ASCA Observations Research Articles

ASCA observation of the compact group of galaxies SCGG 223

Optical observations of the compact group of galaxies SCGG 223 show that the velocity dispersion in it is as high as 1106 km s −1, while the group itself seems to be relaxed. Numerical simulations also suggest that SCGG 223 is virialized. Therefore, it is inferred that the gravitational mass in SCGG 223 may be comparable with that in a typical galaxy cluster, and the intra-group gas may be extremely hot. In order to explore these unusual implications, we observed SCGG 223 in X-rays with the ASCA satellite in February, 1999. The gas temperature and the 0.5 ∼ 10 keV luminosity are found to be kT = 1.4 +0.6 −0.4 keV and 9.85 +6.11 −2.04 × 10 43J s −1, respectively, which show that SCGG 223 belongs to usual galaxy groups. Due to the lack of enough photon counts, we are not able to perform the X-ray spectroscopic imaging to deduce the radial distribution of the gravitating mass in SCGG 223, by the use of which crucial comparison would have been made with the optical virial mass. Thus, for a complete understanding of SCGG 223, further X-ray observations with satellites such as XMM is desirable.

ITAL]ASCA[/ITAL] Observation of an X-Ray–luminous Active Nucleus in Markarian 231

We have obtained a moderately long (105 s) ASCA observation of the Seyfert 1 galaxy Markarian 231, the most luminous of the local ultraluminous infrared galaxy population. In the best-fitting model we do not see the X-ray source directly; the spectrum consists of a scattered power-law component and a reflection component, both of which have been absorbed by a column NH ≈ 3 × 1022 cm-2. About three-quarters of the observed hard X-rays arise from the scattered component, reducing the equivalent width of the iron Kα line. The implied ratio of 1-10 keV X-ray luminosity to bolometric luminosity, LX/Lbol ~ 2%, is typical of Seyfert 1 galaxies and radio-quiet QSOs of comparable bolometric luminosities and indicates that the bolometric luminosity is dominated by the active galactic nucleus (AGN). Our estimate of the X-ray luminosity also moves Mrk 231 in line with the correlations found for AGNs with extremely strong Fe II emission. A second source separated by about 2' is also clearly detected and contributes about 25% of the total flux.

Radio, X‐Ray, and Extreme‐Ultraviolet Coronal Variability of the Short‐Period RS Canum Venaticorum Binary σ2Coronae Borealis

We present the results of a coordinated observing campaign on the short-period (1.14 days) RS CVn binary ?2 Coronae Borealis with the VLA, ASCA, and RXTE. We also discuss earlier observations of the same system obtained by the Extreme Ultraviolet Explorer (EUVE). Dramatic coronal variability is present in all of these observations across the electromagnetic spectrum. ?2 CrB undergoes frequent large flares that occur close together in time. Radio flares are unambiguously correlated with X-ray flares; the X-ray flares peak as much as 1.4 hr before the corresponding radio maxima. Response to flares is more rapid in higher energy X-ray bandpasses, signaling an increase in temperature during the course of the flare. Flares are seen more frequently in the harder RXTE bandpass than in simultaneously taken ASCA observations. There is greater contrast between flaring and nonflaring conditions in the RXTE light curve. Complex flaring is seen in the radio at 3 and 6 cm, consistent with optically thin nonthermal gyrosynchrotron emission for most of the duration of the observation. Bursts of left-circularly polarized emission at 20 cm lasting ?15 minutes appear to be due to a coherent emission process. EUVE spectra reveal coronal material at ne ? 1012 cm-3, with no discernible density differences between flaring and quiescent time intervals. Quiescent ASCA spectra show lower than solar photospheric abundances with iron depleted by a factor of 4 from the solar photospheric value. The abundances increase during a large flare observed with ASCA, with iron enhanced to the solar photospheric value during the rise phase of the flare. Two-temperature fits to extracted spectra show a low-temperature quiescent value of 8 MK and a high-temperature component that varies from 22 MK during quiescence to 50 MK at the peak of the flare. Emission measure distributions measured from the ASCA observations are consistent with the temperatures derived from the discrete two-temperature fits and indicate the presence of very hot (>100 MK) plasma during the rise phase of the ASCA flare. Many of the flares observed with EUVE, ASCA, and RXTE show a double exponential decay phase, further highlighting the importance of this phenomenon in diagnosing flaring conditions. We also find that the observed second decay timescale cannot be explained by some of the currently used flare parameterizations.

