arXiv subjects

Yuichi Terashima

Publications and source records attributed to Yuichi Terashima.

90 records · Page 5Linked to original sources

arXiv2003

X-ray Underluminous Active Galactic Nuclei Relative to Broad Emission Lines in Ultraluminous Infrared Galaxies

We present X-ray spectra of four ultraluminous infrared galaxies (ULIRGs) with detectable broad near-infrared emission lines produced by active galactic nuclei (AGNs): Mrk 463, PKS 1345+12, IRAS 05189-2524, and IRAS 07598+6508. With the exception of IRAS 07598+6508, high quality X-ray spectra obtained with XMM or Chandra show modest (30-340 eV) equivalent widths of the 6.4 keV Fe K alpha emission line and clear signatures for absorption at a level of 4-33 * 10^22 cm^-2 for the main power-law components from the AGNs. These spectral properties are typical of Compton-thin AGNs, and so we estimate absorption-corrected 2-10 keV X-ray luminosities for the AGNs, Lx(2-10keV), using the Compton-thin assumption. We compare the Lx(2-10 keV) values with broad optical/near-infrared emission-line luminosities, and confirm a previous finding by Imanishi & Ueno that the Lx(2-10 keV) to broad-emission-line luminosity ratios in ULIRGs are systematically lower than those of moderately infrared-luminous type-1 AGNs. A comparison of independent energy diagnostic methods suggests that the AGNs are underluminous in the 2-10 keV band with respect to their overall spectral energy distributions, as opposed to the broad emission lines being overluminous. This X-ray under-luminosity should be taken into account when using 2-10 keV X-ray data to investigate the energetic contribution from AGNs to the large infrared luminosities of ULIRGs.

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arXiv2003

The Luminous X-ray Source Population in M51 Observed with Chandra

We present the results of two Chandra observations (separated by 1 year) of the population of X-ray sources in the spiral galaxy M51 (NGC 5194 and NGC 5195). 113 X-ray sources have been detected in an 8'.4x8'.4 region. Nine and 28 sources have luminosities exceeding 10^39 erg/s (ULXs) and 10^38 erg/s in the 0.5-8 keV band, respectively, assuming they are associated with M51. The number of ULXs is much higher than found in most normal spiral and elliptical galaxies. Most of the X-ray sources and all seven of the ULXs in NGC 5194 are located in, or close to, a spiral arm, suggesting a connection with recent star formation. The cumulative luminosity function of the X-ray sources in NGC 5194 with Lx>10^38 erg/s is well described by a power law N(>Lx) \propto Lx^-αwith α=0.91. The X-ray spectra of most of the detected sources are consistent with a power law with a photon index between 1 and 2, with a few sources showing harder or softer spectra. The spectra of most ULXs are consistent with both a power law and a multicolor disk blackbody (MCD) model, while a power law model is preferable to a MCD model in two ULXs. A drastic spectral variability and emission lines in ULXs are also presented. The masses of the compact objects and mass accretion rates in ULXs and other X-ray sources are not well constrained by these observations. If we adopt a MCD interpretation, their MCD parameters imply that most of the X-ray sources are stellar mass (~5-10 M_solar) black holes accreting near or above the Eddington rate.(abridged)

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arXiv2003

X-ray Evidence for a Buried Active Galactic Nucleus in UGC 5101

We present X-ray observations of the ultraluminous infrared galaxy, UGC 5101, thought to contain a buried active galactic nucleus (AGN) based on observations in other wavebands. We detected an absorbed hard component at >3 keV, as well as soft emission in the energy range 0.5-2 keV. The soft X-ray component, possibly due to a modestly dust-obscured, extended starburst, has an absorption-corrected 0.5-2 keV X-ray luminosity of LX(0.5-2 keV) = 1.2 * 10^41 ergs s^-1. The 0.5-2 keV X-ray to infrared luminosity ratio is a factor of 5 lower than typical values for a normal starburst, suggesting that this extended starburst is unlikely to be energetically dominant in UGC 5101. The most plausible origin of the absorbed hard component is the putative buried AGN. The 6.4 keV Fe K alpha emission line has a modest equivalent width (400 eV), suggesting that this hard component is direct emission from the AGN, rather than a scattered component. The absorption-corrected 2-10 keV X-ray luminosity of the buried AGN was estimated to be LX(2-10 keV) = 5 * 10^42 ergs s^-1. The intrinsic 2-10 keV X-ray luminosity and the 2-10 keV X-ray to infrared luminosity ratio are both comparable to values measured for Mrk 463, a Seyfert-2 galaxy of similar infrared luminosity.

