Mission Overview

The Advanced Satellite for Cosmology and Astrophysics, ASCA, is Japan's fourth cosmic X-ray astronomy mission, and the second for which the United States is providing part of the scientific payload. The satellite was successfully launched on February 20, 1993. ASCA operated successfully till July 15 2000 when it was transferred into a safe-hold mode. The satellite re-entered on March 2, 2001 after 7 and half years of scientific observations. ASCA was the first satellite to use CCD detectors for X-ray astronomy.

Lifetime: February 20, 1993 - March 2, 2001
Energy Range: 0.4 - 10 keV
Special Features: First X-ray mission to combine imaging capability with broad pass band, good spectral resolution, and a large effective area


Launch Information
Launch Date: 1993-02-20 at 02:20:00 UTC
Launch Vehicle: M-3SII
Launch Site: Uchinoura Space Center, Japan
Decay Date: 2001-03-02

Trajectory Details
Type: Orbiter
Central Body: Earth
Epoch start: 1993-02-20 00:00:00 UTC

Orbital Parameters
Periapsis 523.5999755859375 km
Apoapsis 615.2999877929688 km
Period 96.08999633789062 minutes
Inclination 31.100000381469727°
Eccentricity 0.009999999776482582

Regions Traversed
Trapped particle belts

Instrumentation

• Four X-ray telescopes each composed of 120 nested gold-coated aluminum foil surfaces (total eff area 1,300 cm2 @ 1 keV, spatial resolution 3´ half power diameter, FOV 24´ @ 1 keV) working in conjunction with one of the following detectors:
   o a Gas Imaging Spectrometer (GIS; 0.8-12 keV)
     Two Imaging Gas Scintillation Proportional Counters (IGSPC)
     FOV 50´,
     spatial resolution ~0.5' at 5.9 keV,
     and energy resolution of 8 % at 5.9 keV,
     Eff area (GIS+XRT) 50 cm2 @ 1 keV
   o Solid-state Imaging Spectrometer (SIS; 0.4-12 keV)
     Two CCD arrays of four 420 X 422 square pixel chips,
     FOV 22´ X 22´,
     Spatial resolution 30“,
     energy resolution of 2 % at 5.9 keV ,
     Eff area (SIS+XRT) 105 cm2

Science

• Broad Fe lines from AGN, probing the strong gravity near the central engine
• Lower than solar Fe abundance in the coronae of active stars
• Spectroscopy of interacting binaries
• Non-thermal X-rays from SN 1006, a site of Cosmic Ray acceleration
• Abundances of heavy elements in clusters of galaxies, consistent with type II supernova origin

Diffuse X-ray background

Since its discovery in 1962 (Giacconi et al. 1962 Phys Rev Lett 9, 439) the origin of the diffuse X-ray background (DXRB) has been one of the main unsolved problems in X-ray astronomy. Its very high relative isotropy and uniformity combined with the perfect fit to a 40 keV bremmstrahlung spectrum over the 2-40 keV band (Marshall et al. 1980 ApJ 235, 4) gave impetus to models of its origin in a hot intergalactic medium. The sharp rise in the spectrum at E < 1 keV (McCammon et al. 1983 ApJ 269, 107) was attributed to a warm galactic component. However, there was no one experiment that observed this spectral transition and there were indications (Wu et al. 1991 ApJ 379, 564) of another spectral component. The COBE upper limits on the Compton y parameter for the microwave background (Mather et al. 1990 ApJ 354, L37) ruled out a hot IGM origin for the E > 2 keV background. Thus point source models of its origin are now considered to be most likely. These models require a large number (> 200 /sq degree) of cosmologically distant ( ~ 1), faint (F(x) < 10^-13 ergs/cm^2/s) sources and indeed the ROSAT deep survey results (Hasinger et al. 1993 A&Ap 275, 1) have found such a population. However, the X-ray spectra of the faint ROSAT sources are in general too steep to fit the spectrum of the E > 1 keV background. In fact, the absence of any observed population of sources with the observed spectrum of the background has been a source of continuing confusion. The solution, reached at the same time by several authors is that the flat observed x-ray background spectrum is due to the superposition of the spectra of sources with a wide range in column density, ranging up to log N(H) ~ 24 over a wide range in redshifts with power law spectral slopes consistent with the HEAO-1/EXOSAT/Ginga values for active galaxies (Zdziarski et al. 1993 ApJ 414, L81; Setti & Woltjer 1989 A& Ap 224, L21; Matt & Fabian 1994 MNRAS 267, 187; Madau et al. 1993 ApJ 410, L7).

