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Based on its strong CO+ emission, it is argued that the M82 starburst galaxy is exposed to a combination of FUV and X- ray radiation. The latter is likely to be the result of the starburst superwind, which leads to diffuse thermal emission, and a compact hard source (but not an AGN). Although a photon-dominated region ( FUV) component is clearly present in the nucleus of M82, and capable of forming CO+, only X- ray irradiated gas of density 10(3) - 10(5) cm (-3) can reproduce the large, similar to (1-4) x 10(13) cm(-2), columns of CO+ that are observed toward the prototypical starburst M82. The total X- ray luminosity produced by M82 is weak, similar to 10(41) ergs s(-1), but this is sufficient to drive the formation of CO+.

Based on its strong CO+ emission, it is argued that the M82 starburst galaxy is exposed to a combination of FUV and X- ray radiation. The latter is likely to be the result of the starburst superwind, which leads to diffuse thermal emission, and a compact hard source (but not an AGN). Although a photon-dominated region ( FUV) component is clearly present in the nucleus of M82, and capable of forming CO+, only X- ray irradiated gas of density 10(3) - 10(5) cm (-3) can reproduce the large, similar to (1-4) x 10(13) cm(-2), columns of CO+ that are observed toward the prototypical starburst M82. The total X- ray luminosity produced by M82 is weak, similar to 10(41) ergs s(-1), but this is sufficient to drive the formation of CO+.

We report on deep (40 ks x 4) observations of the bright broad line radio galaxy 3C 120 using Suzaku. The observations were spaced one week apart, and sampled a range of continuum fluxes. An excellent broadband spectrum was obtained over two decades of frequency (0.6 to 50 keV) within each 40 ks exposure. We clearly resolved the iron K emission-line complex, finding that it consists of a narrow K alpha core (sigma similar or equal to 110 eV or an EW of 60 eV), a 6.9 keV line, and an underlying broad iron line. Our confirmation of the broad line contrasts with the XMM-Newton observation in 2003, where the broad line was not required. The most natural interpretation of the broadline is iron Kline emission from a face-on accretion disk that is truncated at similar to 10 r(g). Above 10 keV, a relatively weak Compton hump was detected (reflection fraction of R similar or equal to 0.6), superposed on the primary X-ray continuum of Gamma similar or equal to 1.75. Thanks to the good photon statistics and low background of the Suzaku data, we clearly confirm the spectral evolution of 3C 120, whereby the variability amplitude decreases with increasing energy. More strikingly, we discovered that the variability is caused by a steep power-law component of Gamma similar or equal to 2.7, possibly related to non-thermal jet emission. We discuss our findings in the context of similarities and differences between radio-loud/quiet objects.

In this paper we test the idea that the low-metallicity hot gas in the centre of NGC 1291 is heated via a dynamical process. In this scenario, the gas from the outer gas-rich ring loses energy through bar-driven shocks and falls to the centre. Heating of the gas to X-ray temperatures comes from the high velocity that it reaches ($\approx$ 700 \kms) as it falls to the bottom of the potential well. This would explain why the stellar metallicity in the bulge region is around solar while the hot gas metallicity is around 0.1 solar. We carried out an observational test to check this hypothesis by measuring the metallicity of HII regions in the outer ring to check whether they matched the hot gas metallicity. For this purpose we obtained medium resolution long slit spectroscopy with FORS1 on the ESO VLT at Paranal and obtained the metallicities using emission line ratio diagnostics. The obtained metallicities are compatible with the bulge stellar metallicities but very different from the hot-gas metallicity. However, when comparing the different time-scales, the gas in the ring had time enough to get enriched through stellar processes, therefore we cannot rule out the dynamical mechanism as the heating process of the gas. However, the blue colours of the outer ring and the dust structures in the bar region could suggest that the origin of the X-ray hot gas is due to the infall of material from further out., 6 pages, 6 figures. A&A accepted

We present results from Chandra observations of 14 ultraluminous infrared galaxies [ULIRGs; log(L(IR)/L.) >= 12] with redshifts between 0.04 and 0.16. The goals of the observations were to investigate any correlation between infrared color or luminosity and the properties of the X-ray emission and to attempt to determine whether these objects are powered by starbursts or active galactic nuclei (AGNs). The sample contains approximately the same number of high- and low-luminosity objects and "warm'' and "cool'' ULIRGs. All 14 galaxies were detected by Chandra. Our analysis shows that the X-ray emission of the two Seyfert 1 galaxies in our sample is dominated by an AGN. The remaining 12 sources are too faint for conventional spectral fitting to be applicable. Hardness ratios were used to estimate the spectral properties of these faint sources. The photon indices, Gamma-values, for our sample plus the Chandra-observed 2003 sample from Ptak et al. peak in the range 1.0-1.5, consistent with expectations for X-ray binaries in a starburst, an absorbed AGN, or hot bremsstrahlung from a starburst or AGN. The values of Gamma for the objects in our sample classified as Seyferts (type 1 or 2) are larger than 2, while those classified as H II regions or LINERs tend to be less than 2. The hard X-ray to far-infrared ratios for the 12 weak sources are similar to those of starbursts, but we cannot rule out the possibility of absorbed, possibly Compton-thick, AGNs in some of these objects. Two of these faint sources were found to have X-ray counterparts to their double optical and infrared nuclei.