Discovery of the Slowest X-Ray Pulsar in the SMC, AX J0049.5–7323, with ASCA

Abstract The discovery of coherent pulsations with ASCA from an X-ray source, AX J0049.5−7323, is reported. The barycentric period was determined to be 755.5 ± 0.6 s, which is the longest among X-ray pulsators in the SMC. The X-ray spectrum has been found to be unchanged through ASCA observations, with a photon index of ∼ 0.8 and a luminosity of ∼ 5 × 1035 erg s−1 (0.7−10 keV). Archival data of the Einstein and the ROSAT satellites indicate that the flux has been ≲ 10−12 erg s−1 cm−2 (≲ 5 × 1035 erg s−1) for over 20 years and exhibits a variability with a factor of ≳ 10. We argue that AX J0049.5−7323 is an X-ray pulsar with a Be star companion.

A Broad F[CLC]e[/CLC] Kα Emission Line in the X-Ray Spectrum of the Quasar 3C 273

The Fe K? line, an important physical diagnostic in the X-ray spectra of active galaxies (AGNs) and quasars, has been notoriously difficult to measure in the high-luminosity, radio-loud quasar 3C 273 (z = 0.158). On the few occasions that it has been detected, its intrinsic width has been thought to be narrow (FWHM < 10,000 km s-1), with an equivalent width (EW) of a few tens of eV. This was consistent with the general trend that as one goes from low- to high-luminosity AGNs, the Fe K? line goes from being strong (EW ~ 200-300 eV) and broad (FWHM ~ 105 km s-1) to being weak and narrow, or absent altogether. Here we present the results of new ASCA and RXTE observations, together with archival ASCA data, and show for the first time that the Fe K? line in 3C 273 is as broad as those seen in Seyfert 1 galaxies. The line is resolved in two of the observations, with a mean Gaussian width of 0.8 ? 0.3 keV, corresponding to a FWHM of ~0.3 ? 0.1c. The smallest and largest equivalent widths measured are 43 and 133 eV, respectively (in the quasar frame). The Compton-reflection continuum is less than 10% of that expected from a centrally illuminated semi-infinite, face-on, Compton-thick disk, confirming previous studies that Compton reflection is negligible in 3C 273. The largest values of the Fe K? line EW are underpredicted if the line originates in the disk, unless a time lag longer than several days between line and continuum and/or an overabundance of Fe is invoked. We cannot unambiguously constrain the disk inclination angle. About 60? is preferred for a cold disk, while a face-on disk is allowed if the ionization state of Fe is H-like.

Discovery of 101-s Pulsations from AX J0057.4–7325 in the SMC with ASCA

Abstract The results from two ASCA observations of AX J0057.4−7325 = RX J0057.3−7325 are presented. Coherent pulsations with a barycentric period of 101.45 ± 0.07 s were discovered in the second observation. The X-ray spectrum was found to be hard (photon index ∼ 0.9) and unchanged through these observations, except for the flux. The ROSAT archival data show that AX J0057.4−7325 exhibits a flux variation with a factor ≳ 10. A discussion on a possible optical counterpart is given.

Complex Spectral Variability from Intensive Multiwavelength Monitoring of Markarian 421 in 1998

We conducted a multifrequency campaign for the TeV blazar Markarian 421 in 1998 April. The campaign started from a pronounced high-amplitude flare recorded by BeppoSAX and Whipple; the ASCA observation started 3 days later. In the X-ray data, we detected multiple flares, occurring on timescales of about 1 day. ASCA data clearly reveal spectral variability. The comparison of the data from ASCA, the Extreme Ultraviolet Explorer, and the Rossi X-Ray Timing Explorer indicates that the variability amplitudes in the low-energy synchrotron component are larger at higher photon energies. In TeV γ-rays, large intraday variations—which were correlated with the X-ray flux—were observed when results from three Cerenkov telescopes were combined. The rms variability of TeV γ-rays was similar to that observed in hard X-rays, above 10 keV. The X-ray light curve reveals flares that are almost symmetric for most cases, implying that the dominant timescale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of 7 days indicates that the characteristic timescale is ~0.5 days. The analysis of ASCA light curves in various energy bands appears to show both soft (positive) and hard (negative) lags. These may not be real, as systematic effects could also produce these lags, which are all much smaller than an orbit. If the lags of both signs are real, these imply that the particle acceleration and X-ray cooling timescales are similar.

