ASCA Observations Research Articles

X‐Ray Spectrum of Supernova 1993J Observed withASCAand Its Evolution 8–572 Days after the Explosion

ASCA observations of the supernova in M81, SN 1993J, 8-572 days after the explosion are presented. Utilizing a one-dimensional image-fitting to the Solid-State Imaging Spectrometer (SIS) data, energy spectra of the supernova in 0.5-8 keV are obtained. The spectrum showed drastic softening with a power-law photon index of 0.3-4, while the X-ray intensity decreased from 0.03 to 0.008 counts s-1 SIS-1. The early phase of spectra requires two thermal emission components of different absorption columns if they are fitted with thermal models. The temperatures of two emission components cannot be well constrained with ASCA continuum spectra. However, the detection of an iron K emission line with ASCA and the hard X-ray spectra observed by OSSE on the Compton Gamma-Ray Observatory suggest that the two components have different temperatures: a few keV and about 100 keV. The properties of the two emission components are consistent with emissions from the front and reverse shocks of the supernova explosion. The drastic softening of the X-ray spectra is explained by the decrease of the absorption column density of the reverse shock component and the change of the major contributor to the soft X-ray band from the front to the reverse shocks.

XMM-Newton observation of a distant X-ray selected cluster of galaxies at $\vec{z}=\mathsf{1.26}$ with possible cluster interaction

We report on the XMM-Newton observation of RX J1053.7+5735, one of the most distant $(z = 1.26)$ selected clusters of galaxies, which also shows an unusual double-lobed X-ray morphology, indicative of possible cluster-cluster interaction. The cluster was discovered during our ROSAT deep pointings in the direction of the Lockman Hole. The XMM-Newton observations were performed with the European Photon Imaging Camera (EPIC) during the performance verification phase. Total effective exposure time was ~100 ksec. The best fit temperature based on a simultaneous fit of spectra from all EPIC cameras (pn+MOS) is $4.9^{+1.5}_{-0.9}$ keV. Metallicity is poorly constrained even using the joint fit of all spectra, with an upper limit on the iron abundance of 0.62 solar. Using the best fit model parameters, we derived an unabsorbed (0.2–10) keV flux of $f_{0.2-10} = 3.0 \times 10^{-14}$ erg cm -2 s -1 , corresponding to a bolometric luminosity of $L_{\rm bol} = 3.4 \times 10^{44}~ h_{50}^{-2}$ erg s -1 . Despite the fact that it was observed at fairly large off-axis angle, the temperature errors are much smaller compared with those of typical measurements based on ASCA or Beppo-Sax observations of high- z ($z > 0.6$) clusters, demonstrating the power of the XMM for determining the X-ray temperature for high- z clusters. The measured temperature and luminosity show that one can easily reach the intrinsically X-ray faint and cool cluster regime comparable with those of $z \sim 0.4$ clusters observed by past satellites. The new cluster temperature and L bol we have measured for RX J1053.7+5735 is consistent with a weak/no evolution of the L bol –$T\rm _x$ relation out to $z \sim 1.3$, which lends support to a low $\Omega_{\rm m}$ universe, although more data-points of $z > 1$ clusters are required for a more definitive statement. The caution has to be also exercised in interpreting the result, because of the uncertainty associated with the dynamical status of this cluster.

Detection of a high frequency break in the X-ray power spectrum of Ark 564

We present a power spectrum analysis of the long ASCA observation of Ark 564 in June/July 2001. The observed power spectrum covers a frequency range of ∼3.5 decades. We detect a high frequency break at ∼2 x 10 - 3 Hz. Thepower spectrum has an rms of ∼30% and a slope of ∼-1 and ∼-2 below and above the break frequency. When combined with the results from a long RXTE observation (Pounds et al. 2001), the observed power spectra of Ark 564 and Cyg X-1 (in the low/hard state) are almost identical, showing a similar shape and rms amplitude. However, the ratio of the high frequency breaks is very small (∼10 3 - 4 ), implying that these characteristic frequencies are not indicative of the black hole mass. This result supports the idea of a small black hole mass/high accretion rate in Ark 564.

