arXiv subjects
Publications and source records attributed to M. Elvis.
Mkn 421 was observed by Chandra twice, on November 5, 1999 as part of the Chandra calibration program, with the ACIS-HETG configuration, and on May 29, 2000 following our Target Of Opportunity request aimed to catch the source in an ultra-high state, with both the ACIS-HETG and the HRC-LETG configurations. In this contribution we present and compare the two Chandra-MEG observations of Mkn 421, which lasted 26 and 19.6 ks respectively.
The presence of a ``warm absorber'' was first suggested to explain spectral variability in an X-ray spectrum of the radio-quiet QSO MR2251-178. A unified picture, in which X-ray warm absorbers and ``intrinsic'' UV absorbers are the same, offers the opportunity to probe the nuclear environment of active galactic nuclei. To test this scenario and understand the physical properties of the absorber, we obtained a UV spectrum of MR2251-178 with the Faint Object Spectrograph (FOS) onboard the Hubble Space Telescope (HST). The HST spectrum clearly shows absorption due to Ly-alpha, NV and CIV, blueshifted by 300 km/sec from the emission redshift of the QSO. The rarity of both X-ray and UV absorbers in radio-quiet QSOs suggests these absorbers are physically related, if not identical. Assuming the unified scenario, we place constraints on the physical parameters of the absorber and conclude the mass outflow rate is essentially the same as the accretion rate in MR2251-178.
We present a deep ($\sim$85 ksec) ASCA observation of the prototype Broad Absorption Line Quasar (BALQSO) PHL 5200. This is the best X-ray spectrum of a BALQSO yet. We find that (1) the source is not intrinsically X-ray weak, (2) the line of sight absorption is very strong with N_H=5 x 10^{23} cm^{-2}, (3) the absorber does not cover the source completely; the covering fraction is about 90%. This is consistent with the large optical polarization observed in this source, implying multiple lines of sight. The most surprising result of this observation is that (4) the spectrum of this BALQSO is not exactly similar to other radio-quiet quasars. The hard X-ray spectrum of PHL 5200 is steep with the power-law spectral index alpha ~1.5. This is similar to the steepest hard X-ray slopes observed so far. At low redshifts, such steep slopes are observed in narrow line Seyfert 1 galaxies, believed to be accreting at a high Eddington rate. This observation strengthens the analogy between BALQSOs and NLS1 galaxies and supports the hypothesis that BALQSOs represent an early evolutionary state of quasars (Mathur 2000). It is well accepted that the orientation to the line of sight determines the appearance of a quasar; age seems to play a significant role as well.
We present results from a Chandra X-ray Observatory study of the field X-ray source populations in 4 different observations: two high-redshift (z~0.5) clusters of galaxies 3C295 and RXJ003033.2+261819; and two non-cluster fields with similar exposure time. Surprisingly, the 0.5-2 keV source surface densities (~900-1200 sources deg**-2 at a flux limit of 1.5x10**-15 erg cm**-2s**-1) measured in an ~8'x8' area surrounding each cluster exceed by a factor of ~2 the value expected on the basis of the ROSAT and Chandra logN-logS, with a significance of ~2 sigma each, or ~3.5 sigma when the 2 fields are combined (i.e. a probability to be a statistical fluctuation of <1% and <0.04%, respectively). The same analysis performed on the non-cluster fields and on the outer chips of the cluster fields does NOT show evidence of such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the detected objects is well fitted by a power-law with Gamma~1.7 similar to AGNs and shows no sign of intrinsic absorption. The few (~10 out of 35) optical identifications available to date confirm that most of them are, as expected, AGNs but the number of redshifts available is too small to allow conclusions on their nature. We discuss possible interpretations of the overdensity in terms of: a statistical variation of Cosmic Background sources; a concentration of AGNs and/or powerful starburst galaxies associated with the clusters; and g ravitational lensing of background QSO's by the galaxy clusters. All explanations are however difficult to reconcile with the large number of excess sources detected. Deeper X-ray observations and more redshifts measurements are clearly required to settle the issue.
We have developed a consistent analytical model to describe the observed evolution of the quasar luminosity function. Our model combines black hole mass distributions based on the Press - Schechter theory of the structure formation in the Universe with quasar luminosity functions resulting from a physics-based emission model that takes into account the time-dependent phenomena occurring in the accretion disks. Quasar evolution and CDM models are mutually constraining, therefore our model gives an estimation of the exponent, n, of the power spectrum, P(k), which is found to be -1.8 < n < -1.6. We were able to reject a generally assumed hypothesis of a constant ratio between Dark Matter Halo and the Black Hole mass, since the observed data could not be fitted under this assumption. We found that the relation between the Dark Matter Halos and Black Hole masses is better described by M_{BH}=M_{DMH}^{0.668}. This model provides a reasonable fit to the observed quasar luminosity function at redshifts higher than ~2.0. We suggest that the disagreement at lower redshift is due to mergers. Based on the agreement at high redshift, we estimated the merger rate at lower redshift, and argue that this rate should depend on the redshift, like (1+z)^3.