We present results from a new XMM-Newton observation of the high-redshift quasar RXJ1028.6-0844 at a redshift of 4.276. The soft X-ray spectral flattening, as reported by a study with ASCA previously (Yuan et al. 2000, ApJ 545, 625), is confirmed to be present, however, with reduced column density when modelled by absorption. The inferred column density for absorption intrinsic to the quasar is 2.1(+0.4-0.3)x10^22 (cm^-2) for cold matter, and higher for ionised gas. The spectral flattening shows remarkable similarity with those of two similar objects, GB1428+4217 (Worsley et al. 2004, MNRAS 350, L67) and PMNJ0525-3343 (Worsley et al. 2004, MNRAS 350, 207). The results improve upon those obtained from a previous short-exposure observation for RXJ1028.6-0844 with XMM-Newton (Grupe et al. 2004, AJ 127, 1). A comparative study of the two XMM-Newton observations reveals a change in the power-law photon index from Gamma ~1.3 to 1.5 on timescales of about one year. A tentative excess emission feature in the rest-frame 5-10keV band is suggested, which is similar to that marginally suggested for GB1428+4217., Comment: 9 pages, accepted for publication in MNRAS; minor changes (added footnote commenting on the use of the F-test, added references)

A fraction of active galactic nuclei do not show the classical Seyfert-type signatures in their optical spectra, i.e. they are optically "elusive". X-ray observations are an optimal tool to identify this class of objects. We combine new Chandra observations with archival X-ray data in order to obtain a first estimate of the fraction of elusive AGN in local galaxies and to constrain their nature. Our results suggest that elusive AGN have a local density comparable to or even higher than optically classified Seyfert nuclei. Most elusive AGN are heavily absorbed in the X-rays, with gas column densities exceeding 10^24 cm^-2, suggesting that their peculiar nature is associated with obscuration. It is likely that in elusive AGN, the nuclear UV source is completely embedded and the ionizing photons cannot escape, which prevents the formation of a classical Narrow Line Region. Elusive AGN may contribute significantly to the 30 keV bump of the X-ray background., Comment: accepted for publication in MNRAS Letters, 6 pages, 3 figures, typos and references corrected

We present the results of the BeppoSAX observations of two BL Lacs, OQ 530 and S5 0716+714, as part of a ToO program for the simultaneous observation at radio, optical, X-ray and TeV energies. Both sources are detected in the LECS and MECS, with S5 0716+714 visible also in the PDS band, up to about 60 keV. The X-ray spectra of both sources are better fitted by a double power-law model, with a steep soft X-ray component flattening at harder energies, with breaks at 0.3 and 1.5 keV, respectively. The concave shape of the spectra in both objects is consistent with soft X-rays being produced by the synchrotron and harder X-rays by the inverse Compton processes. Also the X-ray variability properties confirm this scenario, in particular for S5 0716+714 our observation shows variations by about a factor 3 over one hour below 3 keV and no variability above. Their simultaneous broad band energy spectral distributions can be successfully interpreted within the frame of a homogeneous synchrotron and inverse Compton model, including a possible contribution from an external source of seed photons with the different spectral states of S5 0716+714 being reproduced by changing the injected power. The resulting parameters are fully consistent with the two sources being intermediate objects within the "sequence" scenario proposed for blazars., Comment: 10 pages, 8 figures, accepted by A&A

A Chandra observation of the radio-loud quasar 3C9 at redshift z=2.012 has revealed extended X-ray emission coincident with the radio jet. Of particular interest is the appearance of both jet and counterjet, which argues against the X-ray emission being highly beamed. We present the properties of the jets and discuss possible scenarios for the X-ray emission, contrasting them with the single sided jet found in PKS 0637-752, which has a similar X-ray luminosity. The jet X-ray emission in 3C9 is likely to be due to either nonthermal emission from a sheath with bulk Lorentz factor less than 1.5, or thermal emission from shocked cold gas surrounding the quasar. The thermal possibility implies a high mass for the cold gas unless it is highly clumped.

We report the BeppoSAX observations of 6 Flat Spectrum Radio Quasars. Three of them have a clear detection up to 100 keV with the PDS instrument. For 4 objects the X-ray spectrum is satisfactorily fitted by a power-law continuum with Galactic absorption. 2251+158 show the presence of absorption higher than the galactic value, while the spectrum of the source 0208-512 shows a complex structure, with evidence of absorption at low energy. We construct the Spectral Energy Distributions adding historical data to the broad band X-ray spectra obtained with $Beppo$SAX and reproduce them with a one-zone Synchrotron-Inverse Compton model (including both SSC and External Compton). The implications are briefly discussed., Comment: 5 figures; ApJ, in press

PKS 2155--304 is one of the brightest blazars in the X-ray band. It was repeatedly monitored with BeppoSax during three long campaigns of about 2 days each in November of 1996, 1997 and 1999. The source underwent different states of intensity and was clearly variable with successive flares detected. This paper presents temporal and spectral analysis to study the X-ray variability trends for a blazar. The rich phenomenology is interpreted in the context of a model where relativistic electrons are accelerated through internal shocks taking place in the jets. The most important parameter turns out to be the initial time interval between the two shells ejected from the central engine to produce the flare, which may determine the structure of the shock and in turn the physical quantities of the emitting region to reproduce the observed trends of the X-ray variability (abridged)., 51 pages, 11 figures, ApJ in press, Scheduled for the v572 no2, June 20, 2002 issue