Multiwavelength Observations of Three Short‐Period Active Binary Systems: ER Vulpeculae, CC Eridani, and EI Eridani

We present the results of multiwavelength observing campaigns of three short-period active binary systems: ER Vulpeculae, CC Eridani, and EI Eridani, all with orbital periods less than 2 days. ER Vulpeculae (Porb ~ 0.7 day) was observed simultaneously in the X-ray and radio with ASCA and MERLIN, CC Eridani (Porb ~ 1.56 days) with EUVE, the VLA, and the ATCA, and EI Eridani (Porb ~ 1.95 days) with EUVE and ASCA. The ASCA observations of ER Vul and EI Eri reveal a pattern of low coronal metallicities during quiescence, which increase during flares. There is evidence in ASCA spectra of EI Eri for extremely hot, (T > 100 MK) plasma arising during stellar flares. Multifrequency radio emission from CC Eri shows flux decreasing with increasing frequency, and polarization increasing with frequency. During the observations of CC Eri at 20 cm, two highly circularly polarized flares occurred. One, being left circularly polarized at -70%, is consistent with plasma radiation at the fundamental frequency as on other active binary systems. The second flare is largely right circularly polarized (up to 100%). The results from EUVE observations suggest high coronal electron densities ne > 1012 cm-3, which is in conflict with the implied electron density of the radio-emitting plasma if the two types of emission have a common electron population. Despite our long EUV and X-ray observations of these systems lasting several orbital periods, flaring at these wavelengths was observed only on EI Eri, the longest period system here. We find a very peaked distribution of count rates about the average for all the nonflaring high-energy observations.

Variability Timescales of TeV Blazars Observed in theASCAContinuous Long‐Look X‐Ray Monitoring

Three uninterrupted, long (lasting respectively 7, 10, and 10 days) ASCA observations of the well-studied TeV-bright blazars Mrk 421, Mrk 501, and PKS 2155-304 all show continuous strong X-ray flaring. Despite the relatively faint intensity states in two of the three sources, there was no identifiable quiescent period in any of the observations. Structure function analysis shows that all blazars have a characteristic timescale of ~1 day, comparable to the recurrence time and to the timescale of the stronger flares. On the other hand, examination of these flares in more detail reveals that each of the strong flares is not a smooth increase and decrease but exhibits substructures of shorter flares having timescales of ~10 ks. We verify via simulations that in order to explain the observed structure function, these shorter flares (shots) are unlikely to be fully random, but in some way are correlated with each other. The energy dependent cross-correlation analysis shows that interband lags are not universal in TeV blazars. This is important since in the past only positive detections of lags were reported. In this work, we determine that the sign of a lag may differ from flare to flare; significant lags of both signs were detected from several flares, while no significant lag was detected from others. However, we also argue that the nature of the underlying component can affect these values. The facts that all flares are nearly symmetric and that fast variability shorter than the characteristic timescale is strongly suppressed, support the scenario where the light crossing time dominates the variability timescales of the day-scale flares.

A Phase-connected Timing Solution for the Magnetar Candidate 1E 1841−045

We report on a long-term monitoring campaign of 1E 1841-045, the 12 s anomalous X-ray pulsar and magnetar candidate at the center of the supernova remnant Kes 73. We have obtained approximately monthly observations of the pulsar with the Rossi X-Ray Timing Explorer (RXTE) spanning over 2 years, during which time 1E 1841-045 is found to be rotating with sufficient stability to derive a phase-connected solution. A linear ephemeris is consistent with measurements of the pulse period made over the last 15 years with the Ginga, ASCA, RXTE, and BeppoSAX observatories. Phase residuals suggest the presence of timing noise, as is typically observed from young radio pulsars. These results confirm a rapid, constant spin-down for the pulsar, which continues to maintain a steady flux; this is inconsistent with most accretion scenarios.