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arXiv2003

Detection of the "Active" Nucleus of M32

M32 hosts a 2.5x10^6 solar mass central black hole, but signs of nuclear activity in this galaxy have long eluded detection. We report the first conclusive detection of the nucleus of M32 in X-rays, based on high-resolution, sensitive observations made with Chandra. The 2-10 keV luminosity is merely 9.4x10^35 erg/s, 3x10^-9 of the Eddington luminosity of the black hole. Weak diffuse emission, consistent with thermal gas at a temperature of 0.37 keV, is seen in an annular region ~100 pc from the center. We also present a deep, moderately high-resolution (0.9"), radio continuum observation obtained with the Very Large Array, which places a tight upper bound on the nuclear radio power at 8.4 GHz. We combine these new measurements with upper limits at other wavelengths to discuss implications for the nature of accretion onto the central black hole.

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arXiv2003

A Stringent Limit on the Accretion Luminosity of the Possible Central Black Hole in the Globular Cluster M15

The globular cluster M15 has recently been found to host a possible central black hole with a mass of ~2000 solar masses. A deep, high-resolution Chandra image failed to detect the "nucleus" of the cluster in X-rays. The upper limit on the X-ray luminosity (L_x < 5.6x10^32 erg/s) corresponds to a bolometric Eddington ratio of L_bol/L_Edd < (2-4)x10^-8. Combining this limit with an estimate of the electron density of the intracluster ionized plasma derived from pulsar dispersion measures, we show that the radiative efficiency of the accretion flow, if it accretes at the Bondi rate, must be much lower than that of a standard optically thick, geometrically thin disk.

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arXiv2002

Chandra Snapshot Observations of Low-Luminosity AGNs with a Compact Radio Source

The results of Chandra snapshot observations of 11 LINERs (Low-Ionization Nuclear Emission-line Regions), three low-luminosity Seyfert galaxies, and one HII-LINER transition object are presented. Our sample consists of all the objects with a flat or inverted spectrum compact radio core in the VLA survey of 48 low-luminosity AGNs (LLAGNs) by Nagar et al. (2000). An X-ray nucleus is detected in all galaxies except one and their X-ray luminosities are in the range 5x10^38 to 8x10^41 erg/s. The X-ray spectra are generally steeper than expected from thermal bremsstrahlung emission from an advection-dominated accretion flow (ADAF). The X-ray to Halpha luminosity ratios for 11 out of 14 objects are in good agreement with the value characteristic of LLAGNs and more luminous AGNs, and indicate that their optical emission lines are predominantly powered by a LLAGN. For three objects, this ratio is less than expected. Comparing with properties in other wavelengths, we find that these three galaxies are most likely to be heavily obscured AGN. We use the ratio RX = νLν(5 GHz)/LX, where LX is the luminosity in the 2-10 keV band, as a measure of radio loudness. In contrast to the usual definition of radio loudness (RO = Lν(5 GHz)/Lν(B)), RX can be used for heavily obscured (NH >~ 10^23 cm^-2, AV>50 mag) nuclei. Further, with the high spatial resolution of Chandra, the nuclear X-ray emission of LLAGNs is often easier to measure than the nuclear optical emission. We investigate the values of RX for LLAGNs, luminous Seyfert galaxies, quasars and radio galaxies and confirm the suggestion that a large fraction of LLAGNs are radio loud.