Another of the controversies about the X-ray background, is the apparent rather different normalizations of the soft (0.2-2 keV) and hard (2-10 keV) source counts (the logN-logS laws). It has been known ever since the HEAO-1 and Einstein epochs (Piccinotti et al. 1982 ApJ 253, 485; Gioia et al. 1984 ApJ 283, 495) that the normalization of the hard X-ray logN-logS law is 3 times that of the soft band. However, before ASCA the hard X-ray logN-logS relation was only derived from source counts at F(x) > 10^{-11} ergs/cm^2/s and the direct comparison to the Einstein and ROSAT data relied on a fluctuation analysis from HEAO-1 and Ginga. If the fluctuation analysis were correct then this discrepancy could be explained by the presence of absorbed active galaxies, more of which would be visible in the hard band than in the soft band.

However, without reliable measures of the source counts at low (F(x) < 10^-11 ergs/cm^2/s) fluxes and without a detailed spectral form of the background over the 0.4-10 keV band these explanations were just ``best guesses''. ASCA data has enabled these tests to be made and resulted in a strong indication that the long standing mystery of the origin of the diffuse X-ray background is close at hand.

ASCA spectra of the DXRB (Gendreau et al. 1995 PASJ 47, 5; Gendreau 1995 PhD thesis; Ishisaki 1996 Waseda meeting) have shown that the 0.4-10 keV spectrum can be well modeled as the sum of two components; a hard power law (G = 1.42 +/- 0.02) or hot bremmstrahlung component that dominates at E > 1 keV and a softer component that can be well modeled as a kT ~ 0.16 keV subsolar abundance thermal plasma. The interpretation is that the soft component is the sum of a galactic thermal contribution and a steep spectrum line-less extragalactic component, while the hard emission is very similar to that reported previously by non-imaging proportional counters. The high quality E > 1 keV ASCA spectrum is devoid of spectral features of amplitude > 5% and thus places very strong constraints on any model of the X-ray background (Gendreau 1995 PhD thesis).

Deep surveys by ASCA (Inoue 1996 Waseda meeting) indicate that the hard X-ray logN-logS law continues as a S^-1.5 power law whose normalization is roughly 3 times that of the soft (0.2-2 keV) log N-logS down to flux levels of 10^-13 ergs/cm^2/s . At this level more than 25% of the hard X-ray background has been resolved out into sources. Combined ROSAT and ASCA spectra of a deep field (Chen, Fabian and Gendreau 1996 MNRAS submitted) have shown that the spectra of faint quasars are too steep at E > 1 keV to account for the observed background flux indicating that another population makes up the bulk of the E > 1 keV background. Analysis of the faintest resolved sources, F(X) < 2.5 x 10^-13 ergs/cm^2/s (Ueda 1996, PhD Thesis) indicates that they have a very flat spectrum with a photon index of 1.5 +/- 0.2, consistent with the spectrum of the diffuse background itself.

While optical identification work on the faint hard X-ray sources is still in progress several objects have been found (Makishima et al. 1994 PASJ 46, L77; Ohta et al. 1996 ApJ 458, L57) whose spectra are consistent with recent unified models (Madau et al. 1994 MNRAS 270, L17, Zdziarski et al. 1995 ApJ 438, L63) of the origin of the X-ray background.

