121 research outputs found
Photon Splitting in Magnetar Models of Soft Gamma Repeaters
The recent association of soft gamma repeaters (SGRs) with counterparts in
other wavebands has sparked much interest in these sources. One of the recent
models for these objects is that they originate in the environs of neutron
stars with fields much stronger than the quantum critical field
\teq{B_{cr}=4.413\times 10^{13}} Gauss. Near such neutron stars, dubbed
magnetars, the exotic quantum process of magnetic photon splitting becomes
prolific. Its principal effect is to degrade photon energies and thereby soften
gamma-ray spectra from neutron stars; it has recently been suggested that
splitting may be responsible for limiting the hardness of emission in SGRs, if
these sources originate in neutron stars with supercritical surface fields.
Seed photons in supercritical fields efficiently generate soft gamma-ray
spectra, typical of repeaters. In this paper, the influence of the curved
dipole field geometry of a neutron star magnetosphere on the photon splitting
rate is investigated. The dependence of the attenuation length on the location
and angular direction of the seed photons is explored.Comment: 5 pages including 3 encapsulated figures, as a compressed, uuencoded,
Postscript file. To appear in Proc. of the 1995 La Jolla workshop ``High
Velocity Neutron Stars and Gamma-Ray Bursts'' eds. Rothschild, R. et al.,
AIP, New Yor
Photon splitting in soft gamma repeaters
The exotic quantum process of photon splitting has great potential to explain the softness of emission in soft gamma repeaters (SGRs) if they originate in neutron stars with surface fields above the quantum critical field B_{\rm cr}=4.413\times 10^{13}Gauss. Splitting becomes prolific at such field strengths: its principal effect is to degrade photon energies, initiating a cascade that softens gamma-ray spectra. Uniform field cascade calculations have demonstrated that emission could be softened to the observed SGR energies for fields exceeding about 10^{14}Gauss. Recently, we have determined splitting attenuation lengths and maximum energies for photon escape in neutron star environments including the effects of magnetospheric dipole field geometry. Such escape energies \erg_{esc} suitably approximate the peak energy of the emergent spectrum, and in this paper we present results for \erg_{esc} as a function of photon emission angles for polar cap and equatorial emission regions. The escape