Background Active Galactic Nuclei Research Articles

The Lack of Halo Ultraluminous X-Ray Sources

The premise that Ultraluminous X-ray sources (ULXs) exist beyond the optical extent of nearby galaxies is investigated. A published catalog containing 41 ULX candidates located between 1 and ~3 times the standard D_{25} isophotal radius of their putative host galaxies is examined. Twenty-one of these sources have spectroscopically-confirmed distances. All 21 are background objects giving a 95% probability that at least 37 of the 41 candidates are background sources. Thirty-nine of the 41 sources have X-ray-to-optical flux ratios, -1.6<log(F_{X}/F_{O})<+1.3, consistent with those of background active galactic nuclei. (The remaining two are not detected in optical to a weak limit of m_{B}~21.5$ mag corresponding to log(F_{X}/F_{O})\gtrsim 1.6.) The uniform spatial distribution of the sample is also consistent with a background population. This evidence suggests that ULXs rarely, if at all, exist beyond the distribution of luminous matter in nearby galaxies and, as a consequence, there is no correlation between the population of ULXs and halo objects such as old globular clusters or Pop~III remnants.

A Chandra X-ray observation of the globular cluster Terzan 1

We present a ∼19-ks Chandra Advanced CCD Imaging Spectrometer (ACIS)-S observation of the globular cluster Terzan 1. 14 sources are detected within 1.4 arcmin of the cluster centre with two of these sources predicted to be not associated with the cluster (background active galactic nuclei or foreground objects). The neutron star X-ray transient, X1732−304, has previously been observed in outburst within this globular cluster with the outburst seen to last for at least 12 yr. Here, we find four sources that are consistent with the ROSAT position for this transient, but none of the sources are fully consistent with the position of a radio source detected with the Very Large Array that is likely associated with the transient. The most likely candidate for the quiescent counterpart of the transient has a relatively soft spectrum and an unabsorbed 0.5‐10 keV luminosity of 2.6 × 10 32 erg s −1 , quite typical of other quiescent neutron stars. Assuming standard core cooling, from the quiescent flux of this source we predict long (>400 yr) quiescent episodes to allow the neutron star to cool. Alternatively, enhanced core cooling processes are needed to cool down the core. However, if we do not detect the quiescent counterpart of the transient this gives an unabsorbed 0.5‐10 keV luminosity upper limit of 8 × 10 31 erg s −1 . We also discuss other X-ray sources within the globular cluster. From the estimated stellar encounter rate of this cluster we find that the number of sources we detect is significantly higher than expected by the relationship of Pooley et al.

Keck observations of ROSAT ultra-luminous X-ray source candidates

We present results of optical follow-up observations of candidate ultra-luminous X-ray sources (ULXs). Using Keck optical spectroscopy, 17 of the candidates from the Colbert and Ptak [Colbert, E.J.M., Ptak, A.F. A catalog of candidate intermediate-luminosity X-ray objects. ApJS 143, 25–45, 2002] catalog have been identified; this is one of the largest sets of optical identifications of such objects thus far. Fifteen are background active galactic nuclei (AGN); two are foreground stars in our Galaxy. These findings are consistent with background and foreground object expectations, as derived from log N–log S relations. Also, the results are briefly discussed in terms of the spiral-galaxy/ULX connection.

Cataclysmic variables from a ROSAT/2MASS selection — I. Four new intermediate polars

We report the first results from a new search for cataclysmic variables (CVs) using a combined X-ray (ROSAT)/infrared (2MASS) target selection that discriminates against background active galactic nuclei. Identification spectra were obtained at the Isaac Newton Telescope for a total of 174 targets, leading to the discovery of 12 new CVs. Initially devised to find short-period low-mass-transfer CVs, this selection scheme has been very successful in identifying new intermediate polars. Photometric and spectroscopic follow-up observations identify four of the new CVs as intermediate polars: 1RXS J063631.9+353537 P(orb)similar or equal to 201 min, P-spin= 1008.3408 s or 930.5829 s), 1RXS J070407.9+262501 (P(orb)similar or equal to 250 min, P-spin= 480.708 s) 1RXS J173021.5-055933 (P-orb= 925.27 min, P-spin= 128.0 s), and 1RXS J180340.0+401214 (P-orb= 160.21 min, P-spin= 1520.51 s). RX J1730, also a moderately bright hard X-ray source in the INTEGRAL/IBIS Galactic plane survey, resembles the enigmatic AE Aqr. It is likely that its white dwarf is not rotating at the spin equilibrium period, and the system may represent a short-lived phase in CV evolution.

