arXiv subjects

Makoto Kishimoto

Publications and source records attributed to Makoto Kishimoto.

46 records · Page 3Linked to original sources

arXiv2005

The dust-eliminated shape of quasar spectra in the near-infrared: a hidden part of the big blue bump

The near-infrared shape of the big blue bump component in quasar spectra has been essentially unknown. It usually cannot be observed directly, due to the strong hot dust emission which dominates quasar spectra longward of ~1micron. However this is quite an important part of the spectrum theoretically. At least bare disk models provide quite a robust prediction for the overall continuum shape in the near-infrared. Self-gravity should become important in the outer, near-infrared emitting regions of the putative disk, possibly leaving a signature of disk truncation in the near-infrared. We propose here that this important part of the spectrum can be revealed for the first time by observing polarized flux from normal quasars. At least in some polarized quasars, the emission lines are all unpolarized and so the polarized flux should originate interior to the broad line region, and therefore also interior to the dust emitting region. This can then be used to eliminate the dust emission. We present the results of near-infrared polarimetry for such three quasars (Ton202, 4C37.43, B2 1208+32). The data for Ton202 have the highest S/N, and the near-infrared polarized flux in this case is measured to have quite a blue shape, nu^+0.42+-0.29 in F_nu, intriguingly consistent with the simple multi-temperature black body, bare disk prediction of nu^+1/3. All these data, although still with quite low S/N for the other two objects, demonstrate the unique potential of the technique with future better data. We also present similar data for other quasars and radio galaxies, and briefly discuss the nature of the polarization.

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arXiv2004

Revealing AGN by Polarimetry

Polarimetric study in the UV and optical has been one of the keys to reveal the structure and nature of active galactic nuclei (AGN). Combined with the HST's high spatial resolution, it has directly confirmed the predicted scattering geometry of ~100 pc scale and accurately located the hidden nuclear position in some nearby active galaxies. Recently, we are using optical spectropolarimetry to reveal the nature of the accretion flow in the central engine of quasars. This is to use polarized light to de-contaminate the spectrum and investigate the Balmer edge spectral feature, which is otherwise buried under the strong emission from the outer region surrounding the central engine.

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arXiv2004

The buried Balmer-edge signatures from quasars

In our previous paper, we have reported the detection of a Balmer edge absorption feature in the polarized flux of one quasar (Ton 202). We have now found similar Balmer edge features in the polarized flux of four more quasars (4C09.72, 3C95, B2 1208+32, 3C323.1), and possibly a few more, out of 14 newly observed with the VLT and Keck telescopes. In addition, we also re-observed Ton 202, but we did not detect such a dramatic feature, apparently due to polarization variability (the two observations are one-year apart). The polarization measurements of some quasars are affected by an interstellar polarization in our Galaxy, but the measurements have been corrected for this effect reasonably well. Since the broad emission lines are essentially unpolarized and the polarization is confined only to the continuum in the five quasars including Ton 202 in both epochs, the polarized flux is considered to originate interior to the broad emission line region. The Balmer edge feature seen in the polarized flux is most simply interpreted as an intrinsic spectral feature of the quasar UV/optical continuum, or the ``Big Blue Bump'' emission. In this case, the edge feature seen in absorption indeed indicates the thermal and optically-thick nature of the continuum emitted. However, we also discuss other possible interpretations.

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arXiv2004

Accretion around supermassive black holes: the detection of the Balmer edge signature from quasars

The ultraviolet/optical continuum of quasars is thought to be from an accretion flow around a supermassive black hole, and it dominates the radiative output of quasars. However, the nature of this emission, often called the Big Blue Bump, has not been well understood. Robust evidence for its thermal nature would be a continuum opacity edge feature intrinsic to this component, but this has not been clearly confirmed despite the predictions by many models. We are now developing and exploiting a new method to detect the Balmer edge of hydrogen opacity. The method overcomes for certain objects the major obstacle of the heavy contamination from emissions outside the nucleus by taking the polarized flux spectrum. If our interpretation of the polarized flux is correct, our data show that the Big Blue Bump emission has a Balmer edge in absorption, indicating that the emission is indeed thermal, and the emitter is optically thick.

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arXiv2003

A First Close Look at the Balmer Edge Behavior of the Quasar Big Blue Bump

We have found for the first time a Balmer edge feature in the polarized flux spectrum of a quasar (Ton 202). The edge feature is seen as a discontinuity in the slope, rather than as a discontinuity in the absolute flux. Since the polarized flux contains essentially no broad emission lines, it is considered to arise interior to the broad emission line region, showing the spectrum with all the emissions outside the nucleus scraped off and removed. Therefore, the polarized flux spectrum is likely to reveal features intrinsic to the Big Blue Bump emission. In this case, the existence of the Balmer edge feature, seen in absorption in the shorter wavelength side, indicates that the Big Blue Bump is indeed thermal and optically thick.

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arXiv2002

A First Close Look at the Balmer-edge Behavior of the Quasar Big Blue Bump

We have found for the first time a Balmer edge feature in the Big Blue Bump emission of a quasar. The feature is seen in the polarized flux spectrum of the quasar, where all the emissions from outside the nucleus are scraped off and removed. The existence of the Balmer-edge absorption feature directly indicates that the Big Blue Bump is indeed thermal and optically-thick.