A large sample of X-ray-emitting clusters of galaxies is considered in the context of Milgrom's modified Newtonian dynamics (MOND). The virial discrepancy (the ratio of the dynamical to observable mass) is reduced, on average, from a factor of 4 in the Newtonian analysis to a factor of 2 in the MOND analysis. The reason for this modest reduction is that the internal accelerations in the inner 1 Mpc of the sample clusters are not very small in terms of the critical MOND acceleration: a approximate to 0.5a(0). Although the uncertainty is large, there remains a discrepancy that is not yet accounted for by the directly observed matter in galaxies and diffuse gas. This may be consistent with the fact that modified dynamics cannot resolve the strong lensing discrepancy in those clusters where this phenomenon occurs. If MOND is correct, then the implication is that the budget of possibly detectable matter is not yet complete, particularly in the central regions of rich clusters.

A large sample of X-ray-emitting clusters of galaxies is considered in the context of Milgrom's modified Newtonian dynamics (MOND). The virial discrepancy (the ratio of the dynamical to observable mass) is reduced, on average, from a factor of 4 in the Newtonian analysis to a factor of 2 in the MOND analysis. The reason for this modest reduction is that the internal accelerations in the inner 1 Mpc of the sample clusters are not very small in terms of the critical MOND acceleration: a approximate to 0.5a(0). Although the uncertainty is large, there remains a discrepancy that is not yet accounted for by the directly observed matter in galaxies and diffuse gas. This may be consistent with the fact that modified dynamics cannot resolve the strong lensing discrepancy in those clusters where this phenomenon occurs. If MOND is correct, then the implication is that the budget of possibly detectable matter is not yet complete, particularly in the central regions of rich clusters.

We present a new well defined sample of BL Lac objects selected from the ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming a flux limited sample down to f(X) (0.5 - 2.0 keV) = 8 . 10(-13) ergs cm(-2) s(-1); redshifts are known for 33 objects (and 31 of the complete sample). X-ray spectral properties were determined for each object individually with the RASS data. The luminosity function of RASS selected BL Lac objects is compatible with results provided by objects selected with the Einstein observatory, but the RASS selected sample contains objects with luminosities at least tenfold higher. Our analysis confirms the negative evolution for X-ray selected BL Lac objects found in a sample by the Einstein observatory, the parameterization provides similar results. A subdivision of the sample into halves according to the X-ray to optical flux ratio yielded unexpected results. The extremely X-ray dominated objects have higher redshifts and X-ray luminosities and only this subgroup shows clear signs of strong negative evolution. The evolutionary behaviour of objects with an intermediate spectral energy distribution between X-ray and radio dominated is compatible with no evolution at all. Consequences for unified schemes of X-ray and radio selected BL Lac objects are discussed. We suggest that the intermediate BL Lac objects are the basic BL Lac population. The distinction between the two subgroups can be explained if extremely X-ray dominated BL Lac objects are observed in a state of enhanced X-ray activity.

We have used archival Chandra and XMM- Newton observations of quasars hosting intrinsic narrow UV absorption lines (intrinsic NALs) to carry out an exploratory survey of their X- ray properties. Our sample consists of three intrinsic NAL quasars and one '' mini- BAL'' quasar, plus four quasars without intrinsic absorption lines for comparison. These were drawn in a systematic manner from an optical / UV- selected sample. The X- ray properties of intrinsic NAL quasars are indistinguishable from those of ''normal'' quasars. We do not find any excess absorption in quasars with intrinsic NALs, with upper limits of NH less than or similar to few x 10(22) cm(-2). We compare the X- ray and UV properties of our sample quasars by plotting the equivalent width and blueshift velocity of the intrinsic NALs and the X- ray spectral index against the `` optical - to - X- ray'' slope, alpha(ox). When BAL quasars and other AGNs with intrinsic NALs are included, the plots suggest that intrinsic NAL quasars form an extension of the BAL sequences and tend to bridge the gap between BAL and normal quasars. Observations of larger samples of intrinsic NAL quasars are needed to verify these conclusions. We also test two competing scenarios for the location of the NAL gas in an accretion disk wind. Our results strongly support a location of the NAL gas at high latitudes above the disk, closer to the disk axis than the dense BAL wind. We detect excess X- ray absorption only in Q0014+ 8118, which does not host intrinsic NALs. The absorbing medium very likely corresponds to an intervening system at z = 1.1, which also produces strong absorption lines in the rest- frame UV spectrum of this quasar. In an appendix we discuss the connection between UV and X- ray attenuation and its effect on alpha(ox)., Article, The Astrophysical Journal. 677:863-879 (2008)

Based on its strong CO+ emission, it is argued that the M82 starburst galaxy is exposed to a combination of FUV and X- ray radiation. The latter is likely to be the result of the starburst superwind, which leads to diffuse thermal emission, and a compact hard source (but not an AGN). Although a photon-dominated region ( FUV) component is clearly present in the nucleus of M82, and capable of forming CO+, only X- ray irradiated gas of density 10(3) - 10(5) cm (-3) can reproduce the large, similar to (1-4) x 10(13) cm(-2), columns of CO+ that are observed toward the prototypical starburst M82. The total X- ray luminosity produced by M82 is weak, similar to 10(41) ergs s(-1), but this is sufficient to drive the formation of CO+.