ASCAObservations of the Thermal Composite Supernova Remnant 3C 391

We present the results from ASCA observations of the centrally enhanced supernova remnant (SNR) 3C 391 (G31.9+0.0). We use the ASCA SIS data to carry out an investigation of the spatial and spectral properties of the X-ray emission from this remnant. The collisional equilibrium ionization and nonequilibrium ionization spectral fits indicate that the hot gas within the remnant has basically reached ionization equilibrium. The variation of the hydrogen column density across the remnant is in agreement with the presence of a molecular cloud to the northwest. The comparisons of hydrogen column and X-ray hardness between the northwest and southeast portions of the remnant support a scenario in which the SNR has broken out of a dense region into an adjacent region of lower density. The mean density within the SNR is observed to be much lower than the immediate ambient cloud density. This and the centrally brightened X-ray morphology can be explained either by the evaporation of engulfed cloudlets or by a radiative stage of evolution for the remnant.

Relativistic ionized accretion disc models of MCG-6-30-15

We present results from fitting of ionized accretion disc models to three long ASCA observations of the Seyfert 1 galaxy MCG-6-30-15. All three data sets can be fitted by a model consisting of ionized reflection from the inner region of the accretion disc (with twice solar Fe abundance) and a separate disc-line component from farther out on the disc. The disc-line is required to fit the height of the observed Fe Kα line profile. However, we show that a much simpler model of reflection from a very weakly ionized constant-density disc also fits the data. In this case only a single cold Fe Kα line at 6.4 keV is required to fit the observed line. The ionized disc models predict that O viii Kα, C vi Kα, Fe xvii Lα and Fe xviii Lα lines will appear in the soft X-ray region of the reflection spectrum, but are greatly blurred as a result of Compton scattering. The equivalent width (EW) of O viii Kα is estimated to be about 10 eV and seems to be as strong as the blend of the Fe L lines. This result creates difficulty for the claim of a strong relativistic O viii line in the XMM-Newton grating spectrum of MCG-6-30-15, although we cannot strictly rule it out since MCG-6-30-15 was in an anomalously low state during that observation. We find that increasing the O abundance or breaking the continuum below 2 keV will not significantly strengthen the line. The second Fe Kα line component in the ionized disc model may arise from neutral reflection from a flared disc, or from a second illumination event. The data cannot distinguish between the two cases, and we conclude that single-zone ionized disc models have difficulty fitting these hard X-ray data of MCG-6-30-15.

Detection of Bulk Motions in the Intracluster Medium of the Centaurus Cluster

Several recent numerical simulations of off-center cluster mergers predict that significant angular momentum with associated velocities of a few ?103 km?s-1 can be imparted to the resulting cluster. Such gas bulk velocities can be detected by the Doppler shift of X-ray spectral lines with ASCA spectrometers. Using two ASCA observations of the Centaurus cluster (Abell 3526), we produced a velocity map for the gas in the cluster's central regions. We also detected radial and azimuthal gradients in temperature and metal abundance distributions, which seem to be associated with the infalling subgroup centered at NGC 4709 (Cen 45). More importantly, we found a significant (>99.8% confidence level) velocity gradient along a line near-perpendicular to the direction of the incoming subgroup and with a maximum velocity difference of ~3.4 ? 1.1 ? 103 km s-1. It is unlikely (P < 0.002) that the observed velocity gradient is generated by gain fluctuations across the detectors. While the observed azimuthal temperature and abundance variations can be attributed to the interaction with Cen 45, we argue that the intracluster gas velocity gradient is more likely due to a previous off-center merging event in the main body of the Centaurus cluster.

ASCAObservation of MS 1603.6+2600 (=UW Coronae Borealis): A Dipping Low‐Mass X‐Ray Binary in the Outer Halo?