Recent ROSAT studies have identified a significant population of Active Galactic Nuclei (AGN) that are notably faint in soft X-rays relative to their optical fluxes. Are these AGN intrinsically X-ray weak or are they just highly absorbed? Brandt, Laor & Wills have systematically examined the optical and UV spectral properties of a well-defined sample of these soft X-ray weak (SXW) AGN drawn from the Boroson & Green sample of all the Palomar Green AGN with z<0.5. We present ASCA observations of three of these SXW AGN: PG 1011-040, PG 1535+547 (Mrk 486), and PG 2112+059. In general, our ASCA observations support the intrinsic absorption scenario for explaining soft X-ray weakness; both PG 1535+547 and PG 2112+059 show significant column densities (N_H~10^22-10^23 cm^-2) of absorbing gas. Interestingly, PG 1011-040 shows no spectral evidence for X-ray absorption. The weak X-ray emission may result from very strong absorption of a partially covered source, or this AGN may be intrinsically X-ray weak. PG 2112+059 is a Broad Absorption Line (BAL) QSO, and we find it to have the highest X-ray flux known of this class. It shows a typical power-law X-ray continuum above 3 keV; this is the first direct evidence that BAL QSOs indeed have normal X-ray continua underlying their intrinsic absorption. Finally, marked variability between the ROSAT and ASCA observations of PG 1535+547 and PG 2112+059 suggests that the soft X-ray weak designation may be transient, and multi-epoch 0.1-10.0 keV X-ray observations are required to constrain variability of the absorber and continuum.
In this paper we present the analysis of two broad band (0.1-150 keV) BeppoSAX observations of the Seyfert 1 galaxy NGC 3516. The two observations were taken 4 months apart, on 1996 November 8 and 1997 March 12 respectively. We report a dramatic change in the degree of obscuration of the central source between the two observations, and propose, as possible explanations transient absorption by either a stationary-state cloud of cold gas crossing the line of sight, or a varying-state, initially neutral and dense amount of expanding gas with decreasing density and therefore decreasing opacity. We also report the detection of a second highly ionized absorber/emitter, which causes deep FeXVII-XXII K edges at about 7.8 keV to appear in both BeppoSAX spectra of NGC 3516, and possibly produces the soft X-ray continuum emission and the 1 keV blend of Fe L recombination lines detected during the epoch of heavy nuclear obscuration.
We present photoionization and collisional ionization models, and their application to three important fields: (a) the Warm Absorbers/Emitters in type 1 AGN, (b) the Warm Reflectors in Type 2 AGN, and (c) X-ray absorption of background quasars by intergalactic gas. A number of cases are investigated, and the dependences of the main parameters explored.
We present results from BeppoSAX observations of 10 low redshift quasars. The quasars are part of the Laor et al. (1997) sample of 23 optically selected PG quasars with redshift <0.4 and low Galactic absorption along the line of sight. Significant spectral curvature is detected for the 6 quasars with the highest signal to noise ratio in their low energy spectra. The average curvature can be parameterized as a flattening of the underlying power law by Delta_alpha ~ 0.5 above ~1 keV. We find that quasars with a steeper soft X-ray (0.1-2 keV) spectrum tend to be steeper also at higher (2-10 keV) energies. The distribution of the best fit 2-10 keV slopes is similar to that found for other radio-quiet AGN and characterized by a large dispersion around the mean: alpha_E ~ 1.0+/-0.3. Iron K_alpha lines are detected in 4 quasars. In the narrow-line quasar PG 1115+407, the rest frame line energy (6.69+/-0.11 keV) and equivalent width (580+/-280 eV) are consistent with those found in a few low redshift narrow-line Seyfert 1 galaxies (NLSy1). This, together with the similarity of the 0.1-10 keV X-ray continuum, suggests that this quasar is the higher redshift and luminosity analog of a NLSy1. In the broad line quasar PG 0947+396, the rest-frame line energy suggests fluorescence from cold iron. The line equivalent width (> 400 eV) is however about 2-3 times higher than that usually found in Seyfert 1 galaxies. The high energy power-law slopes and the iron line emission properties seem to be unrelated to the X-ray luminosity.
The quasar PKS 0637-753, the first celestial X-ray target of the Chandra X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to 12 arcsec west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous (~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new ATCA radio images at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space VLBI mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. HST images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet, and a radio component to the east, show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extra-nuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton (SSC) model, but this would imply extreme departures from the conventional minimum-energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.
We have monitored with the RXTE PCA the variability pattern of the 2-20 keV flux in four PG quasars (QSOs) from the Laor et al. (1994) sample. Six observations of each target at regular intervals of 1 day were performed. The sample comprises objects with extreme values of Balmer line width (and hence soft X-ray steepness) and spans about one order of magnitude in luminosity. The most robust result is that the variability amplitude decreases as energy increases. Several options for a possible ultimate driver of the soft and hard X-ray variability, such as the influx rate of Comptonizing relativistic particles, instabilities in the accretion flow or the number of X-ray active sites, are consistent with our results.