AChandraObservation of the Nearby Lenticular Galaxy NGC 5102: Where Are the X‐Ray Binaries?

We present results from a 34 ks Chandra ACIS-S observation of the low-mass X-ray binary (LMXB) population and the hot interstellar medium (ISM) in the nearby (d = 3.1 Mpc) lenticular galaxy NGC 5102, previously shown to have an unusually low X-ray luminosity. We detect 11 X-ray point sources within the D25 optical boundary of the galaxy (93% of the light), one-third to one-half of which are likely to be background active galactic nuclei (AGNs). One of the X-ray sources is coincident with the optical nucleus and may be a low-luminosity AGN. Only two sources with an X-ray luminosity greater than 1037 ergs s-1 in the 0.5-5.0 keV band were detected, one of which is statistically likely to be a background AGN. We expected to detect seven or five such luminous sources if the X-ray binary (XRB) population scales linearly with the B-band or J-band magnitudes, respectively, of the host galaxy. By this measure, NGC 5102 has an unusually low number of XRBs. The deficit of LMXBs is even more striking, because some of these sources may in fact be high-mass X-ray binaries (HMXBs). NGC 5102 is unusually blue for its morphological type and has undergone at least two recent bursts of star formation only ~1.5 × 107 and ~3 × 108 yr ago. We present the results of optical/UV spectral synthesis analysis and demonstrate that a significant fraction (>50%) of the stars in this galaxy are comparatively young (<3 × 109 yr old). We discuss the relationship between the XRB population, the globular cluster (GC) population, and the relative youth of the majority of stars in this galaxy. If the lack of X-ray binaries is related to the relative youth of most of the stars, this would support models of LMXB formation and evolution that require wide binaries to shed angular momentum on a timescale of Gyr. We have also analyzed archival Hubble Space Telescope (HST) images of NGC 5102 and find that it has an unusually low specific frequency of GCs (SN ~ 0.4). The lack of LMXBs could also be explained by the small number of GCs. We have also detected diffuse X-ray emission in the central ~1 kpc of the galaxy with an X-ray luminosity of 4.1 × 1037 ergs s-1 in the 0.1-2.0 keV band. This hot gas is most likely a superbubble created by multiple supernovae of massive stars born during the most recent star burst and is driving the shock into the ISM, which was inferred from previous [O III] λ5007 and Hα observations.

An XMM-Newton survey of M 31

In an analysis of XMM-Newton archival observations of the bright Local Group spiral galaxy M 31 we study the population of X-ray sources (X-ray binaries, supernova remnants) down to a 0.2–4.5 keV luminosity of 4.4 . EPIC hardness ratios and optical and radio information are used to distinguish between different source classes. The survey detects 856 sources in an area of 1.24 square degrees. We correlate our sources with earlier M 31 X-ray catalogues and use information from optical, infra-red and radio wavelengths. As sources within M 31 we detect 21 supernova remnants (SNR) and 23 SNR candidates, 18 supersoft source (SSS) candidates, 7 X-ray binaries (XRBs) and 9 XRB candidates, as well as 27 globular cluster sources (GlC) and 10 GlC candidates, which most likely are low mass XRBs within the GlC. Comparison to earlier X-ray surveys reveal transients not detected with XMM-Newton, which add to the number of M 31 XRBs. There are 567 sources classified as hard, which may either be XRBs or Crab-like SNRs in M 31 or background AGN. The number of 44 SNRs and candidates more than doubles the X-ray-detected SNRs. 22 sources are new SNR candidates in M 31 based on X-ray selection criteria. Another SNR candidate may be the first plerion detected outside the Galaxy and the Magellanic Clouds. On the other hand, six sources are foreground stars and 90 foreground star candidates, one is a BL Lac-type active galactic nucleus (AGN) and 36 are AGN candidates, one source coincides with the Local Group galaxy M 32, one with a background galaxy cluster (GCl) and another is a GCl candidate, all sources not connected to M 31.