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arXiv2002

Chandra Snapshot Observations of LINERs with a Compact Radio Core

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arXiv2002

X-ray Emission from the Type Ic Supernova 1994I Observed with Chandra

We present two high-resolution Chandra X-ray observations of supernova (SN) 1994I which show, for the first time, that the interaction of the blast wave from a Type Ic SN with its surrounding circumstellar material (CSM) can give rise to soft X-ray emission. Given a 0.3-2 keV band X-ray luminosity of L_x ~ 1 x 10^{37} ergs/s between six and seven years after the outburst of SN 1994I, and assuming the X-ray emission arises from the shock-heated CSM, we derive a pre-SN mass-loss rate of \dot{M} ~ 1 x 10^{-5} M_sun/yr (v_w/10 km/s). Combining the results with earlier ROSAT observations, we construct the X-ray lightcurve of SN 1994I. A best-fit X-ray rate of decline of L_x \propto t^{-s} with index s~1 and a CSM density profile of rho_csm \propto r^{-1.9\pm0.1} are inferred, consistent with what is expected for a constant mass-loss rate and constant wind velocity profile for the SN progenitor (rho_csm \propto r^{-2}).

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arXiv2002

Ultraluminous X-ray Sources in M51

We present the results of Chandra observations of off-nuclear X-ray sources in the spiral galaxy M51 (NGC 5194). 113 X-ray sources have been detected in the field of view and 84 among them project within the disk of NGC 5194. Six and twenty eight sources have luminosities exceeding 1x10^39 ergs s^-1 and 1x10^38 ergs s^-1 in the 0.5-8 keV band, respectively. The number of luminous sources is much higher than normal spiral and elliptical galaxies and similar to galaxies experiencing starburst activity. X-ray spectra of most of the detected sources are consistent with a power law with a photon index between 1 and 2, while one source has an extremely hard spectrum and two sources have soft spectra. The spectra of three ultraluminous sources are consistent with both a power law and a multicolor disk blackbody (MCD) model, while a power law model is preferable to a MCD model in two objects. One luminous object which shows a remarkable spectrum, including emission lines, is also found and discussed.

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arXiv2002

X-ray Properties of LINERs and Low-luminosity Seyfert Galaxies Observed with ASCA. I. Observations and Results

This paper presents a comprehensive study of the X-ray properties of low-ionization nuclear emission-line regions (LINERs) and low-luminosity Seyfert galaxies based on observations obtained with ASCA. We analyzed data of 53 observations of 21 LINERs and 17 low-luminosity Seyferts. X-ray emission has been detected in all but one object. The X-ray luminosities in the 2-10 keV band range from 4e39 ergs/s to 5e41 ergs/s, which are 1-3 orders of magnitude smaller than in classical Seyfert galaxies. The X-ray spectra of most objects are well described by a canonical model which consists of (1) a soft component from a thermal plasma with kT<1 keV and (2) a hard component represented by a power law with a photon index~1.8 or thermal bremsstrahlung emission with kT~10 keV. Several objects do not require the soft thermal component, and their continua are well fitted by a single power-law model. Some objects show heavy absorption with column densities in excess of 1e23 cm^-2. We detect in several objects Fe K line emission with equivalent widths ranging from 50 eV to 2 keV. Variability on timescales less than a day is uncommon in our sample. By comparing multiple observations made with ASCA or with published observations from other satellites, we show that at least eight objects are variable on timescales of a week to several years. We find that the morphologies of many objects, both in the soft and hard bands, are consistent with being pointlike relative to the telescope PSF; a few are clearly extended in either or both energy bands.

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arXiv2002

Fe K Emission Lines in Low-luminosity AGNs

We present the results of a systematic analysis of Fe K emission lines in low-luminosity AGNs (LLAGNs) observed with ASCA. We used a sample of LLAGNs with small intrinsic absorption to compare with luminous AGNs and accretion models. Some objects show an Fe line at 6.4 keV or 6.7 keV, and the rest of the objects show no significant Fe K emission line. We made a composite spectrum of sources without detected Fe emission lines to search for weak lines and obtained an upper limit on the equivalent width (EW) of a narrow line at 6.4 keV of 145 eV, which is marginally consistent with EWs seen in Seyfert 1s. Even in those objects with detected Fe lines, a skewed broad component, typically observed in Seyfert 1s, is not seen. The weakness of the Fe line is consistent with the absence of an optically thick disk in the vicinity of a central black hole. Alternatively, it might be due to the ionization of a standard disk. The lack of big blue bump favors the former.

