ASCA Observations Research Articles

Recombining plasma in supernova remnants: Discovery and progress in the last decade

AbstractPresence of recombining (overionized) plasma in several supernova remnants (SNRs), first suggested by ASCA observations, was anchored by the detection of enhanced radiative recombination continua in Suzaku spectra of the SNRs IC 443 and W49B. This discovery was surprising, because X‐ray‐emitting SNRs were thought to have ionizing plasmas in general. Since then, similar spectral features have been detected from more than a dozen SNRs, implying that the recombining plasma is not uncommon for a certain type of SNRs. This paper reviews the observational and theoretical progress achieved in the last decade toward understanding of the origin of the recombining plasmas in SNRs. It is observationally indicated that all the SNRs whose plasma is in overionization are associated with the so‐called mixed‐morphology class, characterized by a centrally peaked X‐ray profile and rim‐brightened radio emission. The latest studies of W49B with NuSTAR and W44 with XMM‐Newton have provided clear evidence that two distinct mechanisms can work for the formation of the recombining plasma, namely adiabatic cooling and thermal conduction, with the dominant channel depending on the evolutionary stage of SNRs.

Probing the nature of AX J0043−737: Not an 87 ms pulsar in the Small Magellanic Cloud

Aims. AX J0043−737 is a source in the ASCA catalogue whose nature is uncertain. It is most commonly classified as a Crab-like pulsar in the Small Magellanic Cloud (SMC) following apparent detection of pulsations at ~87 ms from a single ASCA observation. A follow-up ASCA observation was not able to confirm this, and the X-ray detection of the source has not been reported since. Methods. We studied the nature of the source with a dedicated XMM-Newton observation. We ascertained the source position, searched for the most probable counterpart, and studied the X-ray spectrum. We also analysed other archival observations with the source in the field of view to study its long-term variability. Results. With the good position localisation capability of XMM-Newton, we identify the counterpart of the source as MQS J004241.66–734041.3, an active galactic nucleus (AGN) behind the SMC at a redshift of 0.95. The X-ray spectrum can be fitted with an absorbed power law with a photon-index of Γ = 1.7, which is consistent with that expected from AGNs. By comparing the current XMM-Newton observation with an archival XMM-Newton and two other ASCA observations of the source, we find signatures of long-term variability, another common phenomenon in AGNs. All of the above are consistent with AX J0043−737 being an AGN behind the SMC.

BAT AGN Spectroscopic Survey. V. X-Ray Properties of the Swift /BAT 70-month AGN Catalog

Hard X-ray ($\geq 10$ keV) observations of Active Galactic Nuclei (AGN) can shed light on some of the most obscured episodes of accretion onto supermassive black holes. The 70-month Swift/BAT all-sky survey, which probes the 14-195 keV energy range, has currently detected 838 AGN. We report here on the broad-band X-ray (0.3-150 keV) characteristics of these AGN, obtained by combining XMM-Newton, Swift/XRT, ASCA, Chandra, and Suzaku observations in the soft X-ray band ($\leq 10$ keV) with 70-month averaged Swift/BAT data. The non-blazar AGN of our sample are almost equally divided into unobscured ($N_{\rm H}< 10^{22}\rm cm^{-2}$) and obscured ($N_{\rm H}\geq 10^{22}\rm cm^{-2}$) AGN, and their Swift/BAT continuum is systematically steeper than the 0.3-10 keV emission, which suggests that the presence of a high-energy cutoff is almost ubiquitous. We discuss the main X-ray spectral parameters obtained, such as the photon index, the reflection parameter, the energy of the cutoff, neutral and ionized absorbers, and the soft excess for both obscured and unobscured AGN.