A Multiwavelength Study of the X‐Ray Sources in NGC 5018

The E3 giant elliptical galaxy NGC 5018 was observed with the Chandra X-Ray Observatory's Advanced CCD Imaging Spectrometer for 30 ks on 2001 April 14. Results of the analysis of these X-ray data as well as of complementary optical, infrared, and radio data are reported. Seven X-ray point sources, including the nucleus, were detected. If they are intrinsic to NGC 5018, then all six nonnuclear sources have luminosities exceeding 1039 ergs s-1 in the 0.5-8.0 keV energy band, placing them in the class of ultraluminous X-ray sources. Comparison of X-ray source positions to archival Hubble Space Telescope Wide Field Planetary Camera 2 (HST WFPC2) images reveals that four of the six nonnuclear sources are spatially coincident with bright, MV -8.6 mag objects. These four objects have optical magnitudes and (V - I) colors consistent with globular clusters in NGC 5018, but they also have X-ray-to-optical flux ratios consistent with background active galactic nuclei (AGNs). Strong, unpolarized radio emission has been detected from one of the optically bright counterparts. Another optically bright counterpart was observed to vary by ~1 mag in optical observations taken on 1997 July 28 and 1999 February 4. Extended X-ray emission is detected within an ~15'' radius of the galaxy center at a luminosity of ~1040 ergs s-1 in the X-ray band. Its thermal X-ray spectrum (kT ~ 0.4 keV) and its spatial coincidence with strong Hα emission are consistent with a hot gas origin. The nucleus itself may be a weak X-ray source, LX 3.5 × 1039 ergs s-1, which displays a radio spectrum typical of AGNs.

AnXMM-Newtonview of M101 - II. Global X-ray source properties

We present the global X-ray properties of the point source population in the grand-design spiral galaxy M101, as seen with XMM‐Newton. 108 X-ray sources are detected within the D25 ellipse of M101, of which ∼24 are estimated to be background galaxies. Multiwavelength cross-correlations show that 20 sources are coincident with H II regions and/or supernova remnants (SNRs), seven have identified/candidate background galaxy counterparts, six are coincident with foreground stars and one has a radio counterpart. While the spectral and timing properties of the brightest sources were presented by Jenkins et al., here we apply an X-ray colour classification scheme to split the entire source population into different types, i.e. X-ray binaries (XRBs), SNRs, absorbed sources, background sources and supersoft sources (SSSs). Approximately 60 per cent of the population can be classified as XRBs, although there is source contamination from background active galactic nuclei (AGN) in this category as they have similar spectral shapes in the X-ray regime. 15 sources have X-ray colours consistent with SNRs, three of which correlate with known SNR/H II radio sources. Another two are promising new candidates for SNRs, one is unidentified, and the remainder are a mixture of foreground stars, bright soft XRBs and AGN candidates. We also detect 14 candidate SSSs, with significant detections in the softest X-ray band (0.3‐1 keV) only. 16 sources display short-term variability during the XMM‐Newton observation, twelve of which fall into the XRB category, giving additional evidence of their accreting nature. Using archival Chandra and ROSAT High Resolution Imager data, we find that ∼40 per cent of the XMM sources show long-term variability over a baseline of up to ∼10 yr, and eight sources display potential transient behaviour between observations. Sources with significant flux variations between the XMM and Chandra observations show a mixture of softening and hardening with increasing luminosity. The spectral and timing properties of the sources coincident with M101 confirm that its X-ray source population is dominated by accreting XRBs.

