248 research outputs found
Spectrographic Polarimeter and Method of Recording State of Polarity
A single-shot real-time spectropolarimeter for use in astronomy and other sciences that captures and encodes some or all of the Stokes polarization parameters simultaneously using only static, robust optical components with no moving parts is described. The polarization information is encoded onto the spectrograph at each wavelength along the spatial dimension of the 2D output data array. The varying embodiments of the concept include both a two-Stokes implementation (in which any two of the three Stokes polarization parameters are measured) and a full Stokes implementation (in which all three of the Stokes polarization parameters are measured), each of which is provided in either single beam or dual beam forms
Low radiative efficiency accretion at work in active galactic nuclei: the nuclear spectral energy distribution of NGC4565
We derive the spectral energy distribution (SED) of the nucleus of the
Seyfert galaxy NGC4565. Despite its classification as a Seyfert2, the nuclear
source is substantially unabsorbed. The absorption we find from Chandra data
(N_H=2.5 X 10^21 cm^-2) is consistent with that produced by material in the
galactic disk of the host galaxy. HST images show a nuclear unresolved source
in all of the available observations, from the near-IR H band to the optical U
band. The SED is completely different from that of Seyfert galaxies and QSO, as
it appears basically ``flat'' in the IR-optical region, with a small drop-off
in the U-band. The location of the object in diagnostic planes for low
luminosity AGNs excludes a jet origin for the optical nucleus, and its
extremely low Eddington ratio L_o/L_Edd indicates that the radiation we observe
is most likely produced in a radiatively inefficient accretion flow (RIAF).
This would make NGC4565 the first AGN in which an ADAF-like process is
identified in the optical. We find that the relatively high [OIII] flux
observed from the ground cannot be all produced in the nucleus. Therefore, an
extended NLR must exist in this object. This may be interpreted in the
framework of two different scenarios: i) the radiation from ADAFs is sufficient
to give rise to high ionization emission-line regions through photoionization,
or ii) the nuclear source has recently ``turned-off'', switching from a
high-efficiency accretion regime to the present low-efficiency state.Comment: 7 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Will Jets Identify the Progenitors of Type Ia Supernovae?
We use the fact that a Type Ia supernova has been serendipitously discovered
near the jet of the active galaxy 3C 78 to examine the question of whether jets
can enhance accretion onto white dwarfs. One interesting outcome of such a
jet-induced accretion process is an enhanced rate of novae in the vicinity of
jets. We present results of observations of the jet in M87 which appear to have
indeed discovered 11 novae in close proximity to the jet. We show that a
confirmation of the relation between jets and novae and Type Ia supernovae can
finally identify the elusive progenitors of Type Ia supernovae.Comment: 10 pages, 3 figure
HST emission-line images of nearby 3CR radio galaxies: two photoionization, accretion and feedback modes
We present HST/ACS narrow-band images of a low-z sample of 19 3C radio
galaxies to study the H and [OIII] emissions from the narrow-line
region (NLR). Based on nuclear emission line ratios, we divide the sample into
High and Low Excitation Galaxies (HEGs and LEGs). We observe different line
morphologies, extended line emission on kpc scale, large [OIII]/H
scatter across the galaxies, and a radio-line alignment. In general, HEGs show
more prominent emission line properties than LEGs: larger, more disturbed, more
luminous, and more massive regions of ionized gas with slightly larger covering
factors. We find evidence of correlations between line luminosities and (radio
and X-ray) nuclear luminosities. All these results point to a main common
origin, the active nucleus, which ionize the surrounding gas. However, the
contribution of additional photoionization mechanism (jet shocks and star
formation) are needed to account for the different line properties of the two
classes. A relationship between the accretion, photoionization and feedback
modes emerges from this study. For LEGs (hot-gas accretors), the synchrotron
emission from the jet represents the main source of ionizing photons. The lack
of cold gas and star formation in their hosts accounts for the moderate
ionized-gas masses and sizes. For HEGs (cold-gas accretors), an ionizing
continuum from a standard disk and shocks from the powerful jets are the main
sources of photoionization, with the contribution from star formation. These
components, combined with the large reservoir of cold/dust gas brought from a
recent merger, account for the properties of their extended emission-line
regions.Comment: accepted for publication on ApJ (22 pages, 12 figures
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