arXiv subjects

Taiki Kawamuro

Publications and source records attributed to Taiki Kawamuro.

68 records · Page 4Linked to original sources

arXiv2017

NuSTAR hard X-ray data and Gemini 3D spectra reveal powerful AGN and outflow histories in two low-redshift Lyman-$α$ blobs

We have shown that Lyman-$α$ blobs (LABs) may still exist even at $z\sim0.3$, about 7 billion years later than most other LABs known (Schirmer et al. 2016). Their luminous Ly$α$ and [OIII] emitters at $z\sim0.3$ offer new insights into the ionization mechanism. This paper focuses on the two X-ray brightest LABs at $z\sim0.3$, SDSS J0113$+$0106 (J0113) and SDSS J1155$-$0147 (J1155), comparable in size and luminosity to `B1', one of the best-studied LABs at $z \gtrsim$ 2. Our NuSTAR hard X-ray (3--30 keV) observations reveal powerful active galactic nuclei (AGN) with $L_{2-10{\;\rm keV}}=(0.5$--$3)\times10^{44}$ erg cm$^{-2}$ s$^{-1}$. J0113 also faded by a factor of $\sim 5$ between 2014 and 2016, emphasizing that variable AGN may cause apparent ionization deficits in LABs. Joint spectral analyses including Chandra data constrain column densities of $N_{\rm H}=5.1^{+3.1}_{-3.3}\times10^{23}$ cm$^{-2}$ (J0113) and $N_{\rm H}=6.0^{+1.4}_{-1.1}\times10^{22}$ cm$^{-2}$ (J1155). J0113 is likely buried in a torus with a narrow ionization cone, but ionizing radiation is also leaking in other directions as revealed by our Gemini/GMOS 3D spectroscopy. The latter shows a bipolar outflow over $10$ kpc, with a peculiar velocity profile that is best explained by AGN flickering. X-ray analysis of J1155 reveals a weakly absorbed AGN that may ionize over a wide solid angle, consistent with our 3D spectra. Extinction corrected [OIII] log-luminosities are high, $\sim43.6$. The velocity dispersions are low, $\sim100$--$150$ km s$^{-1}$, even at the AGN positions. We argue that this is a combination of high extinction hiding the turbulent gas, and previous outflows that have cleared the escape paths for their successors.

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arXiv2016

The Complete Infrared View of Active Galactic Nuclei from the 70-month Swift/BAT Catalog

We systematically investigate the near- (NIR) to far-infrared (FIR) photometric properties of a nearly complete sample of local active galactic nuclei (AGN) detected in the Swift/Burst Alert Telescope (BAT) all-sky ultra hard X-ray (14-195 keV) survey. Out of 606 non-blazar AGN in the Swift/BAT 70-month catalog at high galactic latitude of $|b|>10^{\circ}$, we obtain IR photometric data of 604 objects by cross-matching the AGN positions with catalogs from the WISE, AKARI, IRAS, and Herschel infrared observatories. We find a good correlation between the ultra-hard X-ray and mid-IR (MIR) luminosities over five orders of magnitude ($41 < \log (L_{14-195}/{\rm erg}~{\rm s}^{-1})< 46$). Informed by previous measures of the intrinsic spectral energy distribution of AGN, we find FIR pure-AGN candidates whose FIR emission is thought to be AGN-dominated with low starformation activity. We demonstrate that the dust covering factor decreases with the bolometric AGN luminosity, confirming the luminosity-dependent unified scheme. We also show that the completeness of the WISE color-color cut in selecting Swift/BAT AGN increases strongly with 14-195 keV luminosity.