MS 1603.6+2600 is a high-latitude X-ray binary with a 111 min orbital period, thought to be either an unusual cataclysmic variable or an unusual low-mass X-ray binary. In an ASCA observation in 1997 August, we find a burst, whose light curve suggests a Type I (thermonuclear flash) origin. We also find an orbital X-ray modulation in MS 1603.6+2600, which is likely to be periodic dips, presumably due to azimuthal structure in the accretion disk. Both are consistent with this system being a normal low-mass X-ray binary harboring a neutron star, but at a great distance. We tentatively suggest that MS 1603.6+2600 is located in the outer halo of the Milky Way, perhaps associated with the globular cluster Palomar 14, 11 degrees away from MS 1603.6+2600 on the sky at an estimated distance of 73.8 kpc.

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).

A Possible X-Ray Periodicity at Several Tens of Hours of an Ultraluminous Compact X-Ray Source in IC 342

A long (155 hr) ASCA observation was performed of two ultraluminous compact X-ray sources, source 1 and source 2, in the spiral galaxy IC 342. While source 1, which was in a hard spectral state, varied little, source 2, which was in its soft spectral state, varied significantly on a timescale of about 1 day. The rms variation amplitude amounts to 5% in the 2-10 keV band but is less than 4% in 0.7-2 keV. The variation involves statistically significant changes in the parameters describing multicolor disk blackbody emission from this source. The variation is possibly periodic, with a period of either 31 ± 2 or 41 ± 3 hr. Both are consistent with the orbital period of a semidetached binary formed by a black hole and a main-sequence star of several tens of solar masses. These results reinforce the interpretation of these X-ray objects in terms of accreting massive stellar black holes.

Multiwavelength Monitoring of the Narrow‐Line Seyfert 1 Galaxy Arakelian 564. I.ASCAObservations and the Variability of the X‐Ray Spectral Components

We present a 35 day ASCA observation of the Narrow-Line Seyfert 1 galaxy Ark 564, yielding an on-source exposure of ~1 Ms. The ASCA observation was part of a multiwavelength AGN Watch monitoring campaign. The soft X-ray light curve binned to 256 s reveals trough-to-peak flux variations up to a factor of ~16 and changes in the fractional amplitude of variability across the observation. Ark 564 shows small variations in photon index across the observation, with Γ in the range 2.45-2.72. The presence of the soft hump component below 1 keV, previously detected in ASCA data, is confirmed. Time-resolved spectroscopy with approximately daily sampling reveals a distinction in the variability of the soft hump and power-law components over a timescale of weeks, with the hump varying by a factor of ~6 across the 35 day observation compared to a factor of 4 in the power law. This difference in the long-term amplitudes of variation causes changes in the softness ratio across the observation. Flux variations in the power-law component are measured down to a timescale of ~1000 s with accompanying spectral variability. Some correlated events are observed in the soft hump, UV flux, and hard X-ray flux when all are sampled daily. No significant UV to X-ray lags are found, with upper limits of ~1 day. We detect Fe Kα and a blend of Fe Kβ plus Ni Kα, the line energies indicating an origin in highly ionized gas. Variability measurements constrain the bulk of the Fe Kα line to originate within approximately a light week of the nucleus. The large EW of the emission lines may be due to high metallicity in Narrow-line Seyfert 1 galaxies, supporting some evolutionary models for active galactic nuclei.

ASCAX-ray observations of the disc wind in the dwarf nova Z Camelopardalis

We present ASCA observations of the dwarf nova Z Camelopardalis during outburst and during a transition from quiescence to another outburst. At the beginning of the transition the X-ray count rate was an order of magnitude higher and the spectrum much harder than during the outburst. As the transition progressed, the spectrum remained hard as the X-ray flux decreased by a factor of 3, with no spectral softening. Spectral modelling reveals an optically thin, high-temperature component which dominates the transition observation and is also observed during outburst. This is expected from material accreting on to the white dwarf surface. The outburst spectra require additional emission at lower temperatures, through either an additional discrete temperature component, or a combination of a cooling flow model and an ionized absorber. Fits to both observations show large amounts of absorption , two orders of magnitude greater than the measured interstellar value, and consistent with UV measurements of the outburst. This suggests that a disc wind is present even in the earliest stages of outburst, possibly before the outburst heating wave has reached the boundary layer.