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arXiv2001

A Chandra X-ray Study of Cygnus A - II. The Nucleus

We report Chandra ACIS and quasi-simultaneous RXTE observations of the nearby, powerful radio galaxy Cygnus A, with the present paper focusing on the properties of the active nucleus. In the Chandra observation, the hard (> a few keV) X-ray emission is spatially unresolved with a size \approxlt 1 arcsec (1.5 kpc, H_0 = 50 km s^-1 Mpc^-1) and coincides with the radio and near infrared nuclei. In contrast, the soft (< 2 keV) emission exhibits a bi-polar nebulosity that aligns with the optical bi-polar continuum and emission-line structures and approximately with the radio jet. In particular, the soft X-ray emission corresponds very well with the [O III] λ5007 and Hα+ [N II] λλ6548, 6583 nebulosity imaged with HST. At the location of the nucleus there is only weak soft X-ray emission, an effect that may be intrinsic or result from a dust lane that crosses the nucleus perpendicular to the source axis. The spectra of the various X-ray components have been obtained by simultaneous fits to the 6 detectors. The compact nucleus is detected to 100 keV and is well described by a heavily absorbed power law spectrum with Γ_h = 1.52^{+0.12}_{-0.12} (similar to other narrow line radio galaxies) and equivalent hydrogen column N_H (nuc) = 2.0^{+0.1}_{-0.2} \times 10^{23} cm^-2. (Abstract truncated).

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arXiv2001

A Chandra X-ray Study of Cygnus A - III. The Cluster of Galaxies

The results from a recent Chandra ACIS-S study of the cluster surrounding Cygnus A are presented. We have deprojected the X-ray spectra taken from various elliptical shells in order to derive the run of temperature, density, pressure, and abundance for the ICM as a function of radius. We confirm a drop in temperature of the X-ray emitting gas from $\sim 8$ keV more than $\sim 2^{\prime}$ from the center to $\simeq 5$ keV some $30^{\prime\prime}$ from the center, with the coolest gas immediately adjacent to the radio galaxy. ``Belts'' of slightly cooler ($\simeq 4$ keV) X-ray emitting gas run around the minor dimension of the cavity created by the radio source. We find a metallicity gradient in the X-ray emitting gas, with the highest metallicities ($\sim$ solar) in the inner annuli, decreasing to $\sim 0.3$ solar in the outer parts. The total mass within 500 kpc is found to be (2.0--2.8) $\times 10^{14} M_{\odot}$ and approximately 3.5--5% of the mass is X-ray emitting gas [ABRIDGED].

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arXiv2001

A Chandra Observation of M51: Active Nucleus and Nuclear Outflows

We present a Chandra ACIS-S observation of the nuclear region of the nearby spiral galaxy M51 (NGC 5194), which has a low-luminosity Seyfert 2 nucleus. The X-ray image shows the nucleus, southern extranuclear cloud, and northern loop, the morphology of the extended emission being very similar to those seen in radio continuum and optical emission line images. The X-ray spectrum of the nucleus is well represented by a model consisting of soft thermal plasma with kT ~0.5 keV, a very hard continuum, and an Fe Kalpha emission line at 6.45 keV with an equivalent width of >2 keV. The very strong Fe line and the flat continuum indicate that the nucleus is obscured by a column density in excess of 10^24 cm^-2 and the spectrum is dominated by reflected emission from cold matter near the nucleus. The X-ray spectra of the extranuclear clouds are well fitted by a thermal plasma model with kT ~0.5 keV. This spectral shape and morphology strongly suggest that the clouds are shock heated by the bi-polar outflow from the nucleus. The shock velocities of the extranuclear cloud and northern loop inferred from the temperatures of the X-ray gas are 690 km/s and 660 km/s, respectively. By assuming a steady-state situation in which the emission of the extranuclear clouds is powered by the jets, the mechanical energy in the jets is found to be comparable to the bolometric luminosity of the nucleus.