A catalogue of ultraluminous X-ray sources in external galaxies

We present a catalogue of ultraluminous X-ray sources (ULXs) in external galaxies. The aim of this catalogue is to provide easy access to the properties of ULXs, their possible counterparts at other wavelengths ( optical, IR, and radio), and their host galaxies. The catalogue contains 229 ULXs reported in the literature until April 2004. Most ULXs are stellar-mass-black hole X-ray binaries, but it is not excluded that some ULXs could be intermediate-mass black holes. A small fraction of the candidate ULXs may be background Active Galactic Nuclei (AGN) and Supernova Remnants (SNRs). ULXs with luminosity above 10(40) ergs s(-1) are found in both starburst galaxies and in the halos of early-type galaxies.

ChandraObservations of Diffuse Gas and Luminous X‐Ray Sources around the X‐Ray–bright Elliptical Galaxy NGC 1600

We observed the X-ray-bright E3 galaxy NGC 1600 and nearby members of the NGC 1600 group with the Chandra X-Ray Observatory ACIS-S3 to study their X-ray properties. Unresolved emission dominates the observation; however, we resolved some of the emission into 71 sources, most of which are low-mass X-ray binaries associated with NGC 1600. Twenty-one of the sources have LX > 2 ? 1039 ergs s-1 (0.3-10.0 keV; assuming they are at the distance of NGC 1600), marking them as ultraluminous X-ray point source (ULX) candidates; we expect that only 11 ? 2 are unrelated foreground/background sources. NGC 1600 may have the largest number of ULX candidates in an early-type galaxy to date; however, cosmic variance in the number of background active galactic nuclei cannot be ruled out. The spectrum and luminosity function (LF) of the resolved sources are more consistent with sources found in other early-type galaxies than with sources found in star-forming regions of galaxies. The source LF and the spectrum of the unresolved emission both indicate that there are a large number of unresolved point sources. We propose that these sources are associated with globular clusters (GCs) and that NGC 1600 has a large GC specific frequency. Observations of the GC population in NGC 1600 would be very useful for testing this prediction. Approximately 50%-75% of the unresolved flux comes from diffuse gaseous emission. The spectral fits, hardness ratios, and X-ray surface brightness profile all point to two gas components. We interpret the soft inner component (a 25'', kT ~ 0.85 keV) as the interstellar medium of NGC 1600 and the hotter outer component (a 25'', kT ~ 1.5 keV) as the intragroup medium of the NGC 1600 group. The X-ray image shows several interesting structures. First, there is a central region of excess emission that is roughly cospatial with H? and dust filaments immediately west of the center of NGC 1600. There appear to be holes in the X-ray emission to the north and south of the galaxy center that are roughly coincident with the lobes of the NGC 1600 radio source. On larger scales, there is excess emission to the northeast, which we suggest may indicate the center of the group potential. The group galaxy NGC 1603 shows a tail of X-ray emission to its west that is probably due to ram pressure stripping.

An X‐Ray Search for Compact Central Sources in Supernova Remnants. I. SNR s G093.3+6.9, G315.4−2.3, G084.2+0.8, and G127.1+0.5

Most astronomers now accept that stars more massive than about 9?M? explode as supernovae and leave stellar remnants, either neutron stars or black holes, with neutron stars being more prevalent. Recent modeling of the explosions suggests a significant diversity in the key natal properties?rotation rate, velocity, and magnetic field strength?of the resulting neutron stars that account for the association of active radio pulsars, pulsar wind nebulae, and magnetars with supernova remnants (SNRs). The discovery of a central X-ray source in Cas?A, the youngest known Galactic SNR, dramatized the expected diversity. However, less than half of the SNRs within 5 kpc have identified central sources, and only three are identified as the remnants of Type?Ia SNe. Here we report a systematic effort to search for compact central sources in the remaining 23 SNRs of this distance limited sample. Our search was inspired, on empirical considerations, by the enigmatic faint X-ray source in Cas?A; motivated, on theoretical grounds, by the expectation that young neutron stars emit cooling X-ray emission; and made possible by the superb angular resolution offered by the Chandra X-ray mission and the sensitivity of the XMM-Newton mission.?????In this first paper we report Chandra observations of four SNRs (G093.3+6.9, G315.4-2.3, G084.2+0.8, and G127.1+0.5). We have undertaken a systematic optical/IR identification program of the X-ray sources detected in the field of each SNR. Foreground (flare stars, active stars) and background (active galactic nuclei) sources have identifiable IR/optical counterparts. In contrast, the counterparts of neutron stars (or black holes) are expected to be very faint. We are able to account for all the well-detected X-ray sources and thus able to state with some confidence that there are no associated central sources down to a level of one-tenth of that of the Cas?A central source, LX 1031 ergs s-1. We compare our limits with cooling curves for neutron stars and find that any putative neutron stars in these SNRs must be cooling faster than expected for traditional 1.35 M? neutron stars and that any putative pulsar must have low spin-down luminosities 1034 ergs s-1. However, our limits are unable to constrain the presence or absence of more unusual options, such as relatively more massive neutron stars with M 1.45 M?, neutron stars with exotic interiors, or quiescent black holes. In subsequent papers, we will report on the X-ray and optical/IR observations of the remaining members of the 5 kpc sample.