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arXiv2016

X-ray and infrared diagnostics of nearby active galactic nuclei with MAXI and AKARI

Nearby active galactic nuclei were diagnosed in the X-ray and mid-to-far infrared wavelengths, with Monitor of All-sky X-ray Image (MAXI) and the Japanese infrared observatory AKARI, respectively. Among the X-ray sources listed in the second release of the MAXI all-sky X-ray source catalog, 100 ones are currently identified as a non-blazar-type active galactic nucleus. These include 95 Seyfert galaxies and 5 quasars, and they are composed of 73 type-1 and 27 type-2 objects. The AKARI all-sky survey point source catalog was searched for their mid- and far-infrared counterparts at 9, 18, and 90 $μ$m. As a result, 69 Seyfert galaxies in the MAXI catalog (48 type-1 and 21 type-2 ones) were found to be detected with AKARI. The X-ray (3-4 keV and 4-10 keV) and infrared luminosities of these objects were investigated, together with their color information. Adopting the canonical photon index, $Γ= 1.9$, of the intrinsic X-ray spectrum of the Seyfert galaxies, the X-ray hardness ratio between the 3-4 and 4-10 keV ranges derived with MAXI was roughly converted into the absorption column density. After the X-ray luminosity was corrected for absorption from the estimated column density, the well-known X-ray-to-infrared luminosity correlation was confirmed at least in the Compton-thin regime. In contrast, NGC 1365, only one Compton-thick object in the MAXI catalog, was found to deviate from the correlation toward a significantly lower X-ray luminosity by nearly an order of magnitude. It was verified that the relation between the X-ray hardness below 10 keV and X-ray-to-infrared color acts as an effective tool to pick up Compton-thick objects. The difference in the infrared colors between the type-1 and type-2 Seyfert galaxies and its physical implication on the classification and unification of active galactic nuclei were briefly discussed.

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arXiv2016

Suzaku Observations of Moderately Obscured (Compton-thin) Active Galactic Nuclei Selected by Swift/BAT Hard X-ray Survey

We report the results obtained by a systematic, broadband (0.5--150 keV) X-ray spectral analysis of moderately obscured (Compton-thin; $22 \leq \log N_{\rm H} < 24$) active galactic nuclei (AGNs) observed with Suzaku and Swift/Burst Alert Telescope (BAT). Our sample consists of 45 local AGNs at $z<0.1$ with $\log L_{\rm 14-195\1mmkeV} > 42$ detected in the Swift/BAT 70-month survey, whose Suzaku archival data are available as of 2015 December. All spectra are uniformly fit with a baseline model composed of an absorbed cutoff power-law component, reflected emission accompanied by a narrow fluorescent iron-K$α$ line from cold matter (torus), and scattered emission. Main results based on the above analysis are as follows. (1) The photon index is correlated with Eddington ratio, but not with luminosity or black hole mass. (2) The ratio of the iron-K$α$ line to X-ray luminosity, a torus covering fraction indicator, shows significant anti-correlation with luminosity. (3) The averaged reflection strength derived from stacked spectra above 14 keV is larger in less luminous ($\log L_{\rm 10-50\1mmkeV} \leq 43.3$; $R= 1.04^{+0.17}_{-0.19}$) or highly obscured AGNs ($\log N_{\rm H} > 23$; $R = 1.03^{+0.15}_{-0.17}$) than in more luminous ($\log L_{\rm 10-50\1mmkeV} > 43.3$; $R= 0.46^{+0.08}_{-0.09}$) or lightly obscured objects ($\log N_{\rm H} \leq 23$; $R = 0.59^{+0.09}_{-0.10}$), respectively. (4) The [O IV] 25.89 $μ$m line to X-ray luminosity ratio is significantly smaller in AGNs with lower soft X-ray scattering fractions, suggesting that the [O IV] 25.89 $μ$m luminosity underestimates the intrinsic power of an AGN buried in a small opening-angle torus.