ASCA Observations of the Central Regions of M 31

Abstract Using ASCA, spatially integrated X-ray spectra of the central regions of M 31 were studied. Data were accumulated over three different circular regions, with radii of $3', 6'$, and $12'$, all centered on the nucleus. The spectra are relatively similar among the three regions. In the energy range above 1.5 keV, the spectra are reproduced by a combination of a disk black-body component and a black-body component, implying that the emission mainly comes from an assembly of low-mass X-ray binaries. At energies below 1.5 keV, the spectra involve two additional softer components, expressed by thin-thermal plasma emission models of temperatures of $\sim 0.9 \,\mathrm{keV}$ and $\sim 0.3 \,\mathrm{keV}$. Over the central $12'$ (2.4 kpc) region and in the 0.5–10 keV energy band, the binary component has a luminosity of $2.6 \times 10^{39} \,\mathrm{erg} \,\mathrm{s}^{-1}$, while the two softer components both exhibit luminosities of $\sim 2 \times 10^{38} \,\mathrm{erg} \,\mathrm{s}^{-1}$. These results are compared with those from other missions, including Chandra and XMM-Newton in particular. A discussion is presented on the nature of the two softer spectral components besides the binary one.

ASCA observations of deep ROSAT fields - V. The X-ray spectrum of hard X-ray selected QSOs

We present an analysis of the ROSAT and ASCA spectra of 21 broad-line active galactic nuclei (AGN) quasi-stellar objects (QSOs) with detected in the band with the ASCA GIS. The summed spectrum in the ASCA band is well described by a power law with , flatter than the average spectral index of bright QSOs and consistent with the spectrum of the X-ray background in this band. The flat spectrum in the ASCA band could be explained by only a moderate absorption (∼ assuming the typical AGN spectrum, i.e. a power law with . This could in principle suggest that some of the highly obscured AGN, required by most X-ray background synthesis models, may be associated with normal blue QSOs rather than narrow-line AGN. However, the combined ASCA–ROSAT spectrum is well fit by a power-law of with a spectral upturn at soft energies. It has been pointed out that such an upturn may be an artefact of uncertainties in the calibration of the ROSAT or ASCA detectors. Nevertheless, if real, it could imply that the above absorption model suggested by the ASCA data alone is ruled out. Then a large fraction of QSOs could have ‘concave’ spectra i.e. they gradually steepen towards softer energies. This result is in agreement with the BeppoSAX hardness ratio analysis of ∼100 hard X-ray selected sources.

Another Intermediate‐Mass Black Hole in a Starburst Galaxy? The Luminous X‐Ray Source in NGC 3628 Reappears

In a 52 ks-long Chandra ACIS-S observation of the nearby starburst galaxy NGC 3628, obtained to study the starburst-driven outflow from this galaxy, we have detected a very luminous (L_X = 1.1e40 erg/s in the 0.3-8.0 keV energy band) point source located at least 20 arcsec (~970 pc) from the nucleus of the galaxy. No radio, optical or near-IR counterpart to this source has been found. This is most probably the reappearance of the strongly-variable X-ray-luminous source discovered by Dahlem et al (1995), which faded by a factor >27 between December 1991 and March 1994 (at which point it had faded below the detection limit in a ROSAT HRI observation). This source is clearly a member of an enigmatic class of X-ray sources that are considerably more luminous than conventional X-ray binaries but less luminous than AGN, and which are not found at the dynamical center of the host galaxy. The Chandra spectrum is best-fit by an absorbed power law model with a photon index of Gamma = 1.8+/-0.2, similar to that seen in Galactic BH binary candidates in their hard state. Bremsstrahlung models or multi-color disk models (the favored spectral model for objects in this class based on ASCA observations) can provide statistically acceptable fits only if the data at energies E > 5 keV is ignored. This is one of the first X-ray spectra of such an object that is unambiguously that of the source alone, free from the spectral contamination by X-ray emission from the rest of the galaxy that affects previous spectral studies of these objects using ASCA.