Active Galactic Nuclei including the first direct detection of relativistic line broadening of an X-ray emission line in an active galactic nucleus, indicating that the observed iron K line radiation emanates from within tens of Schwarzschild radii of the massive central object. Also the detection of X-ray emission from the radio lobes of Fornax A and Centaurus B with a spectrum consistent with that from inverse Compton from the radio synchrotron electrons, providing the first measurement of the magnetic field strength in the lobes of radio galaxies.

Clusters of Galaxies including highly robust determinations of the mass of clusters of galaxies and the demonstration that most of the intracluster gas in rich clusters has been processed by Type II supernovae at early epochs.

Galaxies (including galactic center) including the discovery that metal abundances in the gas haloes of elliptical galaxies are sub-solar with, in the few cases that have been measured so far, a decrease in abundance with radius. Also evidence that the center of the Milky Way is filled with ionized hot gas whose heating mechanism remains unknown; the detection of strong localized iron K fluorescent radiation suggesting the presence of a low luminosity active nucleus in our Galaxy as recently as a few hundred years ago.

Supernova Remnants including the measurement, using images in prominent X-ray emission lines, of significant variation in supernova remnants of both ionization and chemical composition as a function of position, as well as coherent velocity features that directly measure the expansion of the ejecta; the identification of a site of cosmic ray acceleration in the supernova remnant SN1006.

Stars including measurements of abundances in the coronae of active stars suggesting metal deficiencies when compared to photospheric abundances. Also the completely unexpected discovery of hard X-ray emission, including a flare, from class I protostellar candidates.

X-ray Binaries including the discovery of a ~30 msec period in Cen X-4 demonstrating the theoretically predicted link between low mass X-ray binaries and radio millisecond pulsars.

CVs and supersoft sources

Cataclysmic variables (CVs) are a large, diverse class of semi-detached mass-exchanging binaries consisting of a white dwarf primary and a late-type (G, K, or M) secondary. In non-magnetic CVs, the material lost by the secondary forms a disk around the white dwarf and accretion onto the compact star occurs in a highly sheared boundary layer between the disk and the surface of the white dwarf. In the synchronously rotating AM Her stars or polars, the strong magnetic field (B ~ 10-100 MG) of the primary envelops the secondary, and the material lost by the secondary is funneled along the magnetic field lines and falls radially onto a small spot in the vicinity of the magnetic poles. In the standard picture, the accreting material passes through a standoff shock far enough above the surface of the white dwarf for the hot, post-shock material to cool and come to rest at the stellar surface. In the asynchronously rotating DQ Her stars or intermediate polars, the weaker magnetic field (B~ 0.1-10 MG) of the primary and the larger size (longer period) of the binary allow a disk to form, but the center of this disk is disrupted at that radius where the magnetic field of the white dwarf is strong enough to dominate the flow. The accreting material then leaves the disk and flows along the magnetic field lines to accrete onto the white dwarf in a manner similar to AM Her stars.

ASCA has contributed significantly to our understanding of each of these three classes of CVs via its unique combination of large collecting area (necessary for these relatively dim sources), wide bandpass (necessary for these relatively hard multi-temperature sources), and high spectral resolution (necessary to resolve the K-alpha emission lines of Mg, Si, S, Ar, and Fe).