The Ultraluminous X‐Ray Source Population from the Chandra Archive of Galaxies

One hundred fifty-four discrete non-nuclear Ultra-Luminous X-ray (ULX) sources, with spectroscopically-determined intrinsic X-ray luminosities greater than 1 e39 ergs/s, are identified in 82 galaxies observed with Chandra's Advanced CCD Imaging Spectrometer. Source positions, X-ray luminosities, and spectral and timing characteristics are tabulated. Statistical comparisons between these X-ray properties and those of the weaker discrete sources in the same fields (mainly neutron star and stellar-mass black hole binaries) are made. Sources above approximately le38 ergs per second display similar spatial, spectral, color, and variability distributions. In particular, there is no compelling evidence in the sample for a new and distinct class of X-ray object such as the intermediate-mass black holes. 83% of ULX candidates have spectra that can be described as absorbed power laws with index = 1.74 and column density = 2.24e21 l per square centimeter, or approximately 5 times the average Galactic column. About 20% of the ULX's have much steeper indices indicative of a soft, and likely thermal, spectrum. The locations of ULXs in their host galaxies are strongly peaked towards their galaxy centers. The deprojected radial distribution of the ULX candidates is somewhat steeper than an exponential disk, indistinguishable from that of the weaker sources. About 5--15% of ULX candidates are variable during the Chandra observations (which average 39.5 ks). Comparison of the cumulative X-ray luminosity functions of the ULXs to Chandra Deep Field results suggests approximately 25% of the sources may be background objects including 14% of the ULX candidates in the sample of spiral galaxies and 44% of those in elliptical galaxies implying the elliptical galaxy ULX population is severely compromised by background active galactic nuclei. Correlations with host galaxy properties confirm the number and total X-ray luminosity of the ULXs are associated with recent star formation and with galaxy merging and interactions. The preponderance of ULXs in star-forming galaxies as well as their similarities to less-luminous sources suggest they originate in a young but short-lived population such as the high-mass X-ray binaries with a smaller contribution (based on spectral slope) from recent supernovae. The number of ULXs in elliptical galaxies scales with host galaxy mass and can be explained most simply as the high-luminosity end of the low-mass X-ray binary population.

The Low‐Mass X‐Ray Binary–Globular Cluster Connection. II. NGC 4472 X‐Ray Source Properties and Source Catalogs

We present the results of a Chandra/Hubble Space Telescope (HST) study of the point sources of the Virgo Cluster giant elliptical galaxy NGC 4472. We identify 144 X-ray point sources outside the nuclear region, 72 of which are located within the HST fields. The optical data show 1102 sources, of which 829 have colors consistent with being globular clusters (with only four in the restricted central 10'' region). Thirty matches are found between the two lists?these are likely to be low-mass X-ray binaries (LMXBs) associated with globular clusters, while 42 of the X-ray sources have no optical counterparts to V 25 and I 24, indicating that they are likely to be predominantly LMXBs in the field star population with a small amount of possible contamination from background active galactic nuclei. Thus approximately 40% of the X-ray sources are in globular clusters and ~4% of the globular clusters contain X-ray sources. There is suggestive evidence that the X-ray sources located in blue globular clusters may have harder X-ray spectra than those located in red globular clusters. No statistically significant differences are found between the X-ray properties of the field sources and the X-ray properties of the sources located in globular clusters. This study, along with our previous result from Paper I in this series on the similarity of the spatial profile of the field LMXBs, globular cluster LMXBs, and the globular clusters themselves, suggests that a significant fraction of the observed LMXBs in the field may be created in a globular cluster then ejected into the field by stellar interactions; however, by comparing the results for NGC 4472 with those in several other galaxies, we find tentative evidence for a correlation between the globular cluster specific frequency and the fraction of LMXBs in globular clusters, a correlation that would be most easily explained if some of the field sources were generated in situ. We show that isolated accreting very massive black holes are unlikely to be observable with current X-ray instrumentation and that these sources hence do not contaminate the LMXB population. We discuss the possibility that several point sources near the nucleus and aligned perpendicularly to the radio jet may indicate the presence of a disk wind responsible for the low radiative efficiency observed in the nucleus of this source.