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arXiv2016

Hard X-ray Luminosity Function of Tidal Disruption Events: First Results from MAXI Extragalactic Survey

We derive the first hard X-ray luminosity function (XLF) of stellar tidal disruption events (TDEs) by supermassive black holes (SMBHs), which gives an occurrence rate of TDEs per unit volume as a function of peak luminosity and redshift, utilizing an unbiased sample observed by the Monitor of All-sky X-ray Image (MAXI). On the basis of the light curves characterized by a power-law decay with an index of $-5/3$, a systematic search using the MAXI data in the first 37 months detected four TDEs, all of which have been found in the literature. To formulate the TDE XLF, we consider the mass function of SMBHs, that of disrupted stars, the specific TDE rate as a function of SMBH mass, and the fraction of TDEs with relativistic jets. We perform an unbinned maximum likelihood fit to the MAXI TDE list and check the consistency with the observed TDE rate in the ROSAT all sky survey. The results suggest that the intrinsic fraction of the jet-accompanying events is $0.0007$--$34\%$. We confirm that at $z \lesssim 1.5$ the contamination by TDEs to the hard X-ray luminosity functions of active galactic nuclei is not significant and hence that their contribution to the growth of SMBHs is negligible at the redshifts.

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arXiv2016

Study of Swift/BAT Selected Low-luminosity Active Galactic Nuclei Observed with Suzaku

We systematically analyze the broadband (0.5--200 keV) X-ray spectra of hard X-ray ($>10$ keV) selected local low-luminosity active galactic nuclei (LLAGNs) observed with {\it Suzaku} and {\it Swift}/BAT. The sample consists of ten LLAGNs detected with {\it Swift}/BAT with intrinsic 14--195 keV luminosities smaller than $10^{42}$ erg s$^{-1}$ available in the {\it Suzaku} archive, covering a wide range of the Eddington ratio from $10^{-5}$ to $10^{-2}$. The overall spectra can be reproduced with an absorbed cut-off power law, often accompanied by reflection components from distant cold matter, and/or optically-thin thermal emission from the host galaxy. In all objects, relativistic reflection components from the innermost disk are not required. Eight objects show a significant narrow iron-K$α$ emission line. Comparing their observed equivalent widths with the predictions from the Monte-Carlo based torus model by \cite{Ike09}, we constrain the column density in the equatorial plane to be $\log N^{\rm eq}_{\rm H} > 22.7$ or the torus half opening angle $θ_{\rm oa} < 70^\circ$. We infer that the Eddington ratio ($λ_{\rm Edd}$) is a key parameter that determines the torus structure of LLAGNs: the torus becomes large at $λ_{\rm Edd} \gtrsim 2\times10^{-4}$, whereas at lower accretion rates it is little developed. The luminosity correlation between the hard X-ray and mid-infrared (MIR) bands of the LLAGNs follows the same one as for more luminous AGNs. This implies that other mechanisms than AGN-heated dust are responsible for the MIR emission in low Eddington ratio LLAGNs.

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arXiv2016

A Soft X-Ray Lag Detected in Centaurus A

We performed time lag analysis on the X-ray light curves of Centaurus A (Cen A) obtained by the Gas Slit Camera (GSC) aboard the Monitor of All-sky X-ray Image ({\it MAXI}) in three energy bands (2--4 keV, 4--10 keV, and 10--20 keV). We discovered a soft X-ray lag relative to higher energies (soft lag) on a timescale of days in a flaring episode by employing the discrete correlation function (DCF) and the z-transformed discrete correlation function (ZDCF) method. In the episode, a peak and a centroid in the DCF and the ZDCF was observed at a soft lag of $\sim 5$ days in 2--4 keV versus 4--10 keV and in 4--10 keV versus 10--20 keV, and $\sim 10$ days in 2--4 keV versus 10--20 keV. We found it difficult to explain the observed X-ray variation by a single energy injection with the one-zone synchrotron self-Compton (SSC) model, in which the soft lags in these three energy bands reflect the different cooling times of the relativistic electrons, by assuming the magnetic field and minimum Lorentz factor estimated from a broad-band SED. Alternatively, if the phenomenon is interpreted as cooling of Comptonizing electrons in a corona covering the accretion disk, the temperature of the corona producing the variable X-rays should be $\sim$10 keV for reconciliation with the soft lag in the energy range of 2--20 keV.

