926 research outputs found
Vacuum Breakdown near a Black Hole Charged by Hypercritical Accretion
We consider a black hole accreting spherically from the surrounding medium.
If accretion produces a luminosity close to the Eddington limit the hole
acquires a net charge so that electrons and ions can fall with the same
velocity. The condition for the electrostatic field to be large enough to break
the vacuum near the hole horizon translates into an upper limit for the hole
mass, The astrophysical conditions under
which this phaenomenon can take place are rather extreme, but in principle they
could be met by a mini black hole residing at the center of a star.Comment: 6 pages, accepted for publication in the Astrophysical Journa
On Electrostatic Positron Acceleration In The Accretion Flow Onto Neutron Stars
As first shown by Shvartsman (1970), a neutron star accreting close to the
Eddington limit must acquire a positive charge in order for electrons and
protons to move at the same speed. The resulting electrostatic field may
contribute to accelerating positrons produced near the star surface in
conjunction with the radiative force. We reconsider the balance between energy
gains and losses, including inverse Compton (IC), bremsstrahlung and
non--radiative scatterings. It is found that, even accounting for IC losses
only, the maximum positron energy never exceeds keV. The
electrostatic field alone may produce energies keV at most. We
also show that Coulomb collisions and annihilation with accreting electrons
severely limit the number of positrons that escape to infinity.Comment: 9 pages plus 3 postscript figures, to be published in Ap
A view of PKS 2155-304 with XMM-Newton Reflection Grating Spectrometers
We present the high resolution X-ray spectrum of the BL Lac object PKS
2155-304 taken with the RGS units onboard XMM-Newton in November 2000. We
detect a OVII Kalpha resonant absorption line from warm/hot local gas at 21.59A
(~4.5 sigma detection). The line profile is possibly double peaked. We do not
confirm the strong 20.02 A absorption line seen with Chandra and interpreted as
z~0.05 OVIII Kalpha. A 3sigma upper limit of 14 mA on the equivalent width is
set. We also detect the ~23.5 A interstellar OI 1s-->2p line and derive a
factor <=1.5 subsolar O/H ratio in the ISM along PKS 2155-304 line of sight.Comment: 13 pages, 5 figures, 3 tables, emulateapj style. Accepted by Ap
A kpc-scale X-ray jet in the BL Lac source S5 2007+777
X-ray jets in AGN are commonly observed in FRII and FRI radio-galaxies, but
rarely in BL Lacs, most probably due to their orientation close to the line of
sight and the ensuing foreshortening effects. Only three BL Lacs are known so
far to contain a kpc-scale X-ray jet. In this paper, we present the evidence
for the existence of a fourth extended X-ray jet in the classical
radio-selected source S5 2007+777, which for its hybrid FRI/II radio morphology
has been classified as a HYMOR (HYbrid MOrphology Radio source). Our Chandra
ACIS-S observations of this source revealed an X-ray counterpart to the
19"-long radio jet. Interestingly, the X-ray properties of the kpc-scale jet in
S5 2007+777 are very similar to those observed in FRII jets. First, the X-ray
morphology closely mirrors the radio one, with the X-rays being concentrated in
the discrete radio knots. Second, the X-ray continuum of the jet/brightest knot
is described by a very hard power law, with photon index Gamma_x~1, although
the uncertainties are large. Third, the optical upper limit from archival HST
data implies a concave radio-to-X-ray SED. If the X-ray emission is attributed
to IC/CMB with equipartition, strong beaming (delta=13) is required, implying a
very large scale (Mpc) jet. The beaming requirement can be somewhat relaxed
assuming a magnetic field lower than equipartition. Alternatively, synchrotron
emission from a second population of very high-energy electrons is viable.
Comparison to other HYMOR jets detected with Chandra is discussed, as well as
general implications for the origin of the FRI/II division.Comment: Accepted for publication in ApJ, 19 pages, 3 figure
Progress in understanding Blazars from BeppoSAX observations
Results obtained with BeppoSAX observations of blazars within various
collaborative programs are presented. The spectral similarity "paradigm",
whereby the spectral energy distributions of blazars follow a sequence, leading
to a unified view of the whole population, is briefly illustrated. We
concentrate on recent observations of flares and associated spectral
variability for three objects at the "blue" end of the spectral sequence,
namely PKS 2155-304, Mkn 421 and Mkn 501. The results are discussed in terms of
a general analytic synchrotron self-Compton interpretation of the overall
spectrum. The physical parameters of the quasi-stationary emission region can
be derived with some confidence, while the variability mechanism(s) must be
complex.Comment: 10 pages, 8 figures, to be published in the Proceedings of the 32nd
COSPAR Meeting, Nagoya, 12-19 July 199
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