TheChandra X‐Ray ObservatoryResolves the X‐Ray Morphology and Spectra of a Jet in PKS 0637−752

The core-dominated radio-loud quasar PKS 0637-752 (z = 0.654) was the first celestial object observed with the Chandra X-Ray Observatory, offering the early surprise of the detection of a remarkable X-ray jet. Several observations with a variety of detector configurations contribute to a total exposure time with the Chandra ACIS of about 100 ks. A spatial analysis of all the available X-ray data, making use of Chandra's spatial resolving power of about 04, reveals a jet that extends about 10'' to the west of the nucleus. At least four X-ray knots are resolved along the jet, which contains about 5% of the overall X-ray luminosity of the source. Previous observations of PKS 0637-752 in the radio band had identified a kiloparsec-scale radio jet extending to the west of the quasar. The X-ray and radio jets are similar in shape, intensity distribution, and angular structure out to about 9'', after which the X-ray brightness decreases more rapidly and the radio jet turns abruptly to the north. The X-ray luminosity of the total source is log LX ≈ 45.8 ergs s-1 (2-10 keV) and appears not to have changed since it was observed with ASCA in 1996 November. We present the results of fitting a variety of emission models to the observed spectral distribution, comment on the nonexistence of emission lines recently reported in the ASCA observations of PKS 0637-752, and briefly discuss plausible X-ray emission mechanisms.

ASCA Observations of the Crab-Like Supernova Remnant 3C 58

Abstract We present here X-ray observation of a Crab-like supernova remnant (SNR) 3C 58 with ASCA. We find that the integrated energy spectrum over the nebula is consistent with previous results, showing a power-law spectrum with the photon index of γ = 2.2–2.4, modified by interstellar absorption of about (3–4)' 1021 cm−2. The inclusion of a blackbody component, which is attributable to the central compact source, significantly improves the spectral fit. Stringent upper limits for any line emitting thin hot plasma are established. We find for the first time that the nebular spectrum is harder in the central part of the SNR, becoming softer toward the periphery, while the absorption column is uniform across the nebula. Correspondingly, the nebular size decreases with increasing photon energy, which is a steeper function of the radius than that of the Crab nebula. The results are compared with synchrotron energy-loss models, and the nature of the putative pulsar is discussed. A timing analysis was performed to search for pulsed X-ray emission from the central compact source. No significant pulsations were observed; and we present the upper limit for the pulsed fraction.

Discovery of 172-s Pulsations from a Be/X-Ray Binary Candidate AX J0051.6–7311 in the SMC with ASCA

Abstract The results from three ASCA observations of AX J0051.6–7311 = RX J0051.9–7311 are reported. Coherent pulsations with a barycentric period of 172.40 ± 0.03 s were discovered in the third observation, with an exceptionally long exposure time of ∼ 177 ks. The X-ray spectrum was found to remain unchanged through these observations, with a photon index of of ∼ 0.9. Energy-resolved pulse profiles in the third observation reveal that the pulsations are mostly due to photons with an energy above ∼ 2 keV. Archival data of ROSAT and Einstein Observatory indicate that AX J0051.6–7311 exhibits a flux variation having a factor ≳ 20.

ASCA Observations of Three Shakhbazyan's Compact Groups of Galaxies

Abstract X-ray observations of three Shakhbazyan's Compact Groups of Galaxies are presented for the first time. Extended X-ray emission was detected from SCGG 202 and SCGG 223 with 0.5–2 keV luminosity of 1 ' 1042 erg s−1 and 3 ' 1042 erg s−1 (for H0 = 75 km s−1 Mpc−1), respectively, while no significant emission was detected from SCGG 205 with an upper limit of 0.7 ' 1042 erg s−1. The X-ray spectra of SCGG 202 and SCGG 223 can be described with thin-thermal plasma emission with a temperature of about 1 keV. The X-ray properties of SCGG 202 and SCGG 223 are in good agreement with those of other known groups of galaxies, proving the physical nature of their grouping.