WeestimatedblackholemassesandEddingtonratiosforasampleof X-ray-selectedactivegalacticnuclei(AGNs) inthefieldscoveredbytheGreatObservatoryOriginsDeepSurvey(GOODS).Thespannedrangesinredshift(0:4 < z < 1) and hard X-ray luminosity(10 42 PLX P4 ; 10 43 ergss � 1 )allowus tostudy arepresentative subsampleofthe main contributors to the 2Y10 keV X-ray background. Nuclear and bulge magnitudes in four bands have been measured via a two-dimensional decomposition applied toHSTACS images. Using the black hole versus bulge luminosity relation and the intrinsic nuclear emission, we derived the black hole mass and the AGN bolometric luminosity. Wefindinoursamplethat(1)theX-rayYtoYopticalindicesarelargerthaninopticallyselectedQSOs,asexpecteddue to the X-ray selection); (2) the X-ray bolometric corrections are generally small, suggesting a decrease with the nuclear luminosity; (3) the Eddington ratios are about a factor 10 below the values found at higher redshift and luminosity; (4) the central black holes have rather largemasses; and (5) at least for z P0:8, a scarceness of black holes with

\abridged The nuclei of active galaxies harbor massive young stars, an accreting central black hole, or both. In order to determine the physical conditions that pertain to molecular gas close to the sources of radiation, numerical models are constructed. These models determine the thermal and chemical balance of molecular gas that is exposed to X-rays and far-ultraviolet radiation, as a function of depth. We present a grid of XDR and PDR models that span ranges in density, irradiation and column density. We find that the fine-structure line ratios of e.g. [SiII] 35mum/[CII] 158 mum are larger in XDRs than in PDRs, for a given density, column and irradiation strength. We find that the line ratios HCN/HCO+ and HNC/HCN, as well as the column density ratio CN/HCN, discriminate between PDRs and XDRs. The HCN/HCO+ 1-0 ratio is 1) for XDRs (PDRs) if the density exceeds 10^5 cm^-3 and if the column density is >10^23 cm^-2. For columns 100 erg s^-1 cm^-2), HCN/HCO+ ratios can become larger in XDRs than PDRs as well. The HCN/CO 1-0 ratio is typically smaller in XDRs, and the HCN emission in XDRs is boosted with respect to CO only for columns >10^{23} cm^{-2} and densities =1, line ratios in XDRs. Lines with N>=10, like CO(16-15) and CO(10-9) observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. Column density ratios indicate that CH, CH+, NO, HOC+ and HCO are good PDR/XDR discriminators., 34 pages, 47 figures, accepted by A&A, a version with all figures is available at http://www.strw.leidenuniv.nl/~meijerin/grid/GridCodes_printer.pdf

The optical-UV and X-ray instruments on-board XMM-Ndewton provide an excellent opportunity to perform simultaneous observations of violently variable objects over a broad wavelength range. The UV and X-ray bright BL Lac object PKS 2155-304 has been repeatedly observed with XMM-Ndewton about twice per year. In this paper, we present a detailed analysis of the simultaneous multiwavelength variability of the source from optical to X-rays, based on the currently available XMM-Ndewton observations. These observations probed the intra-day multiwavelength variability at optical-UV and X-ray wavelengths of the source. The UV variability amplitude is substantially smaller than the X-ray one, and the hardness ratios of the UV to X-rays correlates with the X-ray fluxes: the brighter the source, the flatter the UV-X-ray spectra. On 2000 May 30-31 the UV and X-ray light curves were weakly correlated, while the UV variations followed the X-ray ones with no detectable lags on 2000 November 19-21. On 2001 November 30 the source exhibited a major X-ray flare that was not detected in the optical. The intra-day UV and X-ray variability presented here is not similar to the inter-day UV and X-ray variability obtained from the previous coordinated extensive multiwavelength campaigns on the source, indicating that different ``modes'' of variability might be operating in PKS 2155-304 on different timescales or from epoch to epoch., Comment: Accepted by ApJ

The spectral energy distribution (SED) of TeV blazars peaks both at keV and TeV energies. The X-ray emission is generally believed to originate in the synchrotron emission from relativistic electrons (and positrons) in the jet of these sources, while the origin of the gamma-ray emission is still being debated. We report results from a systematic study of X-ray spectral variability of Mrk 421 and Mrk 501 during individual flares that last for several days, making use of some of the high-quality data that have recently become available. The X-ray spectra of the two sources fall on the opposite sides of the synchrotron peak of their respective SEDs, so they together may offer additional insights into the physical origin of X-ray variability. We modeled each of the time-resolved X-ray spectra over a {\em single} flare by adopting a homogeneous spatial distribution and an instantaneous power-law spectral distribution for the emitting particles. We focused on the variation of four key parameters: particle spectral index, maximum Lorentz factor, energy density, and magnetic field. While there is considerable degeneracy in the fits, we show that, in order to account for the X-ray spectral variability observed in Mrk 421, at least three of the parameters are required to vary in most cases, with the spectral index being one of them. The observations of Mrk 501 support the conclusion, although the quality of the data is not as good. We discuss the implications of the results., Comment: 17 pages, 4 figures, 4 tables, accepted by ApJ; one typo (unit of E_tot) corrected