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arXiv2001

UV Imaging Polarimetry of the peculiar Seyfert 2 galaxy Mrk 477

We present the results of UV imaging polarimetry of the Seyfert 2 galaxy Mrk 477 taken by the Faint Object Camera onboard the Hubble Space Telescope (HST). From a previous HST UV image (lambda ~ 2180A), Mrk 477 has been known to have a pointlike bright UV hotspot in the central region, peculiar among nearby Seyfert 2 galaxies. There are also claims of UV/optical variability, unusual for a Seyfert 2 galaxy. Our data show that there is an off-nuclear scattering region ~ 0."6 (~ 500 pc) NE from the hotspot. The data, after the subtraction of the instrumental effect due to this bright hotspot region, might indicate that the scattered light is also detected in the central 0."2 radius region and is extended to a very wide angle. The hotspot location is consistent with the symmetry center of the PA pattern, which represents the location of the hidden nucleus, but our data do not provide a strong upper limit to the distance between the symmetry center and the hotspot. We have obtained high spatial resolution color map of the continuum which shows that the nuclear spiral arm of 0."4 scale (~ 300pc) is significantly bluer than the off-nuclear mirror and the hotspot region. The nature of the hotspot is briefly discussed.

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arXiv2001

UV Imaging Polarimetry of the Seyfert 2 Galaxy Mrk 3

We present UV imaging polarimetry data of the Seyfert 2 galaxy Mrk 3 taken by the Hubble Space Telescope. The polarized flux is found to be extended to ~1 kpc from the nucleus, and the position angles of polarization are centrosymmetric, confirming that the polarization is caused by scattering. We determine the location of the hidden nucleus as the center of this centrosymmetric pattern. From the polarization images taken in two broad bands, we have obtained the color distribution of the polarized flux. Some regions have blue polarized flux, consistent with optically-thin dust scattering, but some bright knots have a color similar to that of Seyfert 1 nucleus. Also, the recent Chandra X-ray observation suggests that the ratio of scattered UV flux to scattered X-ray flux is rather similar to the intrinsic UV/X-ray ratio in a Seyfert 1 nucleus, if the observed extended X-ray continuum is scattered light. While the scattered X-ray would be essentially from electron scattering, the UV slope and UV/X-ray ratio both being similar to Seyfert 1's would lead to two possibilities as to the nature of the UV scatterers. One is that the UV may also be scattered by electrons, in which case the scattering gas is somehow dust-free. The other is that the UV is scattered by dust grains, but the wavelength-independent UV scattering with low efficiency indicated by the UV slope and UV/X-ray ratio would suggest that the grains reside in UV-opaque clouds, or the dust might be mainly composed of large grains and lacks small-grain population.

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arXiv2000

UV Spectropolarimetry of Narrow-line Radio Galaxies

We present the results of UV spectropolarimetry (2000 - 3000A) and far-UV spectroscopy (1500 - 2000A) of two low-redshift narrow-line radio galaxies (NLRGs) taken with the Faint Object Spectrograph onboard the Hubble Space Telescope (HST). Spectropolarimetry of several NLRGs has shown that, by the presence of broad permitted lines in polarized flux spectrum, they have hidden quasars seen through scattered light. Imaging polarimetry has shown that NLRGs including our targets often have large scattering regions of a few kpc to >~10 kpc scale. This has posed a problem about the nature of the scatterers in these radio galaxies. Their polarized continuum has the spectral index similar to or no bluer than that of quasars, which favors electrons as the dominant scattering particles. The large scattering region size, however, favors dust scattering, because of its higher scattering efficiency compared to electrons. In this paper, we investigate the polarized flux spectrum over a wide wavelength range, combining our UV data with previous optical/infrared polarimetry data. We infer that the scattering would be often caused by opaque dust clouds in the NLRGs and this would be a part of the reason for the apparently grey scattering. In the high-redshift radio galaxies, these opaque clouds could be the proto-galactic subunits inferred to be seen in the HST images. However, we still cannot rule out the possibility of electron scattering, which could imply the existence of a large gas mass surrounding these radio galaxies.

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arXiv1999

The Location of the Nucleus of NGC 1068 and the Three-dimensional Structure of Its Nuclear Region

The HST archival UV imaging polarimetry data of NGC 1068 is re-examined. Through an extensive estimation of the observational errors, we discuss whether the distribution of the position angles (PAs) of polarization is simply centrosymmetric or not. Taking into account the effect of a bad focus at the time of the observation, we conclude that, within the accuracy of HST/FOC polarimetry, the PA distribution is completely centrosymmetric. This means that the UV polarization originates only from scattering of the radiation from a central point-like source. However, our analysis shows that the most probable location of the nucleus is only ~0.''08 (~6pc) south from the brightest cloud called ``cloud B''. The error circle of 99% confidence level extends to cloud B and to ``cloud A'' which is about 0.''2 south of cloud B. By this FOC observation, Cloud B is only marginally rejected as the nucleus. Assuming that the UV flux is dominated by electron-scattered light, we have also derived a three-dimensional structure of the nuclear region. The inferred distribution suggests a linear structure which could be related to the radio jet.

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