209 research outputs found
Relativistic Wind Bubbles and Afterglow Signatures
Highly magnetized, rapidly rotating compact objects are widely argued as
central energy sources of -ray bursts (GRBs). After the GRB, such a
magnetar-like object may directly lose its rotational energy through some
magnetically-driven processes, which produce an ultrarelativistic wind
dominated possibly by the energy flux of electron-positron pairs. The
interaction of such a wind with an outward-expanding fireball leads to a
relativistic wind bubble, being regarded as a relativistic version of the
well-studied Crab Nebula. We here explore the dynamics of this wind bubble and
its emission signatures. We find that when the injection energy significantly
exceeds the initial energy of the fireball, the bulk Lorentz factor of the wind
bubble decays more slowly than before, and more importantly, the reverse-shock
emission could dominate the afterglow emission, which yields a bump in
afterglow light curves. In addition, high polarization of the bump emission
would be expected if a toroidal magnetic field in the shocked wind dominates
over the random component.Comment: 7 pages including 1 figure, emulateapj style, expanded version
accepted for publication in Ap
TeV Emission from the Galactic Center Black-Hole Plerion
The HESS collaboration recently reported highly significant detection of TeV
gamma-rays coincident with Sgr A*. In the context of other Galactic Center (GC)
observations, this points to the following scenario: In the extreme
advection-dominated accretion flow (ADAF) regime of the GC black hole (BH),
synchrotron radio/sub-mm emission of 100 MeV electrons emanates from an
inefficiently radiating turbulent magnetized corona within 20 Schwarzschild
radii of the GCBH. These electrons are accelerated through second-order Fermi
processes by MHD turbulence. Closer to the innermost stable orbit of the ADAF,
instabilities and shocks within the flow inject power-law electrons through
first-order Fermi acceleration to make synchrotron X-ray flares observed with
Chandra, XMM, and INTEGRAL. A subrelativistic MHD wind subtending a 1sr cone
with power > 10^{37} erg/s is driven by the ADAF from the vicinity of the GCBH.
As in pulsar powered plerions, electrons are accelerated at the wind
termination shock, at > 10^{16.5} cm from the GCBH, and Compton-scatter the
ADAF and the far infra-red (FIR) dust radiation to TeV energies. The
synchrotron radiation of these electrons forms the quiescent X-ray source
resolved by Chandra. The radio counterpart of this TeV/X-ray plerion, formed
when the injected electrons cool on timescales 10^4 yrs, could explain the
origin of nonthermal radio emission in the pc-scale bar of the radio nebula Sgr
A West.Comment: 6 pages, 2 figures; accepted for publication in ApJ Letters, minor
revison
Implications of a Nonthermal Origin of the Excess EUV Emission from the Coma Cluster of Galaxies
The inverse Compton (IC) interpretation of the excess EUV emission, that was
recently reported from several clusters of galaxies, suggests that the amount
of relativistic electrons in the intracluster medium is highly significant,
W_e>10^{61} erg. Considering Coma as the prototype galaxy cluster of nonthermal
radiation, we discuss implications of the inverse Compton origin of the excess
EUV fluxes in the case of low intracluster magnetic fields of order 0.1 muG, as
required for the IC interpretation of the observed excess hard X-ray flux, and
in the case of high fields of order 1 muG as suggested by Faraday rotation
measurements. Although for such high intracluster fields the excess hard X-rays
will require an explanation other than by the IC effect, we show that the
excess EUV flux can be explained by the IC emission of a `relic' population of
electrons driven into the incipient intracluster medium at the epoch of
starburst activity by galactic winds, and later on reenergized by adiabatic
compression and/or large-scale shocks transmitted through the cluster as the
consequence of more recent merger events. For high magnetic fields B > 1 muG
the interpretation of the radio fluxes of Coma requires a second population of
electrons injected recently. They can be explained as secondaries produced by a
population of relativistic protons. We calculate the fluxes of gamma-rays to be
expected in both the low and high magnetic field scenarios, and discuss
possibilities to distinguish between these two principal options by future
gamma-ray observations.Comment: LaTeX, 6 figures; accepted for publication in Ap
High Energy Cosmic Rays from Local GRBs
We have developed a model that explains cosmic rays with energies E between
\~0.3 PeV and the energy of the second knee at E_2 ~ 3*10^{17} eV as
originating from a recent Galactic gamma-ray burst (GRB) that occurred ~1 Myr
ago within 1 kpc from Earth. Relativistic shocks from GRBs are assumed to
inject power-law distributions of cosmic rays (CRs) to the highest energies.