An excellent example of ASCA's spectroscopic capabilities is afforded by observations of non-magnetic CVs. Acceptable fits to ROSAT PSPC data can be obtained with either two-temperature thermal plasma spectra or single-temperature thermal bremsstrahlung spectra with a narrow emission line at ~ 1 keV (van Teeseling & Verbunt 1994 A&Ap 292, 519; Richman 1996 ApJ 462, 404). With their wide bandpass and high spectral resolution, the ASCA SIS and GIS spectrometers have demonstrated conclusively that the hard X-ray spectrum of the boundary layer emission of non-magnetic CVs is characterized by a two-temperature thermal plasma with kT_1 ~ 1 keV and kT_2 ~ 3-10 keV (Nousek et al. 1994 ApJ 436, L19; Watson & Osborne 1996 AAS/HEAD meeting). In addition to the emission lines of this high-temperature thermal plasma, the 6.4 keV Fe K emission line has been detected for the first time (Kitamura et al. 1996 Waseda meeting; Done & Osborne 1996 Waseda meeting; Watson & Osborne 1996 AAS/HEAD meeting). If this line is due to fluorescence, as seems likely, it implies that the hot gas is in close proximity to relatively cold photoionized material such as the surface of the white dwarf and/or accretion disk. That the hard X-rays come from a compact region near the white dwarf is demonstrated most convincingly by ASCA observations of the eclipsing dwarf nova HT Cas (Figure 22: Mukai et al. 1996 ApJ submitted). The factor of ~ 10 higher count rate for the ASCA SIS+GIS light curve relative to that of the ROSAT HRI is crucial in measuring the eclipse depth and width and hence in constraining the geometry of the X-ray emitting gas: it is found that the X-ray eclipse is deep, compatible with being total, and that the eclipse transition is short, placing a limit of 1.15 times the radius of the white dwarf as the total size of the X-ray emission region.

ASCA's ability to resolve the K-alpha emission lines of cosmically abundant elements has for the first time allowed measurement of the physical conditions in the post-shock region of magnetic CVs. The simultaneous presence of H- and He-like K-alpha emission lines of Mg, Si, S, Ar, and Fe in the ASCA SIS spectrum of the intermediate polar EX Hya (figure 23) clearly signals the presence in this system of a range of temperatures extending from ~ 0.9 keV to ~ 8 keV (Fujimoto & Ishida 1996 ApJ submitted). Via a model of the settling flow below the shock, the intensity ratios of the H- and He-like K-alpha emission lines constrain the shock and base temperature of the accretion column to kT_s = 15 +/- 2 keV and kT_b = 0.8 +/- 0.2 keV, respectively. This measurement for the shock temperature confirms the conclusion from Ginga data that the hard X-ray continua of magnetic CVs are the superposition of a kT_s ~ 15 keV thermal spectrum and its harder reflection spectrum from the surface of the white dwarf (Beardmore et al. 1995 MNRAS 272, 749; Done, Osborne & Beardmore 1995 MNRAS 276, 483). In addition, the shock temperature kT_s = 3GM_wd mu m_H/8R_wd is a function of M_wd/R_wd and R_wd is a function of M_wd via the mass-radius relation, so a very good estimate of the white dwarf mass is provided by ASCA spectroscopic measurements. For EX Hya, M_wd = 0.47 +/- 0.04 Solar masses , a result which is independent of the usual uncertainties involving K corrections and the inclination of the binary.

ASCA has contributed significantly to our understanding of the continua of magnetic CVs as well. ASCA SIS observations of FO Aqr demonstrate the complexity of low-energy continuum of this intermediate polar (Mukai, Ishida, & Osborne 1994 PASJ 46, L87). The spectrum from 0.5 to 10 keV can be fit by a single kT ~ 30 keV thermal bremsstrahlung spectrum suffering varying amounts of absorption. Below ~ 1.5 keV, an unabsorbed component dominates, and above that energy the continuum is photoelectically absorbed by a range of column densities between N_H ~ 3 x 10^22 and 3 x 10^23 atoms/cm^2. That the unabsorbed and absorbed components originate in different regions is indicated by the fact that the strength of the beat period between the spin period of the white dwarf and the orbital period of the binary is a strong function of energy, with little to no beat modulation above ~ 3 keV. The continua of polars are similarly complex. It is known from Ginga observations that the hard X-ray continuum of AM Her can be fit by a thermal bremsstrahlung spectrum with kT ~ 14 keV with a partial covering absorber and reflection from the surface of the white dwarf (Beardmore et al. 1995 MNRAS 272, 749), but the model of the continuum in the ~ 2-5 keV bandpass is not unique. Similar to FO Aqr, the continuum of AM Her in the ASCA bandpass can be fit by a single temperature thermal bremsstrahlung spectrum suffering photoelectric absorption by a range of column densities between N_H ~ 9 x 10^19 and 2 x 10^23 atoms/cm^2 (see figure 24). Below ~ 0.7 keV, a soft component is detected which can be fit by a blackbody spectrum with kT = 36 +/- 3 keV. The peak of this soft component is detected in simultaneous EUVE observations, where it is observed that the blackbody is modified by the bound-free opacity of Ne VI (Paerels et al. 1996 ApJ in press).