BL Lacertae: X-ray spectral evolution and a black-hole mass estimate

We present an analysis of the spectral properties observed in X-rays from active galactic nucleus BL Lacertae using RXTE, Suzaku, ASCA, BeppoSAX, and Swift observations. The total time covered by these observations is approximately 20 years. We show that this source undergoes X-ray spectral transitions from the low hard state (LHS) through the intermediate state (IS) to the high soft state (HSS) during these observations. During the RXTE observations (1997 -- 2001, {180 ks, for a total 145 datasets}), the source was approximately 75%, 20% and only 5 of the time in the IS, LHS, and HSS, respectively. We also used Swift observations (470 datasets, for a total 800 ks), which occurred during 12 years (2005 -- 2016), the (0.3 -- 200 keV) data of BeppoSAX (1997 -- 2000, 160 ks), and the (0.3 -- 10 keV) data of ASCA (1995 -- 1999, 160 ks). Two observations of Suzaku (2006, 2013; 50 ks) in combinations with long-term RXTE and Swift data-sets allow us to describe all spectral states of BL Lac. The spectra of BL Lac are well fitted by the bulk motion Comptonization (BMC) model for all spectral states. We have established the photon index saturation level, Gamma_{sat}=2.2+/-0.1, in the Gamma vs. mass accretion rate (Mdot) correlation. This Gamma-Mdot correlation allows us to estimate the black-hole (BH) mass in BL Lac to be M_{BH}~3x10^7 M_sol for a distance of 300 Mpc. For the BH mass estimate, we use the scaling method taking stellar-mass Galactic BHs 4U~1543--47 and GX~339--4 as reference sources. The Gamma-Mdot correlation revealed in BL Lac is similar to those in a number of stellar-mass Galactic BHs and two recently studied intermediate-mass extragalactic BHs. It clearly shows the correlation along with the very extended $\Gamma$ saturation at ~ 2.2. This is robust observational evidence for the presence of a BH in BL Lac.

ASCA and XMM-Newton observations of the galactic supernova remnant G311.5−0.3

We present an analysis of X-ray observations made with ASCA and XMM-Newton of the Galactic supernova remnant (SNR) G311.5?0.3. Prior infrared and radio observations of this SNR have revealed a shell-like morphology at both wavelengths. The spectral index of the radio emission is consistent with synchrotron emission, while the infrared colors are consistent with emission from shocked molecular hydrogen. Also previous CO observations have indicated an interaction between G311.5?0.3 and an adjacent molecular cloud. Our previous analysis of the pointed ASCA observation made of this SNR detected X-ray emission from the source for the first time but lacked the sensitivity and the angular resolution to rigorously investigate its X-ray properties. We have analyzed an archival XMM-Newton observation that included G311.5?0.3 in the field of view: this is the first time that XMM-Newton data has been used to probe the X-ray properties of this SNR. The XMM-Newton observation confirms that the X-ray emission from G311.5?0.3 is centrally concentrated and supports the classification of this source as a mixed-morphology SNR. In addition, our joint fitting of extracted ASCA and XMM-Newton spectra favor a thermal origin for the X-ray emission over a non-thermal origin. The spectral fitting parameters for our TBABS?APEC fit to the extracted spectra are NH = 4.63+1.87 ?0.85?1022 cm ?2 and kT = 0.68+0.20?0.24 keV. From these fit parameters, we derive the following values for physical parameters of the SNR: ne = 0.20 cm ?3, np = 0.17 cm ?3, MX = 21.4 M? and P/k = 3.18?106 K cm ?3.

The puzzling orbital period evolution of the LMXB AX J1745.6−2901

The orbital period evolution of X-ray binaries provides fundamental clues to understanding mechanisms of angular momentum transfer and loss in these systems. We present an X-ray eclipse timing analysis of the transient low-mass X-ray binary AX J1745.6−2901. This system shows full eclipses and thus is one of the few objects of this class for which accurate orbital evolution studies can be carried out. We report on XMM–Newton and ASCA observations covering 30 complete X-ray eclipses spanning an interval of more than 20 yr. We improve the determination of the orbital period to a relative precision of 2 × 10−8, two orders of magnitudes better than previous measurements. We determine, for the first time, a highly significant rate of decrease of the orbital period | $\dot{P}_{{\rm orb}}=-4.03\pm 0.32{\times} 10^{-11}$ | s s−1. This is at least one order of magnitude larger than expected from conservative mass transfer and angular momentum losses due to gravitational waves and magnetic braking, and might result from either non-conservative mass transfer or magnetic activity changing the quadrupole moment of the companion star. Imprinted on the long-term evolution of the orbit, we observe highly significant eclipse leads–delays of ∼10–30 s, characterized by a clear state dependence in which, on average, eclipses occur earlier during the hard state.