We present a model which relates the Broad Emission Line Regions of AGN to the accretion mechanism. Based on the Keplerian assumption for the high velocity clouds of the gas responsible for the production of the Broad Emission Lines, we identify that gas with an outflowing disk/coronal wind forms a stabilized Shakura-Sunyaev accretion disk. We show that in the framework of this picture the observed range of H-beta FWHM from Broad Line to Narrow Line type 1 AGN, is well reproduced as a function of the accretion rate.
We report optical spectroscopic identifications of 10 hard (2-10 keV) X-ray selected sources discovered by Chandra. The X-ray flux of the sources ranges between 1.5 and 25 x 10-14 cgs, the lower value being 3 times fainter than in previous BeppoSAX and ASCA surveys. Their R band magnitudes are in the range 12.8-22. Six of the Chandra sources are broad line quasars with redshifts between 0.42 and 1.19, while the optical identification of the remaining four is quite varied: two are X-ray obscured, emission line AGN at z=0.272 and z=0.683, one is a starburst galaxy at z=0.016 and one, most unusually, is an apparently normal galaxy at z=0.158. These findings confirm and extend down to fainter X-ray fluxes the BeppoSAX results, in providing samples with a wide range of X-ray and optical properties. The ratio between the soft X-ray and the optical luminosity of the z=0.158 galaxy is a factor at least 30 higher than that of normal galaxies, and similar to those of AGN. The high X-ray luminosity and the lack of optical emission lines suggest an AGN in which either continuum beaming dominates, or emission lines are obscured or not efficiently produced.
We present a deep ASCA observation of a Broad Absorption Line Quasar (BALQSO) PG0946+301. The source was clearly detected in one of the gas imaging spectrometers, but not in any other detector. If BALQSOs have intrinsic X-ray spectra similar to normal radio-quiet quasars, our observations imply that there is Thomson thick X-ray absorption (N_H >~10^{24} cm^{-2}) toward PG0946+301. This is the largest column density estimated so far toward a BALQSO. The absorber must be at least partially ionized and may be responsible for attenuation in the optical and UV. If the Thomson optical depth toward BALQSOs is close to one, as inferred here, then spectroscopy in hard X-rays with large telescopes like XMM would be feasible.
NGC 5548 was observed by BeppoSAX in a single long (8 day) observation from 0.2 to 200 keV. We find (1) the spectral variation of the source is produced by a change of the intrinsic power law slope; (2) a high energy cut-off at $E_c= 115^{+39}_{-27}$ keV with a hint of change of $E_c$ with flux; (3) OVII and OVIII absorption K edges, and a possible blended OVII-OVIII K$α,β$ emission feature at $0.54^{+0.07}_{-0.06}$ keV, inconsistent with a purely photoionized gas in equilibrium. We propose that the temperature of the absorbing and emitting gas is $\sim 10^6$ K so that both collisional ionization and photoionization contribute.
A deep all-sky imaging hard x-ray survey and wide-field monitor is needed to extend soft (ROSAT) and medium (ABRIXAS2) x-ray surveys into the 10-100 keV band (and beyond) at comparable sensitivity (~0.05 mCrab). This would enable discovery and study of >3000 obscured AGN, which probably dominate the hard x-ray background; detailed study of spectra and variability of accreting black holes and a census of BHs in the Galaxy; Gamma-ray bursts and associated massive star formation (PopIII) at very high redshift and Soft Gamma-ray Repeaters throughout the Local Group; and a full galactic survey for obscured supernova remnants. The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed array of 8 x 1m^2 coded aperture telescopes fixed on the International Space Station (ISS) with 160deg x 40deg field of view which images the full sky each 90 min orbit. EXIST has been included in the most recent NASA Strategic Plan as a candidate mission for the next decade. An overview of the science goals and mission concept is presented.
Hard X-ray selection is the most efficient way to discriminate between accretion-powered sources, such as AGN, from sources dominated by starlight. Hard X-rays are also less affected than other bands by obscuration. We have then carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5-10 keV band, finding 180 sources in ~50 deg^2 of sky with flux >5E-14 erg cm-2 s-1. After correction for the non uniform sky coverage this corresponds to resolving about 30 % of the hard Cosmic X-ray Background (XRB). Here we report on a first optical spectroscopic identification campaign, finding 12 AGN out of 14 X-ray error-boxes studied. Seven AGN show evidence for obscuration in X-ray and optical bands, a fraction higher than in previous ROSAT or ASCA-ROSAT surveys (at a 95-99 % and 90 % confidence level respectively), thus supporting the scenario in which a significant fraction of the XRB is made by obscured AGN.
First results on a medium-deep X-ray survey in the "new" 5-10 keV band carried out with the MECS detectors onboard BeppoSAX are presented. The High Energy Llarge Area Survey (HELLAS) is aimed to directly explore a band where the energy density of the X-ray background is more than twice than that in the soft (0.5-2.0 keV) band. The optical identification follow-up of the first ten HELLAS hard X-ray sources indicate that Active Galactic Nuclei are the dominant population at 5-10 keV fluxes of the order of 10e-13 cgs. We discuss the implications of these findings for the AGN synthesis models for the XRB.