Advanced CCD Imaging Spectrometer Research Articles

Monitoring the Evolution of the X‐Ray Remnant of SN 1987A

We report on the results of our monitoring program of the remnant of SN 1987A with the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Two new observations have been performed in AO2, bringing the total to four monitoring observations over the past 2 years. Over this time period, new techniques for correction of "charge transfer inefficiency" (CTI) and for use of charge spreading to provide angular resolution somewhat better than the pixel size of the CCD detector have become available at Pennsylvania State University. We have processed all four observations using subpixel resolution to obtain the highest possible angular resolution and using our CTI correction software to provide more reliable spectral analysis and flux estimations. The high angular resolution images indicate that the X-ray-bright knots are convincingly correlated with the optical spots, primarily at ≲1 keV, while higher energy photons are very well correlated with radio images. Our data also provide marginal evidence for radial expansion of the X-ray remnant at a rate of 5200 ± 2100 km s-1. The X-ray flux appears to linearly increase by ~60% over the 18 month period of these observations. The spectrum is dominated by broad complexes of atomic emission lines and can be fitted with a simple model of a plane-parallel shock with electron temperatures of kT ~ 2-4 keV and a postshock electron density of ne ~ 210-420 cm-3. The implied 0.5-10 keV band luminosity in 2001 April is ~1.3 × 1035 ergs s-1; as of that date, we still observe no direct evidence for the central point source, with an upper limit on the observed luminosity of LX ~ 5.5 × 1033 ergs s-1 in the 2-10 keV band.

Chandra observations of Tycho’s supernova remnant

We present a newChandra observation of Tycho’s supernova remnant with the Advanced CCD Imaging Spectrometer. Multicolor Xray imaging reveals new details of the outer shock and ejecta. At energies between 4 and 6 keV, the outline of the outer shock is clearly revealed in X-rays for the first time. The distribution of the emission from lines of Si and Fe are confirmed to have a different morphology from each other, and the Si ejecta are shown to extend to the blast shock at several locations. Characteristic spectra of the outer shock and ejecta are also presented.

X-Ray Properties of the Young Stellar and Substellar Objects in the IC 348 Cluster: The [ITAL]Chandra[/ITAL] View

We explore the X-ray properties of the young stellar and substellar objects in the open cluster IC 348 as seen in our deep Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer image. First, we give identifications of all X-ray sources and determine upper limits for the X-ray luminosities of the undetected cluster members. Then we analyze the X-ray spectra of the young stellar objects, deriving plasma temperatures between ~0.7 and ~3 keV for the T Tauri stars in IC 348 and higher temperatures, between ~3 and ~7 keV, for flaring sources and two embedded young stellar objects. We find several large X-ray flares, in some of which a clear hardening of the X-ray spectra during the flare peak is seen. Next we use the exceptional optical, infrared, and X-ray data set of this cluster to study various correlations and their implications, and to discuss new answers to some long-standing questions related to X-ray emission from young (sub)stellar objects. The X-ray luminosities of the young low-mass stars are strongly correlated to the stellar bolometric luminosities (LX ~ 10-4 × Lbol). Also, a good correlation between X-ray luminosity and stellar mass is found (LX ∝ M2). For the weak-line T Tauri stars we find a tight correlation between X-ray activity and chromospheric activity (LX ∝ L), supporting the hypothesis that the chromosphere is heated by X-rays from the overlying corona. The observed X-ray properties of the brown dwarfs (and brown dwarf candidates) are very similar to those of late-type stars; we explain this behavior as the consequence of the fact that very young substellar objects are still warm enough to maintain partially ionized atmospheres, which are capable of sustaining electrical currents, while in the cooler neutral atmospheres of L and T dwarfs such currents are shut off (hence no X-ray emission). Finally, we explore the difference between the X-ray luminosity functions of classical and weak-line T Tauri stars. We find that the classical T Tauri stars in IC 348 seem to be on average less X-ray luminous than the weak-line T Tauri stars. However, we suggest that this apparent difference is caused by a selection effect: there is a strong detections bias against those weak-line T Tauri stars that are optically faint and hence X-ray faint; the population of classical T Tauri stars, on the other hand, is essentially completely known because of its very prominent Hα emission. This conclusion is corroborated by another new result: when using a photometrically selected, magnitude-limited, complete sample of T Tauri stars and taking the K-L infrared excess as a tracer of circumstellar material, we find no evidence in IC 348 for a difference in X-ray properties of young stars with and without circumstellar matter, i.e., classical and naked T Tauri stars.