X-ray variability of SS 433: Evidence for supercritical accretion

We study the X-ray variability of SS433 based on data from the ASCA observatory and the MAXI and RXTE/ASM monitoring missions. Based on the ASCA data, we have constructed the power spectrum of SS433 in the frequency range from $10^{-6}$ to 0.1 Hz, which confirms the presence of a flat portion (flat-topped noise) in the spectrum at frequencies $3\times 10^{-5}$ - $10^{-3}$ Hz. The periodic variability (precession, nutation, eclipses) begins to dominate significantly over the stochastic variability at lower frequencies, which does not allow the stochastic variability to be studied reliably. The best agreement with the observations is reached by the model with the flat portion extending to $9.5\times10^{-6}$ Hz and a power-law spectrum with index of 2.6 below that frequency. The jet nutation with a period of about three days suggests that the time for the passage of material through the disk is less than this value. Therefore, at frequencies below $4\times10^{-6}$ Hz, the power spectrum probably does not reflect the disk structure. It may depend on other factors, for example, a variable mass accretion rate supplied by the donor. The flat portion may arise from a rapid decrease in the viscous time in the supercritical or radiative disk zones. It could be related to the variability of the X-ray jets that are formed in the supercritical region.

MAXIINVESTIGATION INTO THE LONG-TERM X-RAY VARIABILITY FROM THE VERY-HIGH-ENERGY γ-RAY BLAZAR Mrk 421

The archetypical very-high-energy gamma-ray blazar Mrk 421 was monitored for more than 3 years with the Gas Slit Camera onboard Monitor of All Sky X-ray Image (MAXI), and its longterm X-ray variability was investigated. The MAXI lightcurve in the 3 -- 10 keV range was transformed to the periodogram in the frequency range $f = 1 \times 10^{-8}$ -- $2 \times 10^{-6}$ Hz. The artifacts on the periodogram, resulting from data gaps in the observed lightcurve, were extensively simulated for variations with a power-law like Power Spectrum Density (PSD). By comparing the observed and simulated periodograms, the PSD index was evaluated as $\alpha = 1.60 \pm 0.25$. This index is smaller than that obtained in the higher frequency range ($f > 1 \times 10^{-5}$ Hz), namely, $\alpha = 2.14 \pm 0.06$ in the 1998 ASCA observation of the object. The MAXI data impose a lower limit on the PSD break at $f_{\rm b} = 5 \times 10^{-6}$ Hz, consistent with the break of $f_{\rm b} = 9.5 \times 10^{-6}$ Hz, suggested from the ASCA data. The low frequency PSD index of Mrk 421 derived with MAXI falls well within the range of the typical value among nearby Seyfert galaxies ($\alpha = 1$ -- $2$). The physical implications from these results are briefly discussed.

A 33 yr CONSTANCY OF THE X-RAY CORONAE OF AR Lac AND ECLIPSE DIAGNOSIS OF SCALE HEIGHT

Extensive X-ray and extreme ultraviolet (EUV) photometric observations of the eclipsing RS CVn system AR Lac were obtained over the years 1997 to 2013 with the Chandra X-ray Observatory Extreme Ultraviolet Explorer. During primary eclipse, HRC count rates decrease by ~40%. A similar minimum is seen during one primary eclipse observed by EUVE but not in others owing to intrinsic source variability. Little evidence for secondary eclipses is present in either the X-ray or EUV data, reminiscent of earlier X-ray and EUV observations. Primary eclipses allow us to estimate the extent of a spherically symmetric corona on the primary G star of about 1.3Rsun, or 0.86Rstar, and indicate the G star is likely brighter than the K component by a factor of 2-5. Brightness changes not attributable to eclipses appear to be dominated by stochastic variability and are generally non-repeating. X-ray and EUV light curves cannot therefore be reliably used to reconstruct the spatial distribution of emission assuming only eclipses and rotational modulation are at work. Moderate flaring is observed, where count rates increase by up to a factor of three above quiescence. Combined with older ASCA, Einstein, EXOSAT, ROSAT and Beppo-SAX observations, the data show that the level of quiescent coronal emission at X-ray wavelengths has remained remarkably constant over 33 years, with no sign of variation due to magnetic cycles. Variations in base level X-ray emission seen by Chandra over 13 years are only ~10%, while variations back to pioneering Einstein observations in 1980 amount to a maximum of 45% and more typically about 15%.