We study accretion in binaries hosting an intermediate mass black hole (IMBH) of 1000 solar masses, and a donor star more massive than 15 solar masses. These systems experience an active X-ray phase characterized by luminosities varying over a wide interval, from, 10 pages, 8 figures, accepted for publication in MNRAS. Few comments added (minor revision)

Starting from XMM-Newton EPIC-PN data, we present the X-ray variability characteristics of PKS 2155-304 using a simple analysis of the excess variance, \xs, and of the fractional rms variability amplitude, fvar. The scatter in \xs\ and \fvar, calculated using 500 s long segments of the light curves, is smaller than the scatter expected for red noise variability. This alone does not imply that the underlying process responsible for the variability of the source is stationary, since the real changes of the individual variance estimates are possibly smaller than the large scatters expected for a red noise process. In fact the averaged \xs and \fvar, reducing the fluctuations of the individual variances, chang e with time, indicating non-stationary variability. Moreover, both the averaged \sqxs (absolute rms variability amplitude) and \fvar show linear correlation with source flux but in an opposite sense: \sqxs correlates with flux, but \fvar anti-correlates with flux. These correlations suggest that the variability process of the source is strongly non-stationary as random scatters of variances should not yield any correlation. \fvar spectra were constructed to compare variability amplitudes in different energy bands. We found that the fractional rms variability amplitude of the source, when significant variability is observed, increases logarithmically with the photon energy, indicating significant spectral variability. The point-to-point variability amplitude may also track this trend, suggesting that the slopes of the power spectral density of the source are energy-independent. Using the normalized excess variance the black hole mass of \pks was estimated to be about $1.45 \times 10^8 M_{\bigodot}$. This is compared and contrasted with the estimates derived from measurements of the host galaxies., Comment: Accepted for publication in The Astrophysical Journal

We present results from Chandra observations of 14 ultraluminous infrared galaxies (ULIRGs; log(L_IR/L_Sun) >= 12) with redshifts between 0.04 and 0.16. The goals of the observations were to investigate any correlation between infrared color or luminosity and the properties of the X-ray emission and to attempt to determine whether these objects are powered by starbursts or active galactic nuclei (AGNs). The sample contains approximately the same number of high and low luminosity objects and ``warm'' and ``cool'' ULIRGs. All 14 galaxies were detected by Chandra. Our analysis shows that the X-ray emission of the two Seyfert 1 galaxies in our sample are dominated by AGN. The remaining 12 sources are too faint for conventional spectral fitting to be applicable. Hardness ratios were used to estimate the spectral properties of these faint sources. The photon indices for our sample plus the Chandra-observed sample from Ptak et al.(2003) peak in the range of 1.0-1.5, consistent with expectations for X-ray binaries in a starburst, an absorbed AGN, or hot bremsstrahlung from a starburst or AGN. The values of photon index for the objects in our sample classified as Seyferts (type 1 or 2) are larger than 2, while those classified as HII regions or LINERs tend to be less than 2. The hard X-ray to far-infrared ratios for the 12 weak sources are similar to those of starbursts, but we cannot rule out the possibility of absorbed, possibly Compton-thick, AGNs in some of these objects. Two of these faint sources were found to have X-ray counterparts to their double optical and infrared nuclei., 40 pages, 5 tables, 14 figures, accepted by ApJ

Motivated by the recent discovery of a hierarchical X-ray flaring phenomenon in Mrk 421, we conducted a systematic search for X-ray flares from Mrk 501, another well-known TeV blazar, by making use of the rich RXTE archival database. We detected flares over a wide range of timescales, from months down to minutes, as in the case of Mrk 421. However, the flares do not seem to occur nearly as frequently in Mrk 501 as in Mrk 421 on any of the timescales. The flaring hierarchy also seems apparent in Mrk 501, suggesting that it might be common among TeV blazars. The results seem to imply a scale-invariant physical origin of the flares (large or small). The X-ray spectrum of the source shows a general trend of hardening toward the peak of long-duration flares, with indication of spectral hysteresis, which is often seen in TeV blazars. However, the data are not of sufficient quality to allow us to draw definitive conclusions about spectral variability associated with more rapid but weaker flares. We critically examine a reported subhour X-ray flare from Mrk 501, in light of intense background-flaring activity at the time of the observation, and concluded that the flare is likely an artifact. On the other hand, we did identify a rapid X-ray flare that appears to be real. It lasted only for about 15 minutes, during which the flux of the source varied by about 30%. Substructures are apparent in its profile, implying variabilities on even shorter timescales. Such rapid variabilities of Mrk 501 place severe constraints on the physical properties of the flaring region in the jet, and the constraints have serious implications for the emission models proposed for TeV blazars.