AChandraX‐Ray Study of Cygnus A. III. The Cluster of Galaxies

We present an analysis of the Chandra Advanced CCD Imaging Spectrometer (ACIS) observation of the intracluster gas associated with the cluster of galaxies surrounding Cygnus A. The dominant gaseous structure is a roughly elliptical (presumably prolate spheroidal in three dimensions) feature with semimajor axis ≃11 (≃100 kpc). This structure apparently represents intracluster gas that has been swept up and compressed by a cavity inflated in this gas by relativistic material that has passed through the ends of the radio jets. The X-ray-emitting gas shows this prolate spheroidal morphology to ≃12 (110 kpc) from the radio galaxy but is spherical on larger scales. The X-ray emission from the intracluster gas extends to at least 8' (≃720 kpc) from the radio galaxy, and a second, extended source of X-ray emission (probably associated with a second cluster of galaxies) is seen some 12' (≃1 Mpc) to the northwest of Cygnus A. The X-ray spectrum of the integrated intracluster gas imaged on the S3 chip (dimensions 8' × 8' = 720 × 720 kpc), excluding the contribution from the radio galaxy and other compact sources of X-ray emission, has a gas temperature, metallicity, and unabsorbed 2-10 keV rest-frame luminosity of 7.7 keV, 0.34 times solar, and 3.5 × 1044 ergs s-1, respectively.

ConstrainingH0fromChandraObservations of Q0957+561

We report the detection of the lens cluster of the gravitational lens (GL) system Q0957+561 from a deep observation with the Advanced CCD Imaging Spectrometer on-board the Chandra X-ray Observatory. Intracluster X-ray emission is found to be centered 4.3 +/- 1.3 arcsec east and 3.5(-0.6,+1.3) arcsec north of image B, nearer than previous estimates. Its spectrum can be modeled well with a thermal plasma model consistent with the emission originating from a cluster at a redshift of 0.36. Our best-fit estimates of the cluster temperature of T_e = 2.09(-0.54,+0.83) keV (90 percent confidence) and mass distribution of the cluster are used to derive the convergence parameter kappa, the ratio of the cluster surface mass density to the critical density required for lensing. We estimate the convergence parameter at the location of the lensed images A and B to be kappa_A = 0.22(+0.14,-0.07) and kappa_B = 0.21(+0.12,-0.07), respectively (90 percent confidence levels). The observed cluster center, mass distribution and convergence parameter kappa provide additional constraints to lens models of this system. Our new results break a mass-sheet degeneracy in GL models of this system and provide better constraints of ~ 29 percent (90 percent confidence levels) on the Hubble constant. We also present results from the detection of the most distant X-ray jet (z = 1.41) detected to date. The jet extends approximately 8 arcsec NE of image A and three knots are resolved along the X-ray jet with flux densities decreasing with distance from the core. The observed radio and optical flux densities of the knots are fitted well with a synchrotron model and the X-ray emission is modeled well with inverse Compton scattering of Cosmic Microwave Background photons by synchrotron-emitting electrons in the jet.

The Structure of the Oxygen-rich Supernova Remnant G292.0+1.8 from [ITAL]Chandra[/ITAL] X-Ray Images: Shocked Ejecta and Circumstellar Medium