THE X-RAY PROPERTIES OF FIVE GALACTIC SUPERNOVA REMNANTS DETECTED BY THESPITZERGLIMPSE SURVEY

(Abbreviated) We present a study of the X-ray properties of five Galactic supernova remnants (SNRs) -- Kes 17 (G304.6$+$0.1), G311.5$-$0.3, G346.6$-$0.2, CTB 37A (G348.5$+$0.1) and G348.5$-$0.0 -- that were detected in the infrared by Reach et al. (2006) in an analysis of data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) that was conducted by the Spitzer Space Telescope. We present and analyze archival ASCA observations of Kes 17, G311.5$-$0.3 and G346.6$-$0.2, archival XMM-Newton observations of Kes 17, CTB 37A and G348.5$-$0.0 and an archival Chandra observation of CTB 37A. All of the SNRs are clearly detected in the X-ray possibly except for G348.5$-$0.0. Our study reveals that the four detected SNRs all feature center-filled X-ray morphologies and that the observed emission from these sources is thermal in all cases. We argue that these SNRs should be classified as mixed-morphology SNRs (MM SNRs): our study strengthens the correlation between MM SNRs and SNRs interacting with molecular clouds and suggests that the origin of mixed-morphology SNRs may be due to the interactions between these SNRs and adjacent clouds. Our ASCA analysis of G311.5$-$0.3 reveals for the first time X-ray emission from this SNR: the X-ray emission is center-filled within the radio and infrared shells and thermal in nature ($kT$ $\sim$ 0.98 keV), thus motivating its classification as an MM SNR. We find considerable spectral variations in the properties associated with the plasmas of the other X-ray-detected SNRs, such as a possible overabundance of magnesium in the plasma of Kes 17. Finally, we also estimate such properties as electron density $n$$_e$, radiative age $t$$_{rad}$ and swept-up mass $M$$_X$ for each of the four X-ray-detected SNRs.

AN ENERGETIC MAGNETAR IN HESS J1713–381/CTB 37B

We obtained a second Chandra timing measurement of the 3.82 s pulsar CXOU J171405.7-381031 in the supernova remnant (SNR) CTB 37B, which shows that it is spinning down rapidly. The average period derivative of (5.88+/-0.08)E-11 over the 1 year time span corresponds to a dipole magnetic field strength B = 4.8E14 G, well into the magnetar range. The spin-down power E-dot = 4.2E34 erg/s is among the largest for magnetars, and the corresponding characteristic age Tau = P/2P-dot = 1030 years is comparable to estimates of the age of the SNR. The period derivative enables us to recover probable pulsations in an ASCA observation taken in 1996, which yields a mean characteristic age of 860 years over the longer 13 year time span. The source is well detected up to 10 keV, and its composite spectrum is typical of a magnetar. CTB 37B hosts HESS J1713-381, the first TeV source that is coincident with a magnetar. While the TeV emission has been attributed to the SNR shell, it is possibly centrally peaked, and we hypothesize that this particularly young, energetic magnetar may contribute to the HESS source. We also searched for pulsations from another source in a HESS SNR, XMMU J173203.3-344518 in HESS J1731-347/G353.6-0.7 but could not confirm pulsations or long-term flux variability, making it more likely that this source is a weakly magnetized central compact object.

Deriving an X-ray luminosity function of dwarf novae based on parallax measurements

We have derived an X-ray luminosity function using parallax-based distance measurements of a set of 12 dwarf novae, consisting of Suzaku, XMM-Newton and ASCA observations. The shape of the X-ray luminosity function obtained is the most accurate to date, and the luminosities of our sample are concentrated between ~10^{30}-10^{31} erg s^{-1}, lower than previous measurements of X-ray luminosity functions of dwarf novae. Based on the integrated X-ray luminosity function, the sample becomes more incomplete below ~3 x 10^{30} erg s^{-1} than it is above this luminosity limit, and the sample is dominated by X-ray bright dwarf novae. The total integrated luminosity within a radius of 200 pc is 1.48 x 10^{32} erg s^{-1} over the luminosity range of 1 x 10^{28} erg s^{-1} and the maximum luminosity of the sample (1.50 x 10^{32} erg s^{-1}). The total absolute lower limit for the normalised luminosity per solar mass is 1.81 x 10^{26} erg s^{-1} M^{-1}_{solar} which accounts for ~16 per cent of the total X-ray emissivity of CVs as estimated by Sazonov et al. (2006).