We report results from a systematic search for X-ray flares from Mrk 421, using archival data from the Rossi X-Ray Timing Explorer (RXTE) satellite. The flares are clearly seen over a wide range of timescales. The quasi-continuous coverage of the source with the All-SkyMonitor (ASM) reveals frequent occurrence of major flares that last for months. On a few occasions, the source was intensively monitored with the more sensitive pointing instruments aboard RXTE. The data from these observations show the presence of X-ray flares of much shorter duration, ranging from weeks down to less than an hour. For the first time, we clearly resolved the most rapid flares from Mrk 421. Moreover, Fourier analyses reveal variability on even shorter timescales, up to about 0.01 Hz. The source appears to behave differently in its spectral properties during different flares, large or small, which is intriguing. While significant hysteresis is observed to be associated with spectral evolution in some cases, little is seen in other cases. Sometimes, the shape of the X-ray spectrum hardly varies across a flare. Therefore, the phenomenology is complex. The observed hierarchical structure of the X-ray flares seems to imply the scale-invariant nature of the phenomenon, perhaps similar in this regard to solar flares or the rapid X-ray flares observed from stellar-mass black holes. Combined with other results, the observed flaring timescales seriously constrain the physical properties of X-ray-emitting regions in the jets of Mrk 421.

We report the results of XMM-Newton observations of the X-ray bright, radio-loud blazar GB B1428+4217 at a redshift of z=4.72. We confirm the presence of soft X-ray spectral flattening at energies, 5 pages, 5 figures, MNRAS accepted. Minor changes to sections 3.1 and 5

Monthly Notices of the Royal Astronomical Society, 350 (3), ISSN:0035-8711, ISSN:1365-2966, ISSN:1365-8711

Between 2002 March 7 and 2002 June 15, intensive X-ray observations were carried out on the extreme BL Lac object H1426+428 with instruments on board the Rossi X-Ray Timing Explorer (RXTE). These instruments provide measurements of H1426+428 in the crucial energy range that characterizes the first peak of its spectral energy distribution. This peak, which is almost certainly due to synchrotron emission, has previously been inferred to be in excess of 100 keV. By taking frequent observations over a four-month campaign, which included similar to450 ks of RXTE time, studies of flux and spectral variability on multiple timescales were performed, along with studies of spectral hysteresis. The 3-24 keV X-ray flux and spectra exhibited significant variability, implying variability in the location of the first peak of the spectral energy distribution. Hysteresis patterns were observed, and their characteristics have been discussed within the context of emission models.

We present new BeppoSAX LECS and MECS observations, covering the energy range 0.1 - 10 keV (observer's frame), of four BL Lacertae objects selected from the 1 Jy sample. All sources display a flat (alpha_x ~ 0.7) X-ray spectrum, which we interpret as inverse Compton emission. One object shows evidence for a low-energy steepening (Delta alpha_x ~ 0.9) which is likely due to the synchrotron component merging into the inverse Compton one around ~ 2 keV. A variable synchrotron tail would explain why the ROSAT spectra of our sources are typically steeper than the BeppoSAX ones (Delta alpha_x} ~ 0.7). The broad-band spectral energy distributions fully confirm this picture and model fits using a synchrotron inverse Compton model allow us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. By combining the results of this paper with those previously obtained on other sources we present a detailed study of the BeppoSAX properties of a well-defined sub-sample of 14 X-ray bright (f_x (0.1 - 10 keV) > 3 x 10^{-12} erg/cm^2/s) 1-Jy BL Lacs. We find a very tight proportionality between nearly simultaneous radio and X-ray powers for the 1-Jy sources in which the X-ray band is dominated by inverse Compton emission, which points to a strong link between X-ray and radio emission components in these objects., 13 pages, 6 figures. Accepted for publication in MNRAS. Postscript file also available at http://www.stsci.edu/~padovani/xrayspectra_papers.html

We report the discovery of extended X-ray emission around the powerful high-redshift quasar GB 1508+5714 at z=4.3, revealed in a long Chandra ACIS observation. The emission feature is 3-4 arcsec away from the quasar core, which corresponds to a projected distance of about 25 kpc. The X-ray spectrum is best fitted with a power law of photon index 1.92+-0.35 (90 per cent c.l.). The X-ray flux and luminosity reach 9.2x10^-15 ergs (0.5-8 keV) and 1.6x10^45 erg s^-1 (2.7-42.4 keV rest frame, Omega_Lambda=0.73, Omega_m=0.27, H_0=71 km s^-1 Mpc^-1), which is about 2 percent of the total X-ray emission of the quasar. We interpret the X-ray emission as inverse Compton scattering of Cosmic Microwave Background photons. The scattering relativistic electron population could either be a quasi-static diffuse cloud fed by the jet, or an outer extension of the jet with a high bulk Lorentz factor. We argue that the lack of an obvious detection of radio emission from the extended component could be a consequence of Compton losses on the electron population, or of a low magnetic field. Extended X-ray emission produced by inverse Compton scattering may be common around high redshift radio galaxies and quasars, demonstrating that significant power is injected into their surrounding by powerful jets., Comment: 5 pages, 3 Figures, accepted by MNRAS, no changes with respect to the earlier version