We present results from the observation of the young Galactic supernova remnant (SNR) G292.0+1.8 with the Advanced CCD Imaging Spectrometer (ACIS) on board the {\it Chandra X-ray Observatory}. In the 0.3 $-$ 8 keV band, the high resolution ACIS images reveal a complex morphology consisting of knots and filaments, as well as the blast wave around the periphery of the SNR. We present equivalent width (EW) maps for the elemental species O, Ne, Mg, and Si, which allow us to identify regions of enhanced metallicity in the SNR. G292.0+1.8 is bright in O, Ne, and Si; weaker in S and Ar; with little Fe. The EW and broad-band images indicate that the metal-rich ejecta are distributed primarily around the periphery of the SNR. The central belt-like structure has normal solar-type composition, strongly suggesting that it is primarily emission from shocked circumstellar medium rather than metal-rich ejecta. We propose that the belt traces its origin to enhanced mass loss in the star's equatorial plane during the slow, red supergiant phase. We also identify thin filaments with normal composition, centered on and extending nearly continuously around the outer boundary of the SNR. These may originate in a shell caused by the stellar winds from the massive progenitor in the red/blue supergiant phases, over-run by the blast wave.

Discovery of 5.16 s pulsations from the isolated neutron star RBS 1223

The isolated neutron star candidate RBS 1223 was observed with the Advanced CCD Imaging Spectrometer aboard the Chandra observatory on 2000 June 24. A timing analysis of the data yielded a periodic modulation with a period s. Using ROSAT HRI archived observations we detected a period s and determined period derivative s . The detection of this period and indicates that RBS 1223 has a “characteristic” age of 6000-12 000 years and huge magnetic field at the surface () typical for anomalous pulsars (AXPs).

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.

AChandraX‐Ray Study of Cygnus A. II. The Nucleus

We report Chandra Advanced CCD Imaging Spectrometer and quasi-simultaneous Rossi X-Ray Timing Explorer (RXTE) observations of the nearby, powerful radio galaxy Cygnus A, with the present paper focusing on the properties of the active nucleus. In the Chandra observation, the hard (>a few keV) X-ray emission is spatially unresolved with a size 1'' (1.5 kpc, H0 = 50 km s-1 Mpc-1) and coincides with the radio and near-infrared nuclei. In contrast, the soft ( 1 ergs cm s-1 and the electron density ne 6 cm-3 given the observed distance of the soft X-ray emission from the nucleus. The electron column density inferred from the X-ray observations is much too low to account for the extended optical scattered light, strongly suggesting that the polarized optical light is scattered by dust. The presence of highly ionized Ne lines in the soft X-ray spectrum requires 20 ergs cm s-1 ξ 300 ergs cm s-1; these lines may originate closer to the nucleus than the extended soft continuum or in a lower density gas. A collisionally ionized thermal model of the extended soft X-rays cannot be ruled out but is unattractive in view of the low metal abundance required (Z = 0.03 Z☉). The hard X-ray to far-infrared ratio for the nucleus of Cygnus A is similar to that seen in Seyfert 1 and unobscured radio galaxies. By means of the correlation between hard X-ray luminosity and nuclear optical absolute magnitude for these classes of object, we estimate MB = -22.4 for Cygnus A, near the borderline between Seyfert galaxies and QSOs.

Modeling charge transfer inefficiency in the Chandra Advanced CCD Imaging Spectrometer

The front-illuminated (FI) CCDs in the Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory ( Chandra) were damaged in the extreme environment of the Earth's radiation belts, causing charge traps that result in enhanced charge transfer inefficiency (CTI) during parallel readout. This causes row-dependent gain, event grade ‘morphing’ (spatial redistribution of charge) and energy resolution degradation. The ACIS back-illuminated (BI) CCDs also exhibit pronounced CTI due to their manufacturing. It is mild enough that position-dependent energy resolution is not seen, but it is present in both parallel and serial registers. This CTI also changes the gain and event grades, in a spatially complicated way as parallel and serial CTI interact. Given these realities, we have developed and tuned a phenomenological model of CTI for both FI and BI CCDs and incorporated it into our Monte Carlo simulations of the ACIS CCDs. It models charge loss and the spatial redistribution of charge (trailing), thus reproducing the spatially dependent gain and grade distribution seen in all ACIS CCDs and the row-dependent energy resolution seen in the FI devices. Here we explore the evidence for CTI, compare our simulations to data, and present a technique for CTI correction based on forward modeling.