PSR J1907+0602: A RADIO-FAINT GAMMA-RAY PULSAR POWERING A BRIGHT TeV PULSAR WIND NEBULA

We present multiwavelength studies of the 106.6 ms γ-ray pulsar PSR J1907+06 near the TeV source MGRO J1908+06. Timing observations with Fermi result in a precise position determination for the pulsar of R.A. = 19h07m547(2), decl. = +06°02'16(2)'' placing the pulsar firmly within the TeV source extent, suggesting the TeV source is the pulsar wind nebula of PSR J1907+0602. Pulsed γ-ray emission is clearly visible at energies from 100 MeV to above 10 GeV. The phase-averaged power-law index in the energy range E > 0.1 GeV is Γ = 1.76 ± 0.05 with an exponential cutoff energy Ec = 3.6 ± 0.5 GeV. We present the energy-dependent γ-ray pulsed light curve as well as limits on off-pulse emission associated with the TeV source. We also report the detection of very faint (flux density of ≃ 3.4 μJy) radio pulsations with the Arecibo telescope at 1.5 GHz having a dispersion measure DM = 82.1 ± 1.1 cm−3 pc. This indicates a distance of 3.2 ± 0.6 kpc and a pseudo-luminosity of L1400 ≃ 0.035 mJy kpc2. A Chandra ACIS observation revealed an absorbed, possibly extended, compact (≲4'') X-ray source with significant nonthermal emission at R.A. = 19h07m5476, decl. = +06°02'146 with a flux of 2.3+0.6−1.4 × 10−14 erg cm−2 s−1. From archival ASCA observations, we place upper limits on any arcminute scale 2–10 keV X-ray emission of ∼1 × 10−13 erg cm−2 s−1. The implied distance to the pulsar is compatible with that of the supernova remnant G40.5 − 0.5, located on the far side of the TeV nebula from PSR J1907+0602, and the S74 molecular cloud on the nearer side which we discuss as potential birth sites.

General relativistic models of the X-ray spectral variability of MCG–6-30-15

We study in detail the GR models of the X-ray spectral variability for various geometries of the X-ray source and with various relativistic effects being the dominant cause of spectral variability. The predicted properties are compared with the Suzaku observational data of the Seyfert 1 galaxy MCG--6-30-15. The data disfavor models with the X-ray source (1) moving vertically on the symmetry axis or (2) corotating with the disc and changing height not far above the disc surface. The most likely explanation for the observed variability is given by the model involving the X-ray source located at a very small, varying distance from a rapidly rotating black hole. This model predicts some enhanced variations in the red wing of the Fe line, which are not seen in the Suzaku observations. However, the enhanced variability of the red wing, while ruled out by the Suzaku data, is consistent with an excess RMS variability, between 5 and 6 keV, reported for some previous ASCA and XMM observations. We speculate that the presence or lack of such a feature is related to the change of the ionization state of the innermost part of the disc, however, investigation of such effects is currently not possible in our model (where a neutral disc is assumed). If the model, completed by description of ionization effects, proves to be fully consistent with the observational data, it will provide a strong indication that the central black hole in MCG--6-30-15 rotates rapidly, supporting similar conclusions derived from the Fe line profile.

Stellar formation in Brightest Cluster Galaxies

AbstractWe study the interplay between stellar population of the Brightest Cluster Galaxies (BCG) and cluster global properties. We use X-ray properties from Chandra, ROSAT and ASCA observations and BCGs spectra from SDSS-DR6. Using STARLIGHT we determined the star formation history of the BCGs and look for relations with the intra-cluster gas properties. Our first results show no correlation between the recent star formation with x-ray temperature and dynamic state. An important star formation is happening in cool-core Cluster. This new stellar population has low metallicity, pointing to an external source of cold gas to form stars.

