730 research outputs found
BRIDGE: A Direct-tree Hybrid N-body Algorithm for Fully Self-consistent Simulations of Star Clusters and their Parent Galaxies
We developed a new direct-tree hybrid N-body algorithm for fully
self-consistent N-body simulations of star clusters in their parent galaxies.
In such simulations, star clusters need high accuracy, while galaxies need a
fast scheme because of the large number of the particles required to model it.
In our new algorithm, the internal motion of the star cluster is calculated
accurately using the direct Hermite scheme with individual timesteps and all
other motions are calculated using the tree code with second-order leapfrog
integrator. The direct and tree schemes are combined using an extension of the
mixed variable symplectic (MVS) scheme. Thus, the Hamiltonian corresponding to
everything other than the internal motion of the star cluster is integrated
with the leapfrog, which is symplectic. Using this algorithm, we performed
fully self-consistent N-body simulations of star clusters in their parent
galaxy. The internal and orbital evolutions of the star cluster agreed well
with those obtained using the direct scheme. We also performed fully
self-consistent N-body simulation for large-N models (). In
this case, the calculation speed was seven times faster than what would be if
the direct scheme was used.Comment: 12 pages, 13 figures, Accepted for PAS
ASCA PV observations of the Seyfert 2 galaxy NGC 4388: the obscured nucleus and its X-ray emission
We present results on the Seyfert 2 galaxy NGC4388 in the Virgo cluster
observed with ASCA during its PV phase. The 0.5-10 keV X-ray spectrum consists
of multiple components; (1) a continuum component heavily absorbed by a column
density NH = 4E23 cm-2 above 3 keV; (2) a strong 6.4 keV line (EW = 500 eV);
(3) a weak flat continuum between 1 and 3 keV; and (4) excess soft X-ray
emission below 1 keV. The detection of strong absorption for the hard X-ray
component is firm evidence for an obscured active nucleus in this Seyfert 2
galaxy. The absorption corrected X-ray luminosity is about 2E42 erg/s. This is
the first time that the fluorescent iron-K line has been detected in this
object. The flat spectrum in the intermediate energy range may be a scattered
continuum from the central source. The soft X-ray emission below 1 keV can be
thermal emission from a temperature kT = 0.5 keV, consistent with the spatially
extended emission observed by ROSAT HRI. However, the low abundance (0.05 Zs)
and high mass flow rate required for the thermal model and an iron-K line
stronger than expected from the obscuring torus model are puzzling. An
alternative consistent solution can be obtained if the central source was a
hundred times more luminous over than a thousand years ago. All the X-ray
emission below 3 keV is then scattered radiation.Comment: 9 pages, 5 Postscript figures, to be published in MNRA
Detection of an X-ray periodicity in the Seyfert galaxy IRAS18325-5926
We report the detection of a 58 ks (16 hr) periodicity in the 0.5-10 keV
X-ray light curve of the Seyfert galaxy IRAS18325-5926 (Fairall49), obtained
from a 5-day ASCA observation. Nearly 9 cycles of the periodic variation are
seen; it shows no strong energy dependence and has an amplitude of about 15 per
cent. Unlike most other well-studied Seyfert galaxies, there is no evidence for
strong power-law red noise in the X-ray power spectrum of IRAS18325-5926.
Scaling from the QPOs found in Galactic black hole candidates suggests that the
mass of the black hole in IRAS18325-5926 is (6-40) million solar masses.Comment: 5 pages, 4 Postscript figures, to be published in MNRA
Evolution of Star Clusters near the Galactic Center: Fully Self-consistent N-body Simulations
We have performed fully self-consistent -body simulations of star clusters
near the Galactic center (GC). Such simulations have not been performed because
it is difficult to perform fast and accurate simulations of such systems using
conventional methods. We used the Bridge code, which integrates the parent
galaxy using the tree algorithm and the star cluster using the fourth-order
Hermite scheme with individual timestep. The interaction between the parent
galaxy and the star cluster is calculate with the tree algorithm. Therefore,
the Bridge code can handle both the orbital and internal evolutions of star
clusters correctly at the same time. We investigated the evolution of star
clusters using the Bridge code and compared the results with previous studies.
We found that 1) the inspiral timescale of the star clusters is shorter than
that obtained with "traditional" simulations, in which the orbital evolution of
star clusters is calculated analytically using the dynamical friction formula
and 2) the core collapse of the star cluster increases the core density and
help the cluster survive. The initial conditions of star clusters is not so
severe as previously suggested.Comment: 19 pages, 19 figures, accepted for publication in Ap
Iron K-alpha Fluorescent Line Profiles from Spiral Accretion Flows in AGNs
We present 6.4 keV iron K-alpha fluorescent line profiles predicted for a
relativistic black hole accretion disk in the presence of a spiral motion in
Kerr geometry, the work extended from an earlier literature motivated by recent
magnetohydrodynamic (MHD) simulations. The velocity field of the spiral motion,
superposed on the background Keplerian flow, results in a complicated redshift
distribution in the accretion disk. An X-ray source attributed to a localized
flaring region on the black hole symmetry axis illuminates the iron in the
disk. The emissivity form becomes very steep because of the light bending
effect from the primary X-ray source to the disk. The predicted line profile is
calculated for various spiral waves, and we found, regardless of the source
height, that: (i) a multiple-peak along with a classical double-peak structure
generally appears, (ii) such a multiple-peak can be categorized into two types,
sharp sub-peaks and periodic spiky peaks, (iii) a tightly-packed spiral wave
tends to produce more spiky multiple peaks, whereas (iv) a spiral wave with a
larger amplitude seems to generate more sharp sub-peaks, (v) the effect seems
to be less significant when the spiral wave is centrally concentrated, (vi) the
line shape may show a drastic change (forming a double-peak, triple-peak or
multiple-peak feature) as the spiral wave rotates with the disk. Our results
emphasize that around a rapidly-rotating black hole an extremely redshifted
iron line profile with a noticeable spike-like feature can be realized in the
presence of the spiral wave. Future X-ray observations, from {\it Astro-E2} for
example, will have sufficient spectral resolution for testing our spiral wave
model which exhibits unique spike-like features.Comment: 30 pages, 10 figures, submitted to ApJ, will be presented at 204th
Meeting of AAS in Denve
On broad iron K-alpha lines in Seyfert 1 galaxies
The X-ray spectrum obtained by Tanaka et al from a long observation of the
active galaxy MCG shows a broad iron K line skewed to low
energies. The simplest interpretation of the shape of the line is that it is
due to doppler and gravitational redshifts from the inner parts of a disk about
a massive black hole. Similarly broad lines are evident in shorter observations
of several other active galaxies. In this paper we investigate other line
broadening and skewing mechanisms such as Comptonization in cold gas and
doppler shifts from outflows. We have also fitted complex spectral models to
the data of MCG to see whether the broad skewed line can be mimicked
well by other absorption or emission features. No satisfactory mechanism or
spectral model is found, thus strengthening the relativistic disk line model.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Pinning down the pairing symmetry of heavy-fermion compound CeIrIn 5
From the thermal transport measurements in rotating magnetic fields H, we pinned down the superconducting gap structure of CeIrIn5. Clear fourfold oscillation was observed when H is rotated within the ab-plane, while no discernible oscillation was observed within the bc-plane. In sharp contrast to previous reports, our results are most consistent with dx2-y2 symmetry, implying that the superconductivity of CeIrIn5 is mediated by antiferromagnetic spin fluctuations as well as that of CeRhIn5 and CeCoIn5. © 2009 IOP Publishing Ltd
- …