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arXiv2000

Hard X-ray Emission and the Ionizing Source in LINERs

We report X-ray fluxes in the 2--10 keV band from LINERs (low-ionization nuclear emission-line regions) and low-luminosity Seyfert galaxies obtained with the ASCA satellite. Observed X-ray luminosities are in the range between 4e39 and 5e41 ergs/s, which are significantly smaller than that of the ``classical'' low-luminosity Seyfert 1 galaxy NGC 4051. We found that X-ray luminosities in 2--10 keV of LINERs with broad Halpha emission in their optical spectra (LINER 1s) are proportional to their Halpha luminosities. This correlation strongly supports the hypothesis that the dominant ionizing source in LINER 1s is photoionization by hard photons from low-luminosity AGNs. On the other hand, the X-ray luminosities of most LINERs without broad Halpha emission (LINER 2s) in our sample are lower than LINER 1s at a given Halpha luminosity. The observed X-ray luminosities in these objects are insufficient to power their Halpha luminosities, suggesting that their primary ionizing source is other than an AGN, or that an AGN, if present, is obscured even at energies above 2 keV.

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arXiv2000

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

The wide-band energy spectrum of NGC6240 over the range 0.5-200keV 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 20keV and the upper limits above 20keV. The spectrum above 10keV is found to be very flat. A strong iron-K emission line discovered in the previous ASCA observation is also confirmed with the RXTE PCA. These results provide further evidence for the dominance of a reflection component, i.e. emission from cool material illuminated by an AGN. By fitting the spectra obtained with RXTE and ASCA simultaneously, we satisfactorily modeled 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 4x10^43 - 6x10^44 ergs/s in the range 2-10keV, which categorizes NGC6240 among the most luminous Seyfert nuclei. The ratio of the 2-10keV X-ray luminosity to the infrared luminosity, L_X(2-10keV)/L_IR, is 0.01 - 0.1, which implies a quite substantial, if not dominant, contribution of AGN to the infrared luminosity.

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arXiv1999

X-ray Properties of the Weak Seyfert 1 Nucleus in NGC 4639

We obtained observations of NGC 4639 with ASCA in order to investigate its mildly active Seyfert 1 nucleus at hard X-ray energies. Koratkar et al. (1995) have previously shown that the nucleus is a pointlike source in the ROSAT soft X-ray band. We detected in the 2-10 keV band a compact central source with a luminosity of 8.3E+40 erg/s. Comparison of the ASCA data with archival data taken with the Einstein and ROSAT satellites shows that the nucleus varies on timescales of months to years. The variability could be intrinsic, or it could be caused by variable absorption. More rapid variability, on a timescale of \~10^4 s, may be present in the ASCA data. The spectrum from 0.5 to 10 keV is well described by a model consisting of a lightly absorbed (N_H = 7.3E+20 cm^-2) power law with a photon index of 1.68. We find no evidence for significant emission from a thermal plasma; if present, it can account for no more than 25% of the flux in the 0.5-2.0 keV band. The limited photon statistics of our data do not allow us to place significant limits on the presence of iron K emission. (abridged)

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arXiv1998

Detection of an iron K Emission Line from the LINER NGC 4579

We present the results of an ASCA observation of the LINER NGC 4579. A point-like X-ray source is detected at the nucleus with a 2-10 keV luminosity of 1.5x10^41 ergs/s assuming a distance of 16.8 Mpc. The X-ray spectrum is represented by a combination of a power-law with a photon index of ~1.7 and soft thermal component with kT~0.9 keV. An iron K emission line is detected at 6.73+/-0.13 keV (rest frame) with an equivalent width of 490 +180/-190 eV and is statistically significant at more than 99.9 % confidence. The line center energy is consistent with Helium-like iron and is significantly higher than 6.4 keV which is expected from fluorescence by "cold" (or a lower ionization state of) iron. The iron line profile shows no significant red tail in contrast to Seyfert 1 galaxies although the statistics are limited. The line center energy, equivalent width, and profile are consistent with an origin in an ionized accretion disk. However the large mass accretion rate necessary to ionize the accretion disk is not consistent with the observed luminosity and normal accretion models.

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