Dozens of Super Soft Sources (SSS) have been discovered with Einstein and ROSAT (for reviews see e.g., Hasinger 1994 AIP Conference Proceedings 308, 611; and in Compact Stars in Binaries Cowley et al., Rappaport & Di Stefano, and Kahabka & Trumper). Steady nuclear burning on the surfaces of accreting white dwarves is a likely model for SSS (van den Heuvel et al. 1992 A&Ap 262, 97; Heise et al. 1994 A&Ap 288, L45).

Thus far, observational information on SSS in the X-ray band has been limited by the spectral resolution of the instruments used. While the ROSAT PSPC has a very suitable energy band (0.1-2 keV) for the study of SSS, its energy resolution (Delta E/E ~ 60 % at 0.5 keV) does not allow detection of spectral structures such as emission lines or absorption edges.

Having a superior energy resolution (Delta E/E ~ 10 % at 0.5 keV), the ASCA Solid State Spectrometer (SIS) is potentially a powerful instrument for the spectroscopic study of SSS, though its energy band (0.4-10 keV) is higher than the typical SSS energy range. RX J0925.7-475 is an unusual SSS having most of its X-ray emission above 0.5 keV (Motch et al. 1994 A&Ap 284, 827), and is thus the most suitable target for the ASCA SIS

RXJ0925.7-475 was observed in the ASCA AO3 phase for 20 ksec. The ROSAT spectrum of RX J0925.7-475 is fitted with the blackbody having kT = 40-55 eV with an inter-stellar absorption of N_H = (1.0-1.9) x 10^22 atoms/cm^2 (Motch et al. 1994 A&Ap 284, 827). Figure 25 is a blackbody model fit to the ASCA spectrum, giving parameters close to the ROSAT best-fit ones. It is obvious that the fit is not acceptable (reduced Chi^2 ~ 10), and several absorption edge-like features are seen.

If the ASCA spectrum is fitted with with a blackbody model with absorption edges, at least three absorption edges are required at around 0.94 keV, 1.04 keV and 1.43 keV (figure 26). Introducing the absorption edges, the blackbody temperature significantly increases to 96 +/- 11 eV, and the bolometric luminosity and radius decreases, compared to the fit without absorption edges. This implies that SSS spectral parameters based on blackbody fits to Einstein or ROSAT spectra are subject to large uncertainties.

ASCA spectrum allows a precise comparison of the observation and theoretical SSS spectral models. Ionization balance of hot white-dwarf atmospheres is determined by the surface gravity and the effective temperature, both of which can be constrained from model fitting to the data. The normalization of the model is directly related to the white-dwarf radius and the distance to the source. The radius, as well as the surface gravity, is almost uniquely determined using the white-dwarf mass-radius relation (e.g. Pringle and Webbink 1975 MNRAS 172, 493). Hence, ASCA observations of SSS enable us to determine the white-dwarf parameters, as well as the distance to the source.

Gamma-ray bursts including the identification of a soft gamma-ray repeater with a neutron star in a supernova remnant

Summary