The steady-state conduction-driven temperature profile in clusters of galaxies

In this paper, I present a theoretical model of a relaxed cluster where the temperature profile (hereafter TP) is structured by electronic thermal conduction. Neglecting cooling and heating terms, the stationary energy conservation equation reduces to a second-order differential equation, the resolution of which requires two boundary conditions, taken here as the inner radius and the ratio between the inner and outer temperature. Thus a two-parameter family of analytical models for the TP is obtained. Once these two constants are chosen, the TP has a fixed analytical expression, which reproduces nicely the observed ‘universal’ TP obtained by Markevitch et al. from ASCA data. Using observed X-ray surface brightnesses for two hot clusters with spatially resolved TP, the local polytropic index and the hot gas fraction profile are predicted and compared with ASCA observations (Markevitch et al. 1999). Moreover, the total density profile derived from observed X-ray surface brightness, hydrostatic equilibrium and the conduction-driven TP is very well fitted by three analytical profiles found to describe the structure of galactic or cluster haloes in numerical simulations of collisionless matter. With the forthcoming availability of spatially resolved high-quality spectroscopic data, the predicted shape of the TP (related to the temperature dependence of the heat flux for a collisionally ionized plasma) will be tested directly against observations.

X-Ray Properties of Groups of Galaxies

▪ Abstract ROSAT observations indicate that approximately half of all nearby groups of galaxies contain spatially extended X-ray emission. The radial extent of the X-ray emission is typically 50–500 h−1100 kpc or approximately 10–50% of the virial radius of the group. Diffuse X-ray emission is generally restricted to groups that contain at least one early-type galaxy. X-ray spectroscopy suggests the emission mechanism is most likely a combination of thermal bremsstrahlung and line emission. This interpretation requires that the entire volume of groups be filled with a hot, low-density gas known as the intragroup medium. ROSAT and ASCA observations indicate that the temperature of the diffuse gas in groups ranges from approximately 0.3 keV to 2 keV. Higher temperature groups tend to follow the correlations found for rich clusters between X-ray luminosity, temperature, and velocity dispersion. However, groups with temperatures below approximately 1 keV appear to fall off the cluster LX-T relationship (and possibly the LX-σ and σ-T cluster relationships, although evidence for these latter departures is at the present time not very strong). Deviations from the cluster LX-T relationship are consistent with preheating of the intragroup medium by an early generation of stars and supernovae. There is now considerable evidence that most X-ray groups are real, physical systems and not chance superpositions or large-scale filaments viewed edge-on. Assuming the intragroup gas is in hydrostatic equilibrium, X-ray observations can be used to estimate the masses of individual systems. ROSAT observations indicate that the typical mass of an X-ray group is ∼1013 h−1100 M[Formula: see text] out to the radius to which X-ray emission is currently detected. The observed baryonic masses of groups are a small fraction of the X-ray determined masses, which implies that groups are dominated by dark matter. On scales of the virial radius, the dominant baryonic component in groups is likely the intragroup medium.

The X-ray spectrum and light curve of Supernova 1995N

We report on multi-epoch X-ray observations of the Type IIn (narrow emission line) supernova SN 1995N with the ROSAT and ASCA satellites. The January 1998 ASCA X-ray spectrum is well fitted by a thermal bremsstrahlung (kT~10 keV, N_H~6e20 cm^-2) or power-law (alpha~1.7, N_H~1e21 cm^-2) model. The X-ray light curve shows evidence for significant flux evolution between August 1996 and January 1998: the count rate from the source decreased by 30% between our August 1996 and August 1997 ROSAT observations, and the X-ray luminosity most likely increased by a factor of ~2 between our August 1997 ROSAT and January 1998 ASCA observations, although evolution of the spectral shape over this interval is not ruled out. The high X-ray luminosity, L_X~1e41 erg/sec, places SN 1995N in a small group of Type IIn supernovae with strong circumstellar interaction, and the evolving X-ray luminosity suggests that the circumstellar medium is distributed inhomogeneously.

ASCAObservation of G299.2−2.9

In this paper, we present the ASCA observation of supernova remnant G299.2-2.9 for the first time and discuss its physical features by our analyses of its X-ray data. The spectrum of G299.2-2.9 is identified as a thermal plasma emission which has already reached the ionization equilibrium. The absorbing column density obtained from the broadband spectral fitting of G299.2-2.9 indicates its distance as 5 kpc, which is more certain than before and is in good agreement with the intermediate value given by Busser and coworkers and that suggested by Slane and coworkers. Hence, it naturally gives a better constraint on the physical parameters of G299.2-2.9. The morphology of G299.2-2.9 is neither central filled nor normal shell type, but a shell type with an incomplete central annulus. We suggest that this special morphology may be caused by the evaporation of the cloudlets.

