Wataru Kawasaki

Optical Identification of the ASCA Large Sky Survey

We present results of optical identification of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources which were detected with the SIS in the 2-7 keV band above 3.5 sigma. The sources are identified with 30 AGNs, 2 clusters of galaxies, and 1 galactic star. Only 1 source is still unidentified. The flux limit of the sample corresponds to 1 x 10^{-13} erg s^{-1} cm^{-2} in the 2-10 keV band. Based on the sample, the paper discusses optical and X-ray spectral properties of the AGNs, contribution of the sources to the Cosmic X-ray Background, and redshift and luminosity distributions of the AGNs. An interesting result is that the redshift distribution of the AGNs suggests a deficiency of high-redshift (0.510^{44} erg s^{-1}) absorbed narrow-line AGNs (so called type 2 QSOs).We present results of optical identification of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources that were detected with the SIS in the 2-7 keV band above 3.5 ?. The flux limit corresponds to ~1 ? 10-13 ergs cm-2 s-1 in the 2-10 keV band. The sources are identified with 30 active galactic nuclei (AGNs), two clusters of galaxies, and one Galactic star. Only one source is still unidentified. All of the X-ray sources that have a hard X-ray spectrum with an apparent photon index of smaller than 1 in the 0.7-10 keV band are identified with narrow-line or weak-broad-line AGNs at redshifts smaller than 0.5. This fact supports the idea that absorbed X-ray spectra of narrow-line and weak-broad-line AGNs make the cosmic X-ray background (CXB) spectrum harder in the hard X-ray band than that of a broad-line AGN, which is the main contributor in the soft X-ray band. Assuming their intrinsic spectra are same as a broad-line AGN (a power-law model with a photon index of 1.7), their X-ray spectra are fitted with hydrogen column densities of log NH(cm-2) = 22-23 at the objects redshift. On the other hand, X-ray spectra of the other AGNs are consistent with that of a nearby type 1 Seyfert galaxy. In the sample, four high-redshift luminous broad-line AGNs show a hard X-ray spectrum with an apparent photon index of 1.3 ? 0.3. The hardness may be explained by the reflection component of a type 1 Seyfert galaxy. The hard X-ray spectra may also be explained by absorption with log NH(cm-2) = 22-23 at the objects redshift, if we assume an intrinsic photon index of 1.7. The origin of the hardness is not clear yet. Based on the log N- log S relations of each population, contributions to the CXB in the 2-10 keV band are estimated to be 9% for less-absorbed AGNs (log NH(cm-2) < 22) including the four high-redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22 < log NH(cm-2) < 23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the flux range from 3 ? 10-11 ergs cm-2 s-1 to 2 ? 10-13 ergs cm-2 s-1. If the four hard broad-line AGNs are included in the absorbed AGNs, the contribution of the absorbed AGNs to the CXB is estimated to be 6%. In optical spectra, there is no high-redshift luminous cousin of a narrow-line AGN in our sample. The redshift distribution of the absorbed AGNs is limited below z = 0.5 excluding the four hard broad-line AGNs, in contrast to the existence of 15 less-absorbed AGNs above z = 0.5. The redshift distribution of the absorbed AGNs suggests a deficiency of AGNs with column densities of log NH(cm-2) = 22-23 in the redshift range 0.5-2, or in the X-ray luminosity range larger than 1044 ergs s-1, or both. If the large column densities of the four hard broad-line AGNs are real, they could complement the deficiency of X-ray absorbed luminous high-redshift AGNs.

Positional Coincidence between the High-Latitude Steady Unidentified Gamma-Ray Sources and Possibly Merging Clusters of Galaxies

We report evidence for the first time that merging clusters of galaxies are a promising candidate for the origin of high Galactic latitude, steady, unidentified EGRET gamma-ray sources. Instead of using past optical catalogs of eye-selected clusters, we made a matched filter survey of galaxy clusters over 4° × 4° areas around seven steady unidentified EGRET sources at |b| > 45°, together with a 100 deg2 area near the south Galactic pole as a control field. In total, 154 Abell-like cluster candidates and 18 close pairs/groups of these clusters, expected to be possibly merging clusters, were identified within estimated redshift zest ≤ 0.15. Five among the seven EGRET sources have one or two cluster pairs/groups (CPGs) within 1° from them. We assess the statistical significance of this result by several methods, and the confidence level of the real excess is maximally 99.8% and 97.8% in a conservative method. In contrast, we found no significant correlation with single clusters. In addition to the spatial correlation, we also found that the richness of CPGs associated with EGRET sources is considerably larger than those of CPGs in the control field. These results imply that a part of the steady unidentified EGRET sources at high latitude are physically associated with close CPGs, not with single clusters. We also discuss possible interpretations of these results. We argue that, if these associations are real, they are difficult to explain by hadronic processes but best explained by the inverse-Compton scattering by high-energy electrons accelerated in shocks of cluster formation, as recently proposed.