Simulating CCDs for the Chandra Advanced CCD Imaging Spectrometer

We have implemented a Monte Carlo algorithm to model and predict the response of various kinds of CCDs to X-ray photons and minimally ionizing particles and have applied this model to the CCDs in the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer. This algorithm draws on empirical results and predicts the response of all basic types of X-ray CCD devices. It relies on new solutions of the diffusion equation, including recombination, to predict the radial charge cloud distribution in field-free regions of CCDs. By adjusting the size of the charge clouds, we can reproduce the event grade distribution seen in calibration data. Using a model of the channel stops developed here and an insightful treatment of the insulating layer under the gate structure developed at MIT, we are able to reproduce all notable features in ACIS calibration spectra. The simulator is used to reproduce ground and flight calibration data from ACIS, thus confirming its fidelity. It can then be used for a variety of calibration tasks, such as generating spectral response matrices for spectral fitting of astrophysical sources, quantum efficiency estimation, and modeling of photon pile-up.

Detection of an Iron Emission Feature from the Lensed Broad Absorption Line QSO H1413+117 at [CLC][ITAL]z[/ITAL][/CLC] = 2.56

We present the X-ray energy spectrum of the lensed broad absorption line QSO H1413+117 (the Cloverleaf) at z = 2.56 observed with the Chandra X-ray observatory. We detected 293 photons in a 40 ks Advanced CCD Imaging Spectrometer (ACIS-S) observation. The X-ray image consists of four lensed image components, thus the photons are from the lensed QSO itself. The overall spectrum can be described with a power-law function heavily absorbed by neutral matter at a redshift consistent with the QSO redshift. This supports the idea that intrinsic absorption is significant for BAL QSOs. The spectral fit significantly (99% confidence) improves when we include an emission line. The centroid energy and intrinsic width (Gaussian σ) of the line are 6.21 ± 0.16 keV and 220 eV (90% errors), respectively, in the QSO rest frame, assuming the absorbed power law as the continuum. The equivalent width of the line in the QSO rest frame is 960 eV. We suggest that the large equivalent width, the centroid energy, and the line broadness can be explained by iron K emission arising from X-ray reprocessing in the BAL flow, assuming it has a conical thin-sheet structure.

The Chandra Deep Field North Survey. V. 1 M[CLC]s[/CLC] Source Catalogs

An extremely deep X-ray survey (about 1 Ms) of the Hubble Deep Field North and its environs (about 450 arcmin^2) has been performed with the Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory. This is one of the two deepest X-ray surveys ever performed; for point sources near the aim point it reaches 0.5-2.0 keV and 2-8 keV flux limits of 3 x 10^{-17} erg/cm^2/s and 2 x 10^{-16} erg/cm^2/s, respectively. Here we provide source catalogs along with details of the observations, data reduction, and technical analysis. Observing conditions, such as background, were excellent for almost all of the exposure. We have detected 370 distinct point sources: 360 in the 0.5-8.0 keV band, 325 in the 0.5-2.0 keV band, 265 in the 2-8 keV band, and 145 in the 4-8 keV band. Two new Chandra sources in the HDF-N itself are reported and discussed. Source positions are accurate to within 0.6-1.7 arcsec (at 90% confidence) depending mainly on the off-axis angle. We also detect two highly significant extended X-ray sources and several other likely extended X-ray sources. We present basic number count results for sources located near the center of the field. Source densities of 7100^{+1100}_{-940} deg^{-2} (at 4.2 x 10^{-17} erg/cm^2/s) and 4200^{+670}_{-580} deg^{-2} (at 3.8 x 10^{-16} erg/cm^2/s) are observed in the soft and hard bands, respectively.