A good long look at the black hole candidates LMC X-1 and LMC X-3

We present results from 170ksec long RXTE observations of LMC X-1 and LMC X-3, taken in 1996 December, where their spectra can be described by a disc black body plus an additional soft (Gamma~2.8) high-energy power-law (detected up to 50keV in LMC X-3). These observations, as well as archival ASCA observations, constrain any narrow Fe line present in the spectra to have an equivalent width 0.2Hz, possibly indicating an outer disc radius of ~1000GM/c^2 in this likely wind-fed system. Furthermore, the coherence function between variability in the > 5keV band and variablity in the lower energy bands is extremely low. We discuss the implications of these observations for the mechanisms.

RXTE observation of NGC 6240: a search for the obscured active nucleus

ABSTRA C T The wide-band energy spectrum of NGC 6240 over the range 0.5‐200 keV is investigated using the RXTE and ASCA data. The RXTE data provide the spectrum beyond the ASCA range (0.5‐10 keV) with significant detection of signals up to 20 keV and upper limits above 20 keV. The spectrum above 10 keV is found to be very flat. A strong iron K emission line discovered in the previous ASCA observation is also confirmed with the RXTE Proportional Counter Array (PCA). These results provide further evidence for the dominance of a reflection component, i.e. emission from cool material illuminated by an active galactic nucleus (AGN). By fitting the spectra obtained with RXTE and ASCA simultaneously, we satisfactorily modelled the AGN spectrum with a Compton reflection component and probably a transmitted AGN component penetrating through a thick absorber. The X-ray luminosity of the AGN is estimated to be in the range 4 10 43 to 6 10 44 erg s 21 in the range 2‐10 keV, which categorizes NGC 6240 among the most luminous Seyfert nuclei. The ratio of the 2‐10 keV X-ray luminosity to the infrared luminosity, LX(2‐10 keV)/LIR ,i s 0.01‐0.1, which implies a quite substantial, if not dominant, contribution of AGN to the infrared luminosity.

X-Ray Observation of the Lynds 1157 Dark Cloud Region with ASCA

An ASCA observation of a region containing a Class 0 protostar IRAS 20386+6751 in the Lynds 1157 dark cloud, has been carried out. The protostar was not detected, and the 95% upper limit to the luminosity depends on the assumed NH: LX(0.5–10 keV) &amp;lt; 1.1 × 1031 erg s−1 for NH = 1 × 1023 cm−2. A Class I protostar in Lynds 1152, IRAS 20353+6742, in the same field was also undetected with an upper limit about three-times as much as the Lynds 1157 level. Besides these non detections, nine new X-ray sources were detected and a spectral analysis was performed for 4 sources. One object (AX J2038+6801) shows a hard spectrum with a temperature of kT ∼ 8 keV or a power-law photon index of ∼ 2.0 and absorbed with NH ∼ 2 × 1022 cm−2. Another fainter one (AX J2036 + 6800) has a soft spectrum with most of the emission falling below 2 keV. We examine the possible nature of these new X-ray sources based on their spectral properties.

ASCA Observations of a Near-by Cluster in Antlia

The Antlia cluster is a near-by poor cluster which has a redshift of z = 0.0087. X-ray observations with ASCA have revealed that the cluster is almost isothermal in the X-ray with a temperature of kT ∼ 2.0 keV. The metal abundance is Z ∼ 0.35 Z⊙, and the 0.5–10.0 keV cluster luminosity is $\sim 3.4 \times 10^{42} h^{-2}_{75} \textrm {erg s}^{-1}$ within r &amp;lt; 250 h-175 kpc. There was no X-ray central excess brightness and little central cool component, which are frequently seen in near-by poor clusters. The Antlia cluster is the nearest poor cluster without these central features.

ASCA spectroscopy of the hard X-ray emission from the colliding wind interaction in 2 Velorum

ABSTRA C T We discuss an ASCA observation of the eccentric WC8 1 O7:5 III binary g 2 Velorum near apastron. The X-ray spectrum is compared with two previous observations obtained when the system was near periastron. All three spectra display a hard-emission component that undergoes strong variability over the orbital cycle. The properties of the hard X-ray emission of g 2 Vel are constrained by taking into account the contribution from contaminating soft X-ray sources in the vicinity of g 2 Vel. We find that the observed variations are in qualitative agreement with the predictions of colliding wind models. We investigate for the first time the effect of uncertainties in the chemical composition of the X-ray emitting plasma on our understanding of the high-energy properties of the wind interaction region. Our results indicate that these uncertainties significantly affect the derived shock temperature and absorption column, but play a smaller role in determining the intrinsic X-ray luminosity of the colliding wind zone. We further find that the intrinsic luminosity from the hard X-ray component in g 2 Vel does not follow the 1=D distance relation expected from simple models