Suzakuobservation of the Phoenix galaxy

Context. In recent years, several Seyfert 2 galaxies have been discovered that change state when observed in X-rays a few years apart, switching from Compton-thin to reflection-dominated or viceversa. Aims. We observed a member of this class of “Changing-look” sources, the Phoenix galaxy, with Suzaku, with the aim of better understanding the nature of the variations. Methods. The Suzaku spectrum was analyzed, and the results compared with previous ASCA and XMM-Newton observations. Results. The source was caught in a Compton-thin state, as in XMM-Newton, but differently from ASCA. Comparing the Suzaku and XMM-Newton observations, a variation in the column density of the absorber on a time scale of years is discovered. A similar change, but on much shorter time scales (i.e. ks) may also explain the count-rate variations during the Suzaku observations. A soft excess is also present, likely due to continuum and line emission from photoionized circumnuclear matter.

ChandraConfirmation of a Pulsar Wind Nebula in DA 495

As part of a multiwavelength study of the unusual radio supernova remnant DA 495, we present observations made with the Chandra X-ray Observatory. Imaging and spectroscopic analysis confirms the previously detected X-ray source at the heart of the annular radio nebula, establishing the radiative properties of two key emission components: a soft unresolved source with a blackbody temperature of 1 MK consistent with a neutron star, surrounded by a nontherma1 nebula 40 in diameter exhibiting a power-law spectrum with photon index Gamma = 1.63, typical of a pulsar wind nebula. Morphologically, the nebula appears to be slightly extended along a direction, in projection on the sky, previously demonstrated to be of significance in radio and ASCA observations; we argue that this represents the orientation of the pulsar spin axis. At smaller scales, a narrow X-ray feature is seen extending out 5 from the point source, but energetic arguments suggest that it is not the resolved termination shock of the pulsar wind against the ambient medium. Finally, we argue based on synchrotron lifetimes in the nebular magnetic field that DA 495 represents the first example of a pulsar wind nebula in which electromagnetic flux makes up a significant part, together with particle flux, of the neutron star's wind.

Discovery of a New X‐Ray Filled Radio Supernova Remnant around the Pulsar Wind Nebula in 3EG J1809−2328

We report the discovery of a partial ~2deg. diameter non-thermal radio shell coincident with Taz, the pulsar wind nebula (PWN) in the error box of the apparently variable gamma-ray source 3EG J1809-2328. We propose that this radio shell is a newly identified supernova remnant (SNR G7.5-1.7) associated with the PWN. The SNR surrounds an amorphous region of thermal X-rays detected in archival ROSAT and ASCA observations putting this system in the mixed-morphology class of supernova remnants. G7.5-1.7 is the fifth such supernova remnant coincident with a bright GeV source, and the fourth containing a pulsar wind nebulae.

Heavy Element Analysis of X-Ray Faint Elliptical Galaxies

Abstract Past ASCA and ROSAT observations could not support the general view that elliptical galaxies are sources of metals in the intracluster medium, since observational results have shown that X-ray faint elliptical galaxies exhibit too low metal abundances of hot interstellar medium (ISM). We investigated the radial profiles of the temperature and abundance of 9 X-ray faint elliptical galaxies using the Chandra data. It is found that the heavy element abundance is as high as $\sim$0.7 solar at the central region. Moreover, toward the periphery of galaxies, the metallicity sharply declines. Therefore, the existence of low-metallicity intergalactic gas around X-ray faint elliptical galaxies is indicated. We also investigated the abundance ratios among several elements of 5 X-ray faint elliptical galaxies using the XMM-Newton data, and obtained a hint that the O$/$Fe ratio of X-ray faint elliptical galaxies is smaller than that of X-ray bright elliptical galaxies. This result perhaps indicates that, in X-ray faint elliptical galaxies, the contribution of SNeIa is smaller than that in X-ray bright ones, and it is speculated that many of the metals in ISM of X-ray faint elliptical galaxies have escaped into the surrounding space.

「あすか」の見た銀河系内の超新星残骸とその関連天体( :X線天文衛星「あすか」)

「あすか」の見た銀河系内の超新星残骸とその関連天体( :X線天文衛星「あすか」)