Panoramic Views of Cluster-Scale Assemblies Explored by Subaru Wide-Field Imaging

We have started PISCES project; a panoramic imaging and spectroscopic survey of distant clusters on Subaru. It exploits the unique wide-field imaging capability of Suprime-Cam, which provides a 34 � × 27 � field of view corresponding to a physical area of 16 ×13Mpc 2 at z ∼ 1. We plan to target 15 clusters at 0.4 z 1.3. We report on our first results concerning the inner structures and large-scale structures of two distant clusters at z = 0.55 and 0.83 together with the earlier results on a z =0 .41 cluster. The photometric redshift technique has been applied to multi-color data in order to remove most of the foreground/background galaxies so as to isolate the cluster member candidates. We have found large-scale filamentary structures around the clusters, extending out to > 5Mpc from the cores, as well as complex inner structures. The galaxy distributions in the inner regions of the clusters look similar to the X-ray intensity maps, suggesting that most of the optical structures trace physically bound systems. We also compared the structures of the three clusters with those of model clusters in a numerical simulation (N-body + semi-analytic model) by parameterising the shapes of the iso-density contours of galaxies, and found a broad agreement. Our results that cluster-scale assembly takes place along filaments during hierarchical clustering need to be confirmed spectroscopically in the near future.

Optical Identification of the Hardest X-Ray Source in the ASCA Large Sky Survey

We report the optical identification of the hardest X-ray source (AX J131501+3141) detected in an unbiased wide-area survey in the 0.5-10 keV band, the ASCA Large Sky Survey (LSS). The X-ray spectrum of the source is very hard and is well reproduced by a power-law component (Γ=1.5−0.6+0.7) with NH=6−2+4×1022 cm-2. We have found a galaxy with R = 15.62 mag near the center of the error circle for the X-ray source. The optical spectrum of the galaxy shows only narrow emission lines whose ratios correspond to those of a type 2 Seyfert galaxy at z = 0.072, implying an absorption-corrected X-ray luminosity of 2 × 1043 ergs s-1 (2-10 keV) and MB = -20.93 mag. A radio point source is also associated with the center of the galaxy. We thus identify the X-ray source with this galaxy as an obscured active galactic nucleus (AGN). The hidden nature of the nucleus of the galaxy in the optical band is consistent with the X-ray spectrum. These results support the idea that the obscured AGNs/QSOs contribute significantly to the cosmic X-ray background in the hard band at the faint flux level.

The Aquarius Superclusters. I. Identification of Clusters and Superclusters

We study the distribution of galaxies and galaxy clusters in a 10° × 6° field in the Aquarius region. In addition to 63 clusters in the literature, we have found 39 new candidate clusters using a matched-filter technique and a counts-in-cells analysis. From redshift measurements of galaxies in the direction of these cluster candidates, we present new mean redshifts for 31 previously unobserved clusters, while improved mean redshifts are presented for 35 other systems. About 45% of the projected density enhancements are due to the superposition of clusters and/or groups of galaxies along the line of sight, but we could confirm for 72% of the cases that the candidates are real physical associations similar to the ones classified as rich galaxy clusters. On the other hand, the contamination due to galaxies not belonging to any concentration or located only in small groups along the line of sight is ~10%. Using a percolation radius of 10 h-1 Mpc (spatial density contrast of about 10), we detect two superclusters of galaxies in Aquarius, at z ~ 0.086 and 0.112, respectively, with five and 14 clusters. The latter supercluster may represent a space overdensity of about 160 times the average cluster density as measured from the Abell et al. cluster catalog and is possibly connected to a 40 h-1 Mpc filament from z ~ 0.11 to 0.14.