Diffusive propagation of CRs from the local GRB explains the CR spectrum near
and above the first knee at E_1 ~ 3*10^{15} eV. The first and the second knees
are explained as being directly connected with the injection of plasma
turbulence in the interstellar medium on a ~1 pc and ~100 pc scales,
respectively. Transition to CRs from extragalactic GRBs occurs at E > E_2. The
origin of the ankle in the CR spectrum at E ~ 4*10^{18} eV is due to photopair
energy losses of UHECRs on cosmological timescales, as also suggested by
Berezinsky and collaborators. Any significant excess flux of extremely high
energy CRs deviating from the exponential cutoff behavior at E> E_{GZK} =
6*10^{19} eV would imply a significant contribution due to recent GRB activity
on timescales t < 10^8 yrs from local extragalactic sources within ~10 Mpc.Comment: 10 pages, 5 figures; to appear in the Proceedings of the Aspen2005
Workshop ``Physics at the End of the Galactic Cosmic Ray Spectrum'' (Aspen,
April 2005
Locating the VHE source in the Galactic Centre with milli-arcsecond accuracy
Very high-energy gamma-rays (VHE; E>100 GeV) have been detected from the
direction of the Galactic Centre up to energies E>10 TeV. Up to now, the origin
of this emission is unknown due to the limited positional accuracy of the
observing instruments. One of the counterpart candidates is the super-massive
black hole (SMBH) Sgr A*. If the VHE emission is produced within ~10^{15} cm
~1000 r_G (r_G=G M/c^2 is the Schwarzschild radius) of the SMBH, a decrease of
the VHE photon flux in the energy range 100--300 GeV is expected whenever an
early type or giant star approaches the line of sight within ~ milli-arcseconds
(mas). The dimming of the flux is due to absorption by pair-production of the
VHE photons in the soft photon field of the star, an effect we refer to as
pair-production eclipse (PPE). Based upon the currently known orbits of stars
in the inner arcsecond of the Galaxy we find that PPEs lead to a systematic
dimming in the 100--300 GeV band at the level of a few per cent and lasts for
several weeks. Since the PPE affects only a narrow energy band and is well
correlated with the passage of the star, it can be clearly discriminated
against other systematic or even source-intrinsic effects. While the effect is
too small to be observable with the current generation of VHE detectors,
upcoming high count-rate experiments like the Cherenkov telescope array (CTA)
will be sufficiently sensitive. Measuring the temporal signature of the PPE
bears the potential to locate the position and size of the VHE emitting region
within the inner 1000 r_G or in the case of a non-detection exclude the
immediate environment of the SMBH as the site of gamma-ray production
altogether.Comment: 7 pages, published in MNRAS 402, pg. 1342-134
Near-Infrared Synchrotron Emission from Cas A
High energy observations of Cas A suggested the presence of synchrotron
radiation, implying acceleration of cosmic rays by young supernova remnants. We
detect synchrotron emission from Cas A in the near-infrared using Two Micron
All Sky Survey (2MASS) and Palomar 200 inch PFIRCAM observations. The remnant
is detected in J, H, and Ks bands, with Ks band brightest and J faint. In the J
and H bands, bright [Fe II] lines (1.24um and 1.64um) are detected
spectroscopically. The Palomar observations include Ks continuum, narrow-band
1.64um (centered on [Fe II]) and 2.12um (centered on H2(1-0)) images. While the
narrow-band 1.64um image shows filamentary and knotty structures, similar to
the optical image, the Ks image shows a relatively smooth, diffuse shell,
remarkably similar to the radio image. The broad-band near-infrared fluxes of
Cas A are generally consistent with, but a few tens of percent higher than, an
extrapolation of the radio fluxes. The hardening to higher frequencies is
possibly due to nonlinear shock acceleration and/or spectral index variation
across the remnant. We show evidence of spectral index variation. The presence
of near-infrared synchrotron radiation requires the roll-off frequency to be
higher than 1.5e14 Hz, implying that electrons are accelerated to energies of
at least 0.2 TeV. The morphological similarity in diffuse emission between the
radio and Ks band images implies that synchrotron losses are not dominant. Our
observations show unambiguous evidence that the near-infrared Ks band emission
of Cas A is from synchrotron emission by accelerated cosmic-ray electrons.Comment: accepted by Ap
Pulsar Jets: Implications for Neutron Star Kicks and Initial Spins
We study implications for the apparent alignment of the spin axes,
proper-motions, and polarization vectors of the Crab and Vela pulsars. The spin
axes are deduced from recent Chandra X-ray Observatory images that reveal jets
and nebular structure having definite symmetry axes. The alignments indicate
these pulsars were born either in isolation or with negligible velocity
contributions from binary motions. We examine the effects of rotation and the
conditions under which spin-kick alignment is produced for various models of
neutron star kicks. If the kick is generated when the neutron star first forms
by asymmetric mass ejection or/and neutrino emission, then the alignment
requires that the protoneutron star possesses an original spin with period
much less than the kick timescale, thus spin-averaging the kick forces.
The kick timescale ranges from 100 ms to 10 s depending on whether the kick is
hydrodynamically driven or neutrino-magnetic field driven. For hydrodynamical
models, spin-kick alignment further requires the rotation period of an
asymmetry pattern at the radius near shock breakout (>100 km) to be much less
than ~100 ms; this is difficult to satisfy unless rotation plays a dynamically
important role in the core collapse and explosion (P_s\lo 1 ms). Aligned kick
and spin vectors are inherent to the slow process of asymmetric electromagnetic
radiation from an off-centered magnetic dipole. We reassess the viability of
this effect, correcting a factor of 4 error in Harrison and Tademaru's
calculation that increases the size of the effect. To produce a kick velocity
of order a few hundred km/s requires that the neutron star be born with an
initial spin close to 1 ms and that spindown due to r-mode driven gravitational
radiation be inefficient compared to standard magnetic braking.Comment: Small changes/additions; final version to be published in ApJ,
Vol.549 (March 10, 2001
- …