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arXiv2016

Suzaku Follow-up of Heavily Obscured Active Galactic Nuclei Detected in Swift/BAT Survey: NGC 1106, UGC 03752, and NGC 2788A

We present the broadband (0.5-100 keV) spectra of three heavily obscured Active Galactic Nuclei (AGNs), NGC 1106, UGC 03752, and NGC 2788A, observed with Suzaku and Swift/Burst Alert Telescope (BAT). The targets are selected from the Swift/BAT 70-month catalog on the basis of high hardness ratio between above and below 10 keV, and their X-ray spectra are reported here for the first time. We apply three models, a conventional model utilizing an analytic reflection code and two Monte-Carlo based torus models with a doughnut-like geometry (MYTorus) and with a nearly spherical geometry (Ikeda torus). The three models can successfully reproduce the spectra, while the Ikeda torus model gives better description than the MYTorus model in all targets. We identify that NGC 1106 and NGC 2788A as Compton-thick AGNs. We point out that the common presence of unabsorbed reflection components below 7.1 keV in obscured AGNs, as observed from UGC 03752, is evidence for clumpy tori. This implies that detailed studies utilizing clumpy torus models are required to reach correct interpretation of the X-ray spectra of AGNs.

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arXiv2015

The sign of active galactic nucleus quenching in a merger remnant with radio jets

We investigate optical, infrared, and radio active galactic nucleus (AGN) signs in the merger remnant Arp 187, which hosts luminous jets launched in the order of $10^5$ yr ago but whose present-day AGN activity is still unknown. We find AGN signs from the optical BPT diagram and infrared [OIV]25.89 $μ$m line, originating from the narrow line regions of AGN. On the other hand, Spitzer/IRS show the host galaxy dominated spectra, suggesting that the thermal emission from the AGN torus is considerably small or already diminished. Combining the black hole mass, the upper limit of radio luminosity of the core, and the fundamental plane of the black hole enable us to estimate X-ray luminosity, which gives $<10^{40}$ erg s$^{-1}$. Those results suggest that the AGN activity of Arp 187 has already been quenched, but the narrow line region is still alive owing to the time delay of emission from the past AGN activity.

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arXiv2014

MAXI investigation into the longterm X-ray variability from the very-high-energy gamma-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 $α= 1.60 \pm 0.25$. This index is smaller than that obtained in the higher frequency range ($f > 1 \times 10^{-5}$ Hz), namely, $α= 2.14 \pm 0.06$ 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 ($α= 1$ -- $2$). The physical implications from these results are briefly discussed.

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arXiv2014

Suzaku Observation of the Black Hole Binary 4U 1630--47 in the Very High State

We report the results from an X-ray and near-infrared observation of the Galactic black hole binary 4U 1630--47 in the very high state, performed with {\it Suzaku} and IRSF around the peak of the 2012 September-October outburst. The X-ray spectrum is approximated by a steep power law, with photon index of 3.2, identifying the source as being in the very high state. A more detailed fit shows that the X-ray continuum is well described by a multi-color disc, together with thermal and non-thermal Comptonization. The inner disc appears slightly truncated by comparison with a previous high/soft state of this source, even taking into account energetic coupling between the disc and corona, although there are uncertainties due to the dust scattering correction. The near-infrared fluxes are higher than the extrapolated disc model, showing that there is a contribution from irradiation in the outer disk and/or the companion star at these wavelengths. Our X-ray spectra do not show the Doppler shifted iron emission lines indicating a baryonic jet which were seen four days previously in an XMM-Newton observation, despite the source being in a similar state. There are also no significant absorption lines from highly ionized irons as are seen in the previous high/soft state data. We show that the increased source luminosity is not enough on its own to make the wind so highly ionized as to be undetectable. This shows that the disc wind has changed in terms of its launch radius and/or density compared to the high/soft state.