An [ITAL]XMM-Newton[/ITAL] Observation of 4U 1755−33 in Quiescence: Evidence of a Fossil X-Ray Jet

We report an XMM-Newton observation of the Low mass X-ray Binary (LMXB) and black hole candidate 4U1755-33. This source had been a bright persistent source for at least 25 yrs, but in 1995 entered an extended quiescent phase. 4U1755-33 was not detected with an upper limit to the 2-10 keV luminosity of 5 x 10(exp 31) d(sup 2) (sub 4kpc) ergs per second (where d(sub 4kpc) is the distance in units of 4 kpc) - consistent with the luminosity of other black hole candidates in a quiescent state. An unexpected result is the discovery of a narrow 7 arc min long X-ray jet centered on the position of 4Ul755-33. The spectrum of the jet is similar to that of jets observed from other galactic and extragalactic sources, and may have been ejected from 4Ul755-33 when it was bright. Jets are a feature of accreting black holes, and the detection of a fossil jet provides additional evidence supporting the black hole candidacy of 4U1755-33. The spectral properties of three bright serendipitous sources in the field are reported and it is suggested these are background active galactic nuclei sources.

ChandraObservations of the Pleiades Open Cluster: X‐Ray Emission from Late B‐ to Early F‐Type Binaries

We present the analysis of a 38.4 and 23.6 ks observation of the core of the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory detected 99 X-ray sources in a 17' × 17' region, including 18 of 23 Pleiades members. Five candidate Pleiades members have also been detected, confirming their cluster membership. Fifty-seven sources have no optical or near-infrared counterparts to limiting magnitudes V = 22.5 and J = 14.5. The unidentified X-ray sources are probably background active galactic nuclei and not stars. The Chandra field of view contains seven intermediate-mass cluster members. Five of these, HII 980 (B6+G), HII 956 (A7+F6), HII 1284 (A9+K), HII 1338 (F3+F6), and HII 1122 (F4+K), are detected in this study. All but HII 1284 have high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties comparable to nonflaring K-type stars. Since all five stars are visual or spectroscopic binaries with X-ray properties similar to F-G stars, the late-type binary companions are probably producing the observed coronal X-ray emission. Strengthening this conclusion is the nondetection by Chandra of two A stars, HII 1362 (A7, no known companion) and HII 1375 (A0+A SB) with X-ray luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7 stars with cooler companions. Despite the low number statistics, the Chandra data appear to confirm the expectation that late B and A stars are not strong intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual increase in coronal temperature with decreasing mass from F4 to K. M stars appear to have somewhat cooler coronae than active K stars.

