CITA Research September 2002 - August 2003
High Energy Astrophysics and Stellar Physics
CITA hosts a broad program of research on fundamental problems in
radiative physics, accretion, and stellar oscillations, in a wide range
of astrophysical settings: neutron stars, black holes, AGN, and
gamma-ray bursts.
X-ray observation of a Seyfert 1 galaxy
David Ballantyne, S. Vaughan and A.C. Fabian (both at Cambridge)
analyzed a 300 ks XMM-Newton observation of the Seyfert 1 galaxy
MCG-6-30-15. The X-ray spectrum exhibits a very strong and broad
iron K line indicative of emission from a relativistic accretion
disk a few Schwarzschild radii away from the black hole. They
proposed a novel origin for the iron line by showing that it could
be split in two: with the broad red wing arising from the
inner-most ionized region of the accretion disk, and the central
line core originating from the outer regions of a warped disk.
X-ray spectra of accreting black holes
David Ballantyne and A.C. Fabian (Cambridge) examined the
possibility that at least part of the iron K line emission commonly
observed in the X-ray spectra of accreting black hole may be caused
by particle excitation rather than photoionization. They considered
beams of thermal and non-thermal electrons, as well as thermal
protons, incident on a layer of iron atoms, and calculated the
amount of Fe K line flux produced by the impact of the incoming
particle. After comparing this flux to that computed from
photoionization (assuming the same total energy flux), they found
that particle impact was a very inefficient means of producing Fe K
line photons.
Absorbing dust in Seyfert 1 galaxies
David Ballantyne, Joe Weingartner and Norm Murray pointed out that
part of the dusty warm absorber in the Seyfert 1 galaxy
MCG-6-30-15 may originate in the dust lane that is observed to cross
the galaxy just below the nucleus. Simple photoionization modeling
showed that dusty interstellar gas can produce soft X-ray absorption
features even if its 100-200 pc away from the active nucleus. They
also discussed various mechanisms to deplete the silicate grains in
such an environment since observations have shown that neutral oxygen
edge may be very weak in this source.
Density inhomogeneities in accretion disks
David Ballantyne, Neal Turner and Omer Blaes (both at the University
of California, Santa Barbara) began an investigation into the effects
of density inhomogeneities in accretion disks on the X-ray reflection
spectrum. Recent numerical simulations of radiation pressure dominated
disks have shown that significant density contrasts can occur in the
fluid, which may persist down to small scales near the surface. They
found that if density changes occurred within two Thomson depths of
the surface then, depending on the illumination strength, significant
changes reflection features, such as the iron K line, may occur.
Neutron-fed blast waves in GRBs
Andrei Beloborodov made detailed calculations of the nuclear composition
of GRB fireballs and showed that a significant part of ejected baryons
are free neutrons with a Lorentz factor 100-1000. Their lifetime is
increased by 100-1000 due to relativistic time dilation, and neutrons
survive till the decelerating stage of the explosion. The neutrons
overtake the decelerating blast wave and deposit mass, momentum, and
heat into the external medium via beta-decay. This qualitatively
changes the mechanism of GRB afterglow emission at radii up to ten
times the mean radius of beta-decay, which covers the main peak of
blast wave emission and a part of its subsequent fall-off tail.
The exponentially decaying neutron precursor changes the standard
picture of self-similar blast waves in GRBs, and this can be tested
by observations of early afterglows by the upcoming Swift mission.
Beta-decay heating of GRB fireballs
Elena Rossi (IoA, Cambridge), Andrei Beloborodov, and Martin Rees
(IoA, Cambridge) investigated the impact of neutron component
on the intrinsic dynamics of GRB fireballs and their thermal history.
Beta-decay of a small fraction of neutrons at early times of the
fireball expansion was found to result in significant heating of
the fireball material. A popular model of GRB emission envisions
a fireball with a strongly inhomogeneous Lorentz factor, which leads
to internal shocks. The decaying neutron component serves as a
decelerating background for the fastest shells in the fireball and
reduces the contrast of Lorentz factors. As a result, neutrons reduce
the amplitude of internal shocks or suppress them completely. This
may change the view of internal shocks as a dominant mechanism of
gamma-ray emission in GRBs.
Currents in twisted magnetospheres of neutron stars
Andrei Beloborodov and Chris Thompson investigated the formation of
currents in twisted magnetospheres of strongly magnetized neutron
stars. The current is dictated by the twist and carried by ions and
electrons which are pulled out of the star by an electric field.
Charges go up to the magnetosphere and, following the closed magnetic
line, fall back to the star. It was shown that the presence of the
gravitational potential barrier between cathode and anode excludes
a steady current. To undestand the time-dependent behavior of the
system a numerical simulation has been done. It gave a surprising
result: particles carrying the current develop enormous energies in
the magnetosphere, much higher than needed to overcome the potential
barrier. If true, neutron-star magnetospheres could not have significant
twists because they dissipate quickly. The simulation, however,
neglects the coupling of currents on neighboring field lines, which
can change the result. This is a subject of current investigation.