X‐Ray Observations of the Supernova Remnant G21.5−0.9

We present the analysis of archival X-ray observations of the supernova remnant (SNR) G21.5-0.9. Based on its morphology and spectral properties, G21.5-0.9 has been classified as a Crab-like SNR. For that reason, it was chosen as a Chandra calibration target. In their early analysis of part of these calibration data, Slane and coworkers discovered a low surface brightness, extended emission. They interpreted this component as the blast wave formed in the supernova explosion. XMM-Newton observations by Warwick and coworkers revealed the nonthermal nature of this emission, suggesting that it is instead an extension of the synchrotron nebula. In this paper, we revisit the Chandra analysis using new calibration data, improving the statistics by a factor of 2. We also include ROSAT and ASCA observations. Our analysis confirms the nonthermal nature of the extended emission. Advanced CCD Imaging Spectrometer images indicate that this component is not limb-brightened and that it shows knotty structures and a bright filament 2' north of the center. We find no evidence of line emission from any part of the remnant. We can reject a collisional equilibrium ionization thermal model at solar abundances and nonequilibrium ionization (NEI) models (such as a plane-parallel shock model with different ionization ages and constant temperature or an NEI model with a single ionization age and a constant temperature). The entire remnant is best fitted with a power-law model with a photon index steepening away from the center. The total unabsorbed flux FX(0.5-10 keV) is 1.1 × 10-10 ergs cm-2 s-1 with an 85% contribution from the 40'' radius core. Timing analysis of the High-Resolution Camera data failed to detect any pulsations. We put a 16% upper limit on the pulsed fraction. We derive the physical parameters of the putative pulsar and compare them with those of other plerions (such as the Crab nebula and 3C 58).

Bulk X-Ray Doppler Velocities in the Supernova Remnant Cassiopeia A

We map the average line-of-sight velocities of the X-ray-emitting mass in the supernova remnant Cassiopeia A using measurements of the centroid of the Si-Heα blend with the Advanced CCD Imaging Spectrometer on the Chandra X-Ray Observatory. This 4'' scale map confirms the broad trends noted in previous studies, namely, that the line-of-sight velocity scale is roughly 2000-3000 km s-1, with relatively more blueshifted material in the southeastern region of the remnant than in the northwest; new details are that the northwestern region consists of two arcs, with the southernmost one representing the most redshifted X-ray-emitting material in Cas A. These results are consistent with contemporaneous results from XMM-Newton. The X-ray patterns resemble the complex velocity patterns measured at optical wavelengths for much denser ejecta, and they support the growing body of evidence that the explosion and subsequent evolution of Cas A were highly asymmetrical.

The Thermal X‐Ray–emitting Shell of Large Magellanic Cloud Supernova Remnant 0540−69.3

We use data from the Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-Ray Observatory to image the shell surrounding the pulsar B0540-69.3 and measure its spectral properties. Weak emission-line features, notably of Fe, Mg, and Si, indicate that the shell is composed primarily of ambient material heated by the blast wave. The shell emission in the east is faint and circular in appearance, with a temperature and ionization age of approximately 5 keV and 1010 cm-3 s, respectively. The emission in the west is brighter and has a more complex, distorted morphology and is fitted with significantly lower average temperatures and higher ionization ages. An image with photon energies above 2 keV shows two hard arcs of emission located diametrically opposite each other at the outer boundary of the remnant shell; the spectra from these regions could include a substantial nonthermal component.

AChandraObservation of M51: Active Nucleus and Nuclear Outflows

We present a Chandra Advanced CCD Imaging Spectrometer (ACIS-S) observation of the nuclear region of the nearby spiral galaxy M51 (NGC 5194), which has a low-luminosity Seyfert 2 nucleus. The X-ray image shows the nucleus, southern extranuclear cloud, and northern loop, the morphology of the extended emission being very similar to those seen in radio continuum and optical emission-line images. The X-ray spectrum of the nucleus is well represented by a model consisting of soft thermal plasma with kT ? 0.5 keV, a very hard continuum, and an Fe K? emission line at 6.45 keV with an equivalent width of greater than 2 keV. The very strong Fe line and the flat continuum indicate that the nucleus is obscured by a column density in excess of 1024 cm-2 and that the spectrum is dominated by reflected emission from cold matter near the nucleus. The X-ray spectra of the extranuclear clouds are well fitted by a thermal plasma model with kT ? 0.5 keV. This spectral shape and morphology strongly suggest that the clouds are shock-heated by the bipolar outflow from the nucleus. The shock velocities of the extranuclear cloud and northern loop inferred from the temperatures of the X-ray gas are 690 and 660 km s-1, respectively. By assuming a steady state situation in which the emission of the extranuclear clouds is powered by the jets, the mechanical energy in the jets is found to be comparable to the bolometric luminosity of the nucleus.

