the halo of the starburst galaxy NGC 253. No diffuse emission was detected from the halo of the edge-on spiral galaxy NGC 5907.     

High-energy neutrinos from powerful radio galaxies

In order to explore mechanisms for the production of radio lobes from radio galaxies, we propose observational tests involving neutrinos at E ≥ 4 TeV. Among the mechanisms that have been suggested are: an explosive burst of energy that has been stored in or near the galactic nucleus; diffusive escape of particles from the vicinity of the galactic nucleus into plasmons; a beam from the galactic core that interacts with the circumgalactic medium; and black holes or spinars ejected from the galactic nucleus by a gravitational slingshot mechanism. As an example, we estimate neutrino fluxes from Cen A. The annual neutrino event rate at energies above 4 TeV is ≈ 10²–10³ for a DUMAND-type dectector if relativistic particles are temporarily stored near the galactic nucleus, and if these have a differential energy spectrum with exponent –2.0. With a similar exponent, but allowing free escape from the galaxy, the fluxes are about 10 times lower.     

On the physical state in the narrow-line region of the classical Seyfert galaxy NGC 7469

Based on the spectrophotometric data, by decomposition of the observed continuum, the power - law continuum characteristics of the central source are obtained. The behaviour of both electron temperature T_e and density n_e in the Narrow - Line Region (NLR) of NGC 7469 is discussed on the thermal and ionization equilibria calculations. T_e in NLR of the Sy1 galaxies are higher than the Sy2 ones, and the possible explanation is the lack of the dense (n_e 10¹⁰ cm⁻³) zone close to the central source in the Sy2 galaxies.     

A survey of radio jets with Chandra and HST

Relativistic jets are a common property of radio-loud Active Galactic Nuclei (AGN). Understanding jet physical properties is an essential precursor to understanding the mechanisms of energy transport, and ultimately, how energy is extracted from the central black hole. In this paper, I highlight recent developments from Chandra and HST observations of kpc-scale jets in AGN, with particular emphasis on our survey of 17 radio jets in a sample of FRII radio galaxies. These observations show that (1) X-ray and optical emission is common from kpc-scale jets, (2) a large fraction of the bolometric luminosity is emitted at X-rays, and (3) in most sources, a candidate emission process for the X-rays is inverse Compton scattering of the Cosmic Microwave Background off the relativistic electrons in the jet. If the latter scenario holds, the implication is that jets are still relativistic on kpc scales.     

The nature of nonthermal X-ray filaments near the galactic center

Recent Chandra and XMM-Newton observations reported evidence of two X-ray filaments G359.88−0.08 (SgrA-E) and G359.54+0.18 (the ripple filament) near the Galactic center. The X-ray emission from these filaments has a nonthermal spectrum and coincides with synchrotron emitting radio sources. Here, we report the detection of a new X-ray feature coincident with a radio filament G359.90−0.06 (SgrA-F) and show more detailed VLA, Chandra and BIMA observations of the radio and X-ray filaments. In particular, we show that radio emission from the nonthermal filaments G359.90−0.06 (SgrA-F) and G359.54+0.18 (the ripple) has a steep spectrum whereas G359.88−0.08 (SgrA-E) has a flat spectrum. The X-ray emission from both these sources could be due to synchrotron radiation. However, given that the 20 km s⁻¹ molecular cloud, with its intense 1.2 mm dust emission, lies in the vicinity of SgrA-F, it is possible that the X-rays could be produced by inverse Compton scattering of far-infrared photons from dust by the relativistic electrons responsible for the radio synchrotron emission. The production of X-ray emission from ICS allows an estimate of the magnetic field strength of 0.08 mG within the nonthermal filament. This should be an important parameter for any models of the Galactic center nonthermal filaments.     

Highly variable X-ray emitting objects in the Rho Oph dark cloud

The Rho Ophiuchi dark cloud region has been the subject of an extensive guest investigation using the $\text{Einstein}$ Observatory. The set of observations comprise 14 IPC fields and 3 HRI fields. The densest part of the cloud has been observed 6 times. Forty seven sources were detected at a level > 3.5 σ and twenty more above 2 σ. The majority of these sources have optical, IR, or even radio continuum counterparts; nine are identified with known T Tauri stars, while several others are identified with stars showing H α in emission. All show a high degree of time variability; flux variations reach factors of 5 in a few hours, or 25 in a day. Apparent luminosities are in the range 10(30) – 10(31)(1) erg.s^?1. The possibility that the X-ray variability is due to flares is examined. If this interpretation is correct, one source has been the seat of the largest stellar flare ever recorded in X rays [L_x = 10(32) erg.s^?1, E_x ?10(36) ergs^-].     

Activity in elliptical galaxies

Elliptical galaxies exhibit a continuum of behaviour between ‘normal’ and highly active objects, with no clear dividing line between the two classes. High-resolution radio observations show that many bright ellipticals contain parsec-scale radio cores which are qualitatively similar to the central engines of radio galaxies but up to a million times less powerful. In contrast, the radio cores of spiral galaxies are qualitatively different from those in ellipticals, providing some clues to why radio galaxies are always ellipticals rather than spirals.