We present the analysis and results from a series of XMM-Newton observations of the distant blazar PMN J0525-3343, at a redshift of 4.4. The X-ray spectrum shows a spectral flattening below ~1 keV confirming earlier results from ASCA and BeppoSAX. The spectrum is well fitted by a power-law continuum with either a spectral break or absorption; no sharp features are apparent in the spectrum. No variability is seen in the individual lightcurves although there is evidence of small longer term variations (~ months in the blazar frame) in both flux and (0.1-2.4)/(2-10) keV flux ratio. Very low levels of optical-UV extinction and the lack of any evidence of a Lyman-limit system at the quasar redshift rule out neutral absorption and we argue that the most plausible explanation is the presence of a warm, ionized absorber. Very strong C IV absorption in the optical spectrum already implies the presence of highly ionized material along the line of sight. Warm absorber models using the photoionization code CLOUDY are consistent with both the X-ray and C IV data, yielding a column density \~10^(22.3-22.5) cm^-2 and ionization parameter ~10^(0.8-1.2) erg cm s^-1., Comment: 6 pages, 5 figures, MNRAS accepted. Minor changes to discussion/conclusions to emphasize lack of Ly-limit system evidence. Replacement of figure 3. Some minor spelling corrections

We present ISO PHOT-S spectra of four galaxies known or suspected to host a central AGN, selected from the initial Iras/Rosat sample of Boller et al. (1992). Two of them had no obvious Seyfert features in their previous optical spectra: IRAS 14201+2956, and IRAS 21582+1018. The latter was bright enough to also allow SWS observations around selected neon lines, to establish its excitation. While both PHOT-S spectra are characteristic of starburst-dominated galaxies, the neon line ratios in IRAS 21582+1018 indicate the presence of a hard excitation source. New, high-resolution, optical spectra show only a weak, broad component around Halpha, classifying now these two objects as Sey 1.9 galaxies. The two other galaxies observed are the NLS1 galaxies Mrk 359 and Mrk 1388. Their ISO spectra however do not reveal the typical, strong PAH features found in the starburst galaxies and are more like those of standard Seyferts. These results show therefore that, although IR observations were expected to be able to always reveal the presence of an active nucleus by piercing through the central obscuration, the result may be ambiguous: the broad band IR energy distribution can still be dominated by starburts located in a circumnuclear region, and the AGN appear only in specific observations (high-excitation lines in the IR, or optical spectra with better quality than classification spectra). The obscuration needs however to be patchy rather than complete, to explain the detection of the high-excitation lines or broad Balmer wings. Only high-energy observations can then establish the strength of the central AGN and the amount of extinction with certainty., 8 pages, 8 figures, accepted in Astronomy & Astrophysics

We report the discovery of a new ultraluminous X-ray source (ULX) in the nearby galaxy NGC 4244 from Chandra archival data. The source, 1WGAJ1216.9+3743, is one of the least luminous and softest ULXs discovered so far. Its X-ray spectrum is the best available for a representative of the soft ULXs, a class of sources recently discovered by Chandra and XMM-Newton. 1WGAJ1216.9+3742 appears point-like in the Chandra image and has a complex spectral shape: a multicolor disk model, suitable for brighter ULXs, is inadequate for this source. 1WGAJ1216.9+3743 spectrum is heavily absorbed (NH ~ (1-4)E21 cm^-2),and very soft. The best-fit power-law model gives a photon index ~ 5 and implies a luminosity L(0.5-10 keV) ~ 1E39 erg/s. A comparison with previous detections shows that, despite the variability displayed by the source during the Chandra observation, 1WGAJ1216.9+3743 count rate, spectral shape and absorption are practically unchange over a 9-year period. We performed also deep optical imaging of the field containing the X-ray source, and found a possible R ~ 23.7 counterpart., 7 pages, 3 figures, emulateapj5 style, accepted version, scheduled for ApJ v582 n2 January 10, 2003 issue

We present the first results from 60ks of observations of Arp 220 using the ACIS-S instrument on Chandra. We report the detection of several sources near the galaxy's nucleus, including a point source with a hard spectrum that is coincident with the western radio nucleus B. This point source is mildly absorbed (N_H ~ 3 x 10^22 cm^-2) and has an estimated luminosity of 4 x 10^40 erg/s. In addition, a fainter source may coincide with the eastern nucleus A. Extended hard X-ray emission in the vicinity raises the total estimated nuclear 2-10 keV X-ray luminosity to 1.2 x 10^41 erg/s, but we cannot rule out a hidden AGN behind columns exceeding 5 x 10^24 cm^-2. We also detect a peak of soft X-ray emission to the west of the nucleus, and a hard point source 2.5 kpc from the nucleus with a luminosity of 6 x 10^39 erg/s., Accepted for publication in ApJ

We have identified a population of blank field sources (or `blanks') among the ROSAT bright unidentified X-ray sources with faint optical counterparts. The extreme X-ray over optical flux ratio of blanks is not compatible with the main classes of X-ray emitters except for extreme BL Lacertae objects. From the analysis of ROSAT archival data we found no indication of variability and evidence for only three sources, out of 16, needing absorption in excess of the Galactic value. We also found evidence for an extended nature for only one of the 5 blanks with a serendipitous HRI detection; this source (1WGAJ1226.9+3332) was confirmed as a z=0.89 cluster of galaxies. Palomar images reveal the presence of a red (O-E~2) counterpart in the X-ray error circle for 6 blanks. The identification process brought to the discovery of another high z cluster of galaxies, one (possibly extreme) BL Lac, two ultraluminous X-ray sources in nearby galaxies and two apparently normal type1 AGNs. These AGNs, together with 4 more AGN-like objects seem to form a well defined group: they present unabsorbed X-ray spectra but red Palomar counterparts. We discuss the possible explanations for the discrepancy between the X-ray and optical data, among which: a suppressed big blue bump emission, an extreme dust to gas (~40-60 the Galactic ratio), a high redshift (z>3.5) QSO nature, an atypical dust grain size distribution and a dusty warm absorber. These AGN-like blanks seem to be the bright (and easier to study) analogs of the sources which are found in deep Chandra observations. Three more blanks have a still unknown nature., 23 pages, 8 figures, accepted by ApJ main journal