ChandraObservations of Markarian 273: Unveiling the Central Active Galactic Nucleus and the Extended Hot Gas Halo

We report X-ray observations of the field containing the ultraluminous IRAS galaxy Mrk 273. The data were obtained using the Advanced CCD Imaging Spectrometer S3 instrument on board Chandra. The high-resolution X-ray image for the first time reveals a compact hard X-ray nucleus in Mrk 273. Its position is coincident with the northern nucleus identified in the optical, infrared, and radio and in molecular CO maps. Its X-ray energy distribution is well described by a heavily obscured active galactic nucleus (AGN) spectrum with an absorbed power law plus a narrow Fe Kα emission line at 6.4 keV. The neutral hydrogen column density is about 4 × 1023 cm-2, implying an absorption-corrected X-ray luminosity (0.1-10 keV) for the nucleus of LX ≈ 6.5 × 1043 ergs s-1 for H0 = 50 km s-1 Mpc-1. The X-ray properties therefore firmly establish the northern nucleus of Mrk 273 as a Seyfert 2 AGN. There are also bright soft X-ray clumps and diffuse soft X-ray emissions surrounding the central hard X-ray nucleus within the 10'' of the nuclear region. Its spectrum can be fitted by a MEKAL thermal model with a temperature of about 0.8 keV and high metallicity (Z ~ 1.5 Z☉) plus emission lines from α elements and ions. We find that a soft X-ray clump, about 4'' (projected separation of about 4 kpc) southwest of the northern hard X-ray nucleus, is coincident with a nebula with strong [O III] λ5007 emissions. Further outside the central region, the Chandra observations reveal a very extended hot halo in Mrk 273. The X-ray halo encompasses the entire optical tidal tail and plume, with a projected diameter of about 108 × 68 kpc. The total soft X-ray luminosity (0.1-2.4 keV) of the hot halo is LX ≈ 1.9 × 1041 ergs s-1, in the range of the soft X-ray luminosity of bright elliptical galaxies. The temperature of the hot gas is about 0.62 keV with a low metallicity (Z ~ 0.1 Z☉). We discuss the nature of the AGN in Mrk 273 and the implications of our results on the origin of X-ray halos in elliptical galaxies. We also discuss the properties of Mrk 273x, a background AGN at redshift z = 0.46 in the Mrk 273 field. The AGN has an X-ray luminosity of LX ≈ 2.43 × 1044 ergs s-1 in the 0.5-10 keV band. Its X-ray properties resemble those of Seyfert 1 galaxies, while its optical properties are similar to Seyfert 2 galaxies. Such mixed classifications in the optical and X-ray may be a challenge for the unification scheme of AGNs.

ChandraX‐Ray Sources in M101

A deep (98.2 ks) Chandra Cycle 1 observation has revealed a wealth of discrete X-ray sources as well as diffuse emission in the nearby face-on spiral galaxy M101. From this rich data set we have created a catalog of the 110 sources from the S3 chip detected with a significance of greater than 3 σ. This detection threshold corresponds to a flux of ~10-16 ergs cm-2 s-1 and a luminosity of ~1036 ergs s-1 for a distance to M101 of 7.2 Mpc. The sources display a distinct correlation with the spiral arms of M101 and include a variety of X-ray binaries, supersoft sources, supernova remnants, and other objects of which only ~27 are likely to be background sources. There are only a few sources in the interarm regions, and most of these have X-ray colors consistent with that of background active galactic nuclei. The derived log N- log S relation for the sources in M101 (background subtracted) has a slope of -0.80 ± 0.05 over the range of 1036-1038 ergs s-1. The nucleus is resolved into two nearly identical X-ray sources, each with a 0.5-2.0 keV flux of 4 × 1037 ergs s-1. One of these sources coincides with the optical nucleus, and the other coincides with a cluster of stars 110 pc to the south. The field includes 54 optically identified supernova remnants (SNRs), of which 12 are detected by Chandra. Two of the SNR sources are variable and hence must be compact objects. In total, eight of the X-ray sources show evidence for short-term temporal variation during this observation. Two of these variable sources are now brighter than the ROSAT detection threshold, but they were not detected in the previous ROSAT observations taken in 1992 and 1996. There are also two variable sources previously seen with ROSAT that apparently have faded below the Chandra detection threshold. The brightest source in the field shows extreme long-term and short-term temporal variability. At its peak brightness it has a super-Eddington luminosity greater than 1039 ergs s-1. There are 10 supersoft sources (SSSs) in the field, which can be divided into two distinct subclasses: the brighter class (three objects) has a luminosity of ~1038 ergs s-1 and a blackbody temperature of ~70 eV, whereas the other class (seven objects) is an order of magnitude fainter and has a blackbody temperature of only ~50 eV.