Jet structure in Gamma Ray Bursts
Nicole Lloyd-Ronning focused most of her research on investigating the
jet structure of Gamma-Ray Bursts. With Bing Zhang and Xinyu Dai
(Penn State), she explored the possibility that GRBs adhere to a
quasi-universal jet structure, in which all GRBs are have
approximately the same energy release as a function of jet angle, but
the difference in their observed properties is due to different
observer viewing angles (or - in other words - orientation of the GRB
jet toward the observer). They found that both power-law and Gaussian
models for energy as a function of angle can adequately reproduce the
observations, and that this quasi-universal picture is indeed a viable
model for GRB jets.
Investigation of electron capture rates
In collaboration with K. Langanke (Univ. of Aarhus),
G. Martinez-Pinedo (Institut d'Estudis Espacials de Cataluny,
Barcelona, IEEC), J. M. Sampaio (Univ. of Aarhus), D. J. Dean (ORNL),
W. R. Hix (University of Tennessee, Knoxville, UTK), O. E. B. Messer
(UTK), A. Mezzacappa (ORNL), H.-T. Janka (MPA), and M. Rampp (MPA),
Liebendoerfer investigated the electron capture rates on heavy
nuclei. They found that Pauli blocking does not occur to the extent
assumed in previous core collapse simulations. Simulations with the
improved rates showed that these ignored reactions actually dominate
throughout core collapse. As electron capture rates and neutrino
transport during stellar core collapse determine the degree of
deleptonization of the inner core, these findings might have important
consequences for the onset of a supernova explosion.
Impact of weak interactions
P. Hauser (Univ. of Basel), G. Martinez-Pinedo (IEEC), M. Liebendoerfer,
W. R. Hix (UTK), and F.-K. Thielemann (Univ. of Basel) investigated
the impact of weak interactions in the vicinity of the mass cut
in nucleosynthesis calculations for supernova explosions. Previous
calculations assumed an unchanged progenitor composition. It turns out
that the large neutrino fluxes drive the electron fraction to higher
values, sometimes even above Ye=0.5. As the data from observational
analysis of supernova ejecta on metal-poor stars accumulate, a new
generation of supernova nucleosynthesis calculations may shed new light
on the conditions during the supernova explosion.
Neutron star merger
S. Rosswog (Univ. of Leicester) and M. Liebendoerfer carried out a
three-dimensional simulation of a neutron star merger with a realistic
equation of state and a new neutrino leakage scheme. A high resolution
smoothed particle hydrodynamics method was used for this second merger
calculation that includes neutrino physics. The common features and
differences in the neutrino signal with respect to the signal from a
supernova have been worked out.
Formation of massive stars in AGN discs
Levin has developed a theory of how massive stars may form at the
outer edges of AGN discs. He has argued that mergers of the compact
remnants of these massive stars with the supermassive black hole may
be detectable by LISA.
Non-thermal X-ray Spectra of Magnetars
Thompson and Maxim Lyutikov (McGill) carried out Monte Carlo simulations
of the X-ray spectrum of a magnetar with a twisted external magnetic field.
Such a configuration maintains a significant optical depth to
scattering at the cyclotron resonance of the current-carrying charges.
If the charges are mildly relativistic (as is expected in the gravitational
potential of a neutron star) then an input blackbody spectrum is modified
by multiple scattering, and aquires an extended high-energy power law tail.
Non-linear Relativistic Magnetofluids in Strong Gravitational Fields
Parker Troischt, a graduate student at UNC-Chapel Hill, performed a
series of calculations under the supervision of Thompson. He
demonstrated how the exact, non-linear wave solutions of force-free
magnetohydrodynamics are modified by a strong gravitational field.
Beyond the WKB limit, there is a non-linear coupling between torsional
and compressive waves. The effect of zero-frequency components of
the wave on the non-linear coupling between force-free waves was also
worked out, taking into account the finite compressibility of a
relativistic magnetofluid.
X-ray Outburst and Glitch Activity in an Anomalous X-ray Pulsar
Thompson collaborated with Kaspi (McGill) and Woods (MSFC) on the analysis
and interpretation of the recent X-ray activity of the Anomalous X-ray
Pulsar 1E 2259$+$586. This activity is strongly reminiscent of the
burst activity of SGRs, but on a smaller scale. The source also underwent
a large-amplitude glitch, probably triggered by the same instability that
produced the SGR-like bursts and enhanced X-ray emission. The large
magnitude of the glitch suggests that either it was localized in the core
of the neutron star; or that the superfluid rotation rate was below
that of the solid crust and magnetosphere. As Thompson had suggested
in earlier theoretical work, internal magnetic torques would have this
effect.
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