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arXiv2014

MAXI observations of GRBs

Monitor of all-sky image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs) including the bursts with soft spectra, such as X-ray flashes (XRFs). MAXI/GSC is sensitive to the energy range from 2 to 30 keV. This energy range is lower than other currently operating instruments which is capable of detecting GRBs. Since the beginning of the MAXI operation on August 15, 2009, GSC observed 35 GRBs up to the middle of 2013. One third of them are also observed by other satellites. The rest of them show a trend to have soft spectra and low fluxes. Because of the contribution of those XRFs, the MAXI GRB rate is about three times higher than those expected from the BATSE log N - log P distribution. When we compare it to the observational results of the Wide-field X-ray Monitor on the High Energy Transient Explorer 2, which covers the the same energy range to that of MAXI/GSC, we find a possibility that many of MAXI bursts are XRFs with Epeak lower than 20 keV. We discuss the source of soft GRBs observed only by MAXI. The MAXI log N - log S distribution suggests that the MAXI XRFs distribute in closer distance than hard GRBs. Since the distributions of the hardness of galactic stellar flares and X-ray bursts overlap with those of MAXI GRBs, we discuss a possibility of a confusion of those galactic transients with the MAXI GRB samples.

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arXiv2013

The 37-month MAXI/GSC source catalog in the high Galactic-latitude sky

We present the catalog of high Galactic-latitude ($|b|>10^{\circ}$) X-ray sources detected in the first 37-month data of Monitor of All-sky X-ray Image (MAXI) / Gas Slit Camera (GSC). To achieve the best sensitivity, we develop a background model of the GSC that well reproduces the data based on the detailed on-board calibration. Source detection is performed through image fit with the Poisson likelihood algorithm. The catalog contains 500 objects detected in the 4--10 keV band with significance of $s_{\rm D, 4-10 keV} \geq 7$. The limiting sensitivity is $\approx 7.5\times10^{-12}$ ergs cm$^{-2}$ s$^{-1}$ ($\approx 0.6$ mCrab) in the 4--10 keV band for 50% of the survey area, which is the highest ever achieved as an all-sky survey mission covering this energy band. We summarize the statistical properties of the catalog and results from cross matching with the Swift/BAT 70-month catalog, the meta-catalog of X-ray detected clusters of galaxies, and the MAXI/GSC 7-month catalog. Our catalog lists the source name (2MAXI), position and its error, detection significances and fluxes in the 4--10 keV and 3--4 keV bands, their hardness ratio, and basic information of the likely counterpart available for 296 sources.

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arXiv2013

Broad Band X-Ray Spectra of Two Low-luminosity Active Galactic Nuclei NGC 1566 and NGC 4941

We report the first broad band X-ray spectra of the low luminosity active galactic nuclei (LLAGN) NGC 1566 (type 1.5) and NGC 4941 (type 2) observed with Suzaku and Swift/BAT covering the 0.5-195 keV band. Both targets have hard X-ray luminosities of ~ 10^{41-42} ergs/s in the 15-55 keV band. The spectra of the nucleus are well reproduced by a sum of partially or fully covered transmitted emission and its reflection from the accretion disk, reprocessed emission from the torus accompanied by a strong narrow iron-Kαline, and a scattered component (for NGC 4941). We do not significantly detect a broad iron-Kαline from the inner accretion disk in both targets, and obtain an upper limit on the corresponding solid angle of Ω/2π< 0.3 in NGC 1566. The reflection strength from the torus is moderate, Ω/2π=0.45^{+0.13}_{-0.10} in NGC 1566 and Ω/2π=0.64^{+0.69}_{-0.27} in NGC 4941. Comparison of the equivalent width of the narrow iron-Kαline with a model prediction based on a simple torus geometry constrains its half-opening angle to be θ_oa ~ 60-70 degree in NGC 4941. These results agree with the obscured AGN fraction obtained from hard X-ray and mid-infrared selected samples at similar luminosities. Our results support the implication that the averaged covering fraction of AGN tori is peaked at L 10^{42-43} ergs/s but decreases toward lower luminosities.

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