ChandraDetects a Rapid Flare in the Gravitationally Lensed Mini–Broad Absorption Line QSO RX J0911.4+0551

The mini-broad absorption line (BAL) quasar RX J0911.4+0551 was observed with the Advanced CCD Imaging Spectrometer of the Chandra X-Ray Observatory for ~29 ks as part of a gravitational lens (GL) survey aimed at measuring time delays. Timing analysis of the light curve of the lensed image A2 shows a rapid flux variation with a duration of about 2000 s. A Kolmogorov-Smirnov test shows that the probability that a constant intensity source would produce the observed variability is less than ~0.2%. We discuss possible origins for the observed short-term X-ray variability. Our gravitational lens models for the RX J0911.4+0551 GL system predict a time delay of less than a day between images A1 and A2. The rapid variability combined with the predicted short time delay make RX J0911.4+0551 an ideal system to apply the GL method for estimating the Hubble constant. We describe the prospects of measuring H0 within single X-ray observations of GL systems with relatively short time delays. Modeling of the spectrum of the mini-BAL quasar RX J0911.4+0551 suggests the presence of an intrinsic absorber. Partial covering models are slightly preferred over models that contain absorption due to intrinsic ionized or neutral gas.

Deep [ITAL]CHANDRA X-RAY OBSERVATORY[/ITAL][ITAL]Chandra X-Ray Observatory[/ITAL] Imaging Study of the Very Young Stellar Cluster IC 348

We have obtained a deep (53 ks) X-ray image of the very young stellar cluster IC 348 with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. In our image with a sensitivity limit of ∼1 × 1028 ergs s-1 (more than 10 times deeper than our ROSAT images of IC 348), 215 X-ray sources are detected. While 115 of these sources can be identified with known cluster members, 58 X-ray sources are most likely new, still unidentified cluster members. About 80% of all known cluster members with masses between ∼0.15 and 2 M⊙ are visible as X-ray sources in our image. We discover X-ray emission at levels of ∼1028 ergs s-1 from four of 13 known brown dwarfs and from three of 12 brown dwarf candidates in IC 348. We also detect X-ray emission from two deeply embedded objects, presumably class I protostars, south of the cluster center.

ITAL]CHANDRA[/ITAL][ITAL]Chandra[/ITAL] and [ITAL]ASCA[/ITAL] X-Ray Observations of the Type II-L Supernova SN 1979C in NGC 4321

We report on the X-ray observation of the radio-selected supernova SN 1979C carried out with ASCA in 1997 December and serendipitously available from a Chandra Guaranteed Time Observation in 1999 November. The supernova, of type SN II-linear (SN II-L), was first observed in the optical and occurred in the weakly barred, almost face-on spiral galaxy NGC 4321 (M100). The galaxy, a member of the Virgo S Cluster, is at a distance of 17.1 Mpc and contains at least three other supernovae discovered in this century. The useful exposure time was ~25 ks for the Solid-State Imaging Spectrometer (SIS), ~28 ks for the Gas Scintillation Imaging Spectrometer, and ~2.5 ks for the Chandra Advanced CCD Imaging Spectrometer. No point source was detected at the radio position of SN 1979C in a 3' diameter, half-power response circle in the ASCA data. The background- and galaxy-subtracted supernova (SN) signal had a 3 σ upper limit to the flux of 6.3 × 10-14 ergs cm-2 s-1 in the full ASCA SIS band (0.4–10.0 keV) and a 3 σ upper limit of less than (3–4) × 10-14 erg cm-2 s-1 in the 2–10 keV band. In the Chandra data, a source at the position of SN 1979C is marginally detected at energies below 2 keV at a flux consistent with the ROSAT HRI detection in 1995. At energies above 2 keV, no source is detected with an upper limit of ~3 × 10-14 ergs cm-2 s-1. These measurements give the first ever X-ray flux limit of a Type II-L SN above 2 keV, which is an important diagnostic of the outgoing shock wave plowing through the circumstellar medium.