Anomalous Radio‐Wave Scattering from Interstellar Plasma Structures

This paper considers scattering screens that have arbitrary spatial variations of scattering strength transverse to the line of sight, including screens that are spatially well confined, such as disks and filaments. We calculate the scattered image of a point source and the observed pulse shape of a scattered impulse. The consequences of screen confinement include: (1) Source image shapes that are determined by the physical extent of the screen rather than by the shapes of much-smaller diffracting microirregularities. These include image elongations and orientations that are frequency dependent. (2) Variation with frequency of angular broadening that is much weaker than the trademark \nu^{-2} scaling law (for a cold, unmagnetized plasma), including frequency-independent cases; and (3) Similar departure of the pulse broadening time from the usually expected \nu^{-4} scaling law. We briefly discuss applications that include scattering of pulses from the Crab pulsar by filaments in the Crab Nebula; image asymmetries from Galactic scattering of the sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active galactic nuclei by intervening galaxies. We also address the consequences for inferences about the shape of the wavenumber spectrum of electron density irregularities, which depend on scaling laws for the image size and the pulse broadening. Future low-frequency (< 100 MHz) array observations will also be strongly affected by the Galactic structure of scattering material. Our formalism is derived in the context of radio scattering by plasma density fluctuations. It is also applicable to optical, UV and X-ray scattering by grains in the interstellar medium.

Further Investigation of the Time Delay, Magnification Ratios, and Variability in the Gravitational Lens 0218+357

High-precision VLA flux density measurements for the lensed images of 0218+357 yield a time delay of 10.1 days (95% confidence). This is consistent with independent measurements carried out at the same epoch by Biggs and colleagues in 1999, lending confidence to the robustness of the time delay measurement. However, since both measurements make use of the same features in the light curves, it is possible that the effects of unmodeled processes, such as scintillation or microlensing, are biasing both time delay measurements in the same way. Our time delay estimates result in confidence intervals that are somewhat larger than those of Biggs et al., probably because we adopt a more general model of the source variability, allowing for constant and variable components. When considered in relation to the lens mass model of Biggs et al., our best-fit time delay implies a Hubble constant of H0 = 71 km s-1 Mpc-1 for ?0 = 1 and ?0 = 0 (95% confidence; filled beam). This confidence interval for H0 does not reflect systematic error, which may be substantial, as a result of uncertainty in the position of the lens galaxy. We also measure the flux ratio of the variable components of 0218+357, a measurement of a small region that should more closely represent the true lens magnification ratio. We find ratios of 3.2 (95% confidence; 8 GHz) and 4.3 (15 GHz). Unlike the reported flux ratios on scales of 01, these ratios are not significantly different. We investigate the significance of apparent differences in the variability properties of the two images of the background active galactic nucleus. We conclude that the differences are not significant and that time series much longer than our 100 day time series will be required to investigate propagation effects in this way.

A Deep [ITAL]ROSAT[/ITAL] HRI Observation of NGC 1313

We describe a series of observations of NGC 1313 using the ROSAT HRI with a combined exposure time of 183.5 ks. The observations span an interval between 1992 and 1998; the purpose of observations since 1994 was to monitor the X-ray flux of SN 1978K, one of several luminous sources in the galaxy. No diffuse emission is detected in the galaxy to a level of ~1–2 × 1037 ergs s-1 arcmin-2. A total of eight sources are detected in the summed image within the D25 diameter of the galaxy. The luminosities of five of the eight range from ~6 × 1037 to ~6 × 1038 ergs s-1; these sources are most likely accreting X-ray binaries, similar to sources observed in M31 and M33. The remaining three sources all emit above 1039 ergs s-1. We present light curves of the five brightest sources. Variability is detected at the 99.9% level in four of these. We identify one of the sources as an NGC 1313 counterpart of a Galactic X-ray source. The light curve, though crudely sampled, most closely resembles that of a Galactic black hole candidate such as GX 339-4 but with considerably higher peak X-ray luminosity. An additional seven sources lie outside the D25 diameter and are either foreground stars or background active galactic nuclei.