We present the BeppoSAX observations of 3C 279 performed in 1997 January 13-23 simultaneously with gamma-ray pointings with EGRET on board CGRO and optical observations at the Torino Observatory (1997 January 11-18). ISO data close to this epoch are also available. We compare the derived Spectral Energy Distribution with those obtained at all other epochs with adequate multiwavelength coverage. Simple spectral models fitted to the multiepoch SEDs suggest that changes in intensity can be ascribed mainly to the variation of the bulk Lorentz factor of the plasma in the jet., 34 pages (draft format), 6 figures, Accepted for publication in ApJ

High resolution X-ray and low frequency radio imaging now allow us to examine in detail the interaction and physical properties of the radio source 3C84 and the surrounding thermal gas. The radiative and dynamical properties of the inner X-ray holes, which coincide with the radio lobes, indicate that the ratio of the energy factor k and filling factor f is in the range 180, Comment: 7 pages, accepted for publication in MNRAS

We present BeppoSAX LECS, MECS, and PDS spectra of eleven X-ray selected BL Lacertae objects. Combining these sources with the ones presented elsewhere we have a sample of 21 BL Lacs from the Einstein Medium Sensitivity and Einstein Slew Survey. The sample shows strong correlations of several physical parameters with the peak frequency of the synchrotron branch of the spectral energy distribution. In particular the peak frequency is correlated to the X-ray spectral shape: objects with the peak near to the X-ray band show harder and straighter X-ray spectra than those of the low energy peaked sources. This work shows that the recently proposed unification scenario for different types of blazars can hold also within the class of high frequency peaked BL Lac objects., Comment: 14 pages, 9 figures, accepted by A&A

A follow-up X-ray study was made of the west lobe of the radio galaxy Fornax A (NGC 1316) that was based on new ASCA observations made in 1997 for 98 ks and that incorporated the previous observation in 1994 for 39 ks. The 0.7-10 keV spectrum of the emission can be described by a power law with an energy index of 0.74 +/- 0.10 which agrees with the synchrotron radio index of 0.9 +/- 0.2. Therefore, the X-rays are reconfirmed to arise via the inverse Compton scattering of the cosmic microwave photons, as Kaneda et al. and Feigelson et al. concluded. The surface brightness of the inverse Compton X-rays exhibits a relatively flat distribution over the west lobe, indicative of an approximately spherical emissivity distribution with a radius of similar to 11 ' (75 kpc). In contrast, the 1.4 GHz radio image by Ekers et al. exhibits a rim-brightened surface brightness, consistent with a shell-like emissivity distribution whose inner and outer boundaries are 4 ' and 11 ', respectively. These morphological differences between radio and X-rays suggest that the relativistic electrons are distributed homogeneously over the lobe volume, whereas the magnetic field is amplified toward the lobe rim region., arXiv:astro-ph/0010503v1http://arxiv.org/abs/astro-ph/0010503

We present multiwaveband photometric and optical spectropolarimetric observations of the R=15.9 narrow emission line galaxy R117_A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from McHardy et al 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infra-red and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K=20.8 within the X-ray errorbox and so conclude that R117_A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117_A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117_A., 8 pages, 9 figures, MNRAS accepted

Mrk 421 was observed for about 2 days with BeppoSAX in 1998 April as part of a worldwide multiwavelength campaign. A large, well-defined flare was observed in X-rays. The same flare was observed simultaneously at TeV energies by the Whipple Observatory gamma-ray telescope. These data provide (1) the first evidence that the X-ray and TeV intensities are well correlated on timescales of hours and (2) the first exactly simultaneous X-ray and TeV spectra. The results imply that the X-ray and TeV photons derive from the same region and from the same population of relativistic electrons. 'The physical parameters deduced from a homogeneous synchrotron self-Compton model for the spectral energy distribution yield electron cooling times close to the observed variability timescales.

A large sample of X-ray-emitting clusters of galaxies is considered in the context of Milgrom's modified Newtonian dynamics (MOND). The virial discrepancy (the ratio of the dynamical to observable mass) is reduced, on average, from a factor of 4 in the Newtonian analysis to a factor of 2 in the MOND analysis. The reason for this modest reduction is that the internal accelerations in the inner 1 Mpc of the sample clusters are not very small in terms of the critical MOND acceleration: a approximate to 0.5a(0). Although the uncertainty is large, there remains a discrepancy that is not yet accounted for by the directly observed matter in galaxies and diffuse gas. This may be consistent with the fact that modified dynamics cannot resolve the strong lensing discrepancy in those clusters where this phenomenon occurs. If MOND is correct, then the implication is that the budget of possibly detectable matter is not yet complete, particularly in the central regions of rich clusters.