Characteristic X‐Ray Variability of TeV Blazars: Probing the Link between the Jet and the Central Engine

We have studied the rapid X-ray variability of three extragalactic TeV γ-ray sources: Mrk 421, Mrk 501, and PKS 2155-304. Analyzing the X-ray light curves obtained from ASCA and/or Rossi X-Ray Timing Explorer observations between 1993 and 1998, we have investigated the variability in the time domain from 103 to 108 s. For all three sources, both the power spectrum density (PSD) and the structure function (SF) show a rollover with a timescale of the order of 1 day or longer, which may be interpreted as the typical timescale of successive flare events. Although the exact shape of turnover is not well constrained and the low-frequency (long timescale) behavior is still unclear, the high-frequency (short timescale) behavior is clearly resolved. We found that, on timescales shorter than 1 day, there is only small power in the variability, as indicated by a steep power spectrum density of f-2~-3. This is very different from other types of mass-accreting black hole systems, for which the short-timescale variability is well characterized by a fractal, flickering-noise PSD (f-1~-2). The steep PSD index and the characteristic timescale of flares imply that the X-ray-emitting site in the jet is of limited spatial extent: D ≥ 1017 cm distant from the base of the jet, which corresponds to ≥102 Schwarzschild radii for 107-10 M☉ black hole systems.

ChandraObservations of the X‐Ray Point Source Population in Centaurus A

We present results from two Chandra X-ray Observatory observations of the X-ray point source population in the nearby radio galaxy Centaurus A (NGC 5128). Using a wavelet decomposition detection algorithm, we detect 246 individual point sources above a limiting luminosity of ~2 × 1036 ergs s-1, 82 of which are detected in both data sets where the fields of view overlap. Thirty-eight sources were detected in only one observation but were within the field of view of both pointings, implying considerable variability. We identify eight foreground stars in our observations. We also identify nine of the sources with known globular clusters in Centaurus A. All previously observed ROSAT sources within our field of view are detected. The number of luminous (Lx > 1037 ergs s-1) X-ray binaries per unit optical luminosity in Cen A is roughly consistent with that observed for the M31 bulge. There are approximately 50% more X-ray binaries per unit optical luminosity in Centaurus A than in the whole of M31, however. We find considerably fewer luminous X-ray binaries per unit optical luminosity in Cen A than in two other elliptical galaxies, M84 and NGC 4697, recently observed by Chandra. This result directly confirms the variance in the X-ray binary population with host galaxy optical luminosity previously inferred from spectral analysis of ROSAT and ASCA observations of elliptical galaxies.

Rotationally modulated X-ray emission from the single O starζOphiuchi

Archived measurements by the X-ray Telescope on board the ASCA satellite of the single runaway O9.5V star, ζOphiuchi, are analysed. The data set is unique as it covers just more than one full rotational period of the star. We report a clearly detected periodic X-ray flux variability with amplitude ~20% in the ASCA passband (0.5-10 keV). The detected period possibly indicates a connection with the recurrence time () of the discrete absorption components (DACs) in UV spectra of the star, thought to be due to the presence of large scale structures in the stellar wind modulated by rotation. We attribute the X-ray fluctuation with an uneven distribution of X-ray absorbing material. We also report that an analysis of similar ASCA observations of ζPuppis failed to confirm earlier reported variability of X-rays from this star based on Rosat observations.