Probing the evolution of early‐type galaxies using multicolour number counts and redshift distributions

We investigate pure luminosity evolution models for early-type (elliptical and S0) galaxies (i.e., no number density change or morphology transition), and exam- ine whether these models are consistent with observed number counts in the B, I and K bands and redshift distributions of two samples of faint galaxies selected in the I and K bands. The models are characterized by the star formation time scaleSF and the time tgw when galactic wind blows in addition to several other conventional pa- rameters. We find the single-burst model (�SF = 0.1 Gyr and tgw = 0.353 Gyr), which is known to reproduce the photometric properties of early-type galaxies in clusters, is inconsistent with redshift distributions of early-type galaxies in the field environment due to overpredictions of galaxies at z > � 1.4 even with strong extinction which is at work until tgw. In order for dust extinction to be more effective, we changeSF and tgw as free parameters, and find that models withSF > 0.5 Gyr and tgw > 1.0 Gyr can be made consistent with both the observed redshift distributions and number counts, if we introduce strong extinction (E(B V ) � 1 as a peak value). These results suggest that early-type galaxies in the field environment do not have the same evolutionary history as described by the single-burst model.

A Deep Optical Observation for an Enigmatic Unidentified Gamma-Ray Source 3EG J1835+5918

We report on a deep optical imaging observation using the Subaru Telescope for a very soft X-ray source, RX J1836.2+5925, which has been suspected to be an isolated neutron star associated with the brightest as-yet unidentified EGRET source outside the Galactic plane, 3EG J1835+5918. An extended source having a complex, bipolar shape is found at B ∼26, which might be an extended pulsar nebular whoseflux is about 5–6 orders of magnitude lower than gamma-ray flux, although finding a galaxy of this magnitude by chance in the error circle is of order unity. We have found two even fainter, possible point sources at B ∼ 28, although their detections are not firm because of a low signal-to-noise ratio. If the extended object of B ∼26 is a galaxy and not related to 3EG J1835+5918, a lower limit on the X-ray/optical flux ratio can be set as fX/f B 2700, giving further strong support for the neutronstar identification of 3EG J1835+5918. Interestingly, if either of the two sources at B ∼28 is the real counterpart of RX J1836.2+5925 and thermal emission from the surface of an isolated neutron star, the temperature and distance to the source become ∼ 4 ×10 5 Ka nd∼ 300pc, respectively, showing a striking similarity of its spectral energy distribution to the proto-type radio-quiet gamma-ray pulsar Geminga. No detection of nonthermal hard X-ray emission is consistent with the ASCA upper limit, if the nonthermal flux of 3EG J1835+5918/RX J1836.2+5925 is at

A CLUSTER SURVEY AROUND THE UNIDENTIFIED EGRET SOURCES

Based on optical galaxy data, we executed a systematic search for galaxy clusters around the 15 steady unidentified EGRET GeV gamma-ray sources in high Galactic-latitude sky ([b] > ). We found a strong correlation with 3.7 level between close cluster pairs (merging cluster candidates) and the unidentified EGRET sources, though, in contrast, no correlation with single clusters. This result implies that merging clusters of galaxies are a possible candidate for the origin of high galactic-latitude, steady unidentified EGRET gamma-ray sources.

A Serendipitous Search for GRB Afterglows by Subaru/Suprime‐Cam: A Test of GRB Beaming

Gamma‐ray bursts (GRBs) are the most mysterious object in astronomy, and one of the key issues is whether they are isotropic explosions or strongly collimated jet‐like ones. A direct consequence of the collimation (or beaming) is that we should be able to find many afterglows without observed prompt gamma‐ray emission, since the collimation gets wider and wider at later stages. We describe a proposed serendipitous search for optical GRB afterglows as a test of GRB beaming, by the Suprime‐Cam on the 8.2m Subaru telescope, which is a very unique facility for this test by its wide field of view and high sensitivity for faint objects. We will show that the detection probability of such afterglows is of order unity by this observation, if GRBs are beamed by a factor of 4π/ΔΩ > 100, which is a typical beaming factor currently discussed. Therefore, positive detection is possible and it would have a strong impact on our understanding of GRBs. Even if undetected, it would set the strongest constraint on the GRB be...

Muti-Color Imaging of Clusters of Galaxies with Mosaic CCD Cameras

At present, the photometric data for clusters at z≲ 0.2 mainly come from photographic photometry. The lack of CCD data for such clusters is simply due to the fact that no CCD camera had been available until recently that covers the wide extension of clusters within a reasonable amount of observing time. We have developed a large mosaic CCD camera and conducted multicolor imaging observations of z ≲ 0.2 clusters using the 40-inch Swope telescope at Las Campanas Observatory.