ASCA Observations Research Articles

ASCA Observations of the Crab-Like Pulsar PSR B0540 – 69

AbstractWe report on the spectral and temporal properties of the 50 ms pulsar PSR B0540–69 using ASCA archival data obtained during 1993 to 1995. From the spectral analysis it was found that the spectra of the whole (nebular and pulsed) emission and pulsed emission in the range 1-10 keV can be represented by a single power law of photon index, Γ = 2.00 ± 0.02 and Γpulsed= 1.7 ± 0.3 respectively. The parameters for pulse frequency change during 1993-1995 were obtained using the 9 pulse frequency measurements with ASCA. The parameters derived from the ASCA observations are consistent with the previous measurements, suggesting high stability of this pulsar, ΔΩ/Ω ≲ 0.5 × 10−7over the past 10 years. These results confirm similarity of this pulsar with the Crab pulsar.

The Changing X-Ray Properties of γ Cas

The origin of the hot X-ray emission from γ Cas has been a subject of long debate, with some authors advocating that it arises from a supposed undetected degenerate companion. Parmar et al. (1996) concluded that X-ray properties alone limited one’s ability to understand this source. Thus, Smith et al. (1998, “SRC”) obtained ≈1 day’s simultaneous X-ray and UV spectroscopic and photometric coverage with the RXTE/PCA and HST/GHRS space-borne instruments on 14-15 March 1996. The observed 2–10 keV X-ray light curve was found to have two distinct components; short duration bursts (‘shots)’ superimposed on a slowly varying background, termed the “basal” flux, which modulates on a timescales of hours. SRC found two maxima in the RXTE light curve separated by 10 hours. These features coincided with the appearance of two “dips” in the UV continuum light curve sampled near λ1400. From these light curves and other arguments, SRC concluded that γ Cas’s X-ray and UV continuum flux originated near the surface of the star and were modulated in intensity as the star rotated with a period of 1.123 days. This period was deduced by phasing of the X-ray minima obtained from our RXTE data with ASCA observations taken 10 days earlier and from comparisons of UV minima observed with the GHRS with similar minima seen in IUE observations taken two months earlier.

X-ray lobes of W50/SS433 system

The X-ray lobel lie on both sides (east and west) of SS433, and are presumably produced by the interaction of the jets of SS433 with the surrounding SNR W50. The non-thermal nature of the east lobe has been reported by Yamauchi et al. (1994) using the ASCA data. With the further ASCA observations, we studied the spectra and morphology of both the east and west lobes in detail. Their spectra have no emission lines, and are best described by power-law models with interstellar absorption. The X-ray emission becomes softer on the both sides with the distance from SS433. The surface brightness peaks closer to SS433 in the high-energy band than in the low energy band, suggesting that the high energy electrons in the X-ray lobes lose energies by synchrotron emission as they travel away from SS433. We estimate the magnetic field and the total energy of the high energy electrons to be a few μG and ∼ 10 46 erg respectively, where equipartition in energy between the relativistic electrons and magnetic field is assumed.

ASCA observation of the Cygnus Loop supernova remnant

We present here the results of the mapping observation of the Cygnus Loop with the Gas Imaging Spectrometer (GIS) onboard the ASCA observatory. The data covered the entire region of the Cygnus Loop. Spatial resolution of the GIS is moderate whereas the energy resolving power is much better than those used in the previous observations. The ASCA soft-band image shows the well-known shell-like feature whereas the ASCA hard-band image shows rather center-filled morphology with a hard X-ray compact source at the blow-up southern region.

X-ray evidence for multiphase hot gas with nearly solar Fe abundances in the brightest groups of galaxies

We analyse the ASCA spectra accumulated within |100 kpc radii of 12 of the brightest groups of galaxies. Upon fitting isothermal models (1T) jointly to the ASCA SIS and GIS spectra we obtain fits for most groups that are of poor or at best marginal quality and give very subsolar metallicities similar to previous studies, 〈Z〉=0.29±0.12 Z⊙. Two-temperature models (2T) provide significantly better fits for 11 out of the 12 groups, and in every case have metallicities that are substantially larger than obtained for the 1T models, 〈Z〉=0.75±0.24 Z⊙. Though not very well constrained, for most of the groups absorption in excess of the Galactic value is indicated for the cooler temperature component of the 2T models. A simple multiphase cooling flow model gives results analogous to the 2T models including large metallicities, 〈Z〉=0.65±0.17 Z⊙. The nearly solar Fe abundances and also solar α/Fe ratios indicated by the 2T and cooling flow models are consistent with models of the chemical enrichment of ellipticals, groups, and clusters which assume ratios of Type Ia to Type II supernovae and an initial mass function (IMF) similar to those of the Milky Way. Thus we have shown that the very subsolar Fe abundances and Si/Fe enhancements obtained from most previous studies within r|100 kpc of galaxy groups are an artefact of fitting isothermal models to the X-ray spectra, which also has been recently demonstrated for the brightest elliptical galaxies. Owing to the importance of these results for interpreting X-ray spectra, in an appendix we use simulated ASCA observations to examine in detail the ‘Fe bias’ and ‘Si bias’ associated with the spectral fitting of ellipticals, groups and clusters of galaxies.

ASCA observations of two new X-ray pulsars near SMC X-3

ASCA carried out target of opportunity observations centered on the position of SMC X-3 following the report of a detection of emission from SMC X-3 with RXTE. However, no source was detected at the position of SMC X-3 itself, instead two new pulsars in the SMC were found within the ASCA field of view with periods of 46.6 and 91.1 sec.

ASCA observations of blazars and multiband analysis

We present data for 18 blazars observed with ASCA, half of which were also observed contemporaneously with EGRET as parts of multi-wavelength campaigns. the ASCA X-ray spectra of High-energy peaked BL Lacs (HBLs) are soft, and they form the highest energy tail of the low energy (synchrotron) component. The X-ray spectra of the quasar-hosted blazars (QHBs) are hard and consistent with the lowest energy end of the high energy (Compton) component. For Low-energy peaked BL Lacs (LBLs), the X-ray spectra are intermediate. We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Synchrotron-Self-Compton (SSC) emission. For many QHBs, on the other hand, the γ-rays cannot be solely due to the SSC mechanism. We consider an alternative scenario for QHBs where the SSC component dominates in the X-ray band, but it is below the observed γ-ray spectrum. We infer the magnetic field B to be 0.1 – 1 Gauss, and Lorentz factors γ b of electrons radiating at the peak of the ν F(ν) spectrum of ∼ 10 3 for QHBs. This is much lower than γ b ∼ 10 5 for HBLs.

ASCA observations of the Seyfert 2 galaxy NGC 7582: Implications for the unified scheme of AGNs

ASCA observations of the Seyfert 2 galaxy NGC 7582 revealed that it was highly variable on the timescale of ∼ 2×10 4 s in the hard X-ray band (2–10 keV), while the soft X-ray (0.5-2 keV) flux remained constant. The normalized variability amplitude in the hard X-ray band was σ RMS ≈ 0.3, which is the same level as that of Seyfert 1 galaxies of the same luminosity ∼ 3×10 42 erg s −1 (2–10 keV). The spectral analysis suggests that this object is seen through an obscuring torus with the thickness of N H ∼ 1.0 × 10 23 cm −2. The hard X-ray is an absorbed direct continuum from a hidden Seyfert 1 nucleus, while the soft X-ray is dominated by the scattered central continuum from a spatially extended region. We have an obscured/absorbed and a scattered view of this source as expected from the unification model for Seyfert galaxies. An interesting point is that the inferred column density increased by ∼ 4 × 10 22 cm −2 from 1994 to 1996, suggesting a “patchy” torus structure, namely the torus might be composed of many individual clouds. The observed iron line near 6.4 keV with EW of 170 eV is consistent with the torus picture.

ASCA observations of two SNRs and NEI analysis

Based on the data from the ASCA observation of SNRs Kes79 and W49B, we present here the analysis of their X-ray spectra and morphologies. The Kes79 spectrum can be well fitted by a single NEI component, and the narrow-band images of that source show an inhomogeneous distribution of heavy elements. The heavy elements are richest in the positions S, SE and SW of Kes79, where there may exist interaction between shocks and molecular clouds implied by radio observations. For W49B we present here the non-equilibrium ionization (NEI) analysis based on its emission line diagnostics, and the spectral fit using two NEI components. The reverse shock in W49B may be still hot and we don't find evidence for a hotter blast wave in ASCA spectra.

ASCA observation of a near-by cluster, Antlia

Results from ASCA observation of the Antlia cluster are presented. The Antlia cluster is a poor cluster very near to our galaxy which has a redshift of 0.0087. There was no X-ray central excess brightness frequently seen in near-by poor clusters, such as Fornax and Centaurus, so the gravitational potential seems to have only a single component. This is the second near-by poor cluster of such type next to A1060 and it became clear that there is a variety in the potential structure between the clusters.

ASCA detection of faint sources in the Lockman Hole and its comparison with ROSAT sources

We conducted the ASCA observations of the Lockman Hole, aiming to detect faint sources and to determine the CXB origin. We have detected 18 discrete sources in 5σ significance from 0.7-2/2–7/0.7-7 keV band images obtained with ASCA. Correlated with the ROSAT catalog, 13 sources out of 16 (2 are out of bounds) have counterparts in the catalog, and 11 are identified as AGNs. We further determined the ASCA counting rate for all cataloged ROSAT sources utilizing the source position in the ROSAT catalog. Correlation between ROSAT counts (0.5-2 keV) and ASCA counts (2–10 keV) shows a large scatter. This suggests time variability of sources and/or the various degree of absorption / soft-excess on the spectra of AGNs which contribute the CXB.

ASCA observation of A1674

We observed a rich but X-ray faint cluster of galaxies A1674 with ASCA. The cluster belongs to richness class 3, but its X-ray luminosity is as low as 5 × 10 43 erg/s. ASCA observation of A1674 revealed its low gas temperature of 3 keV and asymmetric profile of the X-ray emission. Moreover, we find its heavy metal abundance is low; we obtained the upper limit of 0.2 for it. We calculate the total mass of the gas, galaxies, and gravitational matter in A1674, and find the cluster has high galaxy/gravitational mass ratio and low gas/gravitational mass ratio, comparing with other clusters. It is strange, at first sight, that the cluster having large amount of metal source (galaxies) like A1689 indicates such a low metal abundance in the X-ray spectrum. However, by inspecting the color-magnitude diagram of the cluster galaxies, we got a hint that the cluster contains small number of elliptical population. If the low elliptical fraction is confirmed, this cluster provides a valuable proof for the widely believed model in which metal in the intra-cluster gas is dominantly produced by the ellipticals, not by spirals.

ASCA observations of the Sculptor supercluster

The Sculptor supercluser is a supercluster of galaxies with a redshift of 0.11, near the south Galactic pole. We observed central regions of the supercluster with ASCA in June 1996, Dec. 1996 and Dec. 1997. X-ray emission from several clusters in the supercluster was detected and properties of the clusters such as luminocity, temperature and metal abundance were analysed. No difference in X-ray properties were found between clusters in the supercluster and other clusters. We also searched for X-ray emission from hot diffuse plasma associated with the supercluster, and obtained an upper limit to the emission as 9.2 × 10 −9 ergs s −1 cm −2 str −1 in 2 to 10 keV.

X-ray variability and soft X-ray spectrum of narrow line Seyfert 1 sources

ASCA observations of narrow line Seyfert 1 galaxies (NLS1) are presented. We focus on the black hole size of the NLS1 sources by employing two independent methods for the mass estimation; one is using X-ray variability, the other is using blackbody fit to the soft component. Although the coincidence is not good for some sources, the mass estimated in these ways ranges from 10 4 to 10 7 M ⊙, systematically smaller than those for typical (broad line) Seyfert 1. We consider the small mass black hole is the principal cause of the several extreme characteristics of the NLS1s. We also mention the emission line model for the X-ray spectral features found in some NLS1 sources.

ASCA observation of the X-ray transient GRS 1009-45

We report on the result of the X-ray observation of X-ray Nova Velorum 1993 (GRS 1009-45), made with ASCA on November 10–11, 1993. The energy spectrum was extremely soft and accompanied by a hard tail, characteristics of the Galactic black-hole binaries in the soft state. It is well represented by a two component model consisting of a multicolor disk model or a general relativistic accretion disk model, both describing emission from an optically-thick accretion disk, and a power-law component with a photon index of ∼ 2.5. Spectral analysis of the soft component allows estimation of the mass of the central object. For the optically-estimated distance (> 1 kpc) and inclination (> 37°), the mass is estimated to be > 3.1 M ⊙. These results strongly support that the compact object is a black hole.

A Scaling Law for Magnetic Flux Tubes on an AGN Accretion Disk

An ASCA observation of Seyfert galaxy NGC 3227 showed flares with a linear increase and exponential decrease similar to that of solar flares. We derive a scaling law relating the loop length of a magnetic flux tube to rise and decay times of the flare using cooling mechanisms suitable for the central engine of a Seyfert galaxy. The predicted loop lengths are consistent with physical constraints on the plasma dynamics, suggesting that the same mechanism which explains solar flares may explain variability in Seyfert galaxies.

ASCA observations of Galactic rotation-powered pulsars

AbstractWe have examined several archivalASCAobservations of Galactic radio pulsars, including PSRs B1046–58 and B1610–50. X-ray emission is detected from PSR B1046–58 with a significance of ∼5σ. We find no evidence for pulsations. We argue that the emission is from a spatially unresolved synchrotron nebula powered by the pulsar. The location of the X-ray counterpart within the 95% position error ellipse of the gamma-ray source 3EG J1048–5840 strengthens the claim of Kaspi et al. (2000) that PSR B1046–58 emitϒ-rays. X-ray emission from PSR B1610–50 is not detected. We use the X-ray luminosity upper limit to constrain the pulsar’s velocity ≲ 200km s−1, arguing against an association with the nearby supernova remnant Kes 32. Our results for these radio pulsars contradict previous reports of detections of large (tens of arc minutes) associated synchrotron nebulae.

New ASCA Observations of two Anomalous X-ray Pulsars

AbstractNewASCAobservations of two anomalous X-ray pulsars (AXP) 4U 0142+61 and 1E 1048.1-5937, made in 1998, when compared to earlier observations in 1994 show remarkable stability in the intensity, spectral shape and pulse profile. The energy spectra consist of two components, a power-law and a blackbody emission from the neutron star surface. In IE 1048.1-5937, we have identified three epochs with different spin-down rates and discuss its implications for the magnetar hypothesis of the AXPs. We also note that the spin-down rate and its variations in IE 1048.1-5937 are much larger than what normally can be produced by an accretion disc with very low mass accretion rate corresponding to its low X-ray luminosity.

ASCA Observation of the Quiescent X-Ray Counterpart to SGR 1627-41.

We present a 2-10 keV ASCA observation of the field around the soft gamma repeater SGR 1627-41. A quiescent X-ray source, whose position is consistent both with that of a recently discovered BeppoSAX X-ray source and with the Interplanetary Network localization for this soft gamma repeater, was detected in this observation. In 2-10 keV X-rays, the spectrum of the X-ray source may be fit equally well by a power-law, blackbody, or bremsstrahlung function, with unabsorbed flux approximately 5x10-12 ergs cm-2 s-1. We do not confirm a continuation of a fading trend in the flux, and we find no evidence for periodicity, both of which were noted in the earlier BeppoSAX observations.

ASCA observations of the supernova remnant G156.2+5.7

We report on the X-ray spectra in the wide energy band from the north rim and the central regions of the supernova remnant G156.2+5.7 obtained with ASCA. No clear shell structure is found: the surface brightness in the north rim is nearly the same as that in the center. The spectra from the north rim and the central regions are composed, at least, of 2 components: one is a thin thermal component with a temperature of about 0.4 keV and the other is a hard component represented by either a thin thermal emission model with a temperature higher than a few keV or a power-law model with a photon index of ∼2. Si and S abundances in the central region are found to be 3–10 times larger than those in the north rim region. Results of simultaneous fits of the ASCA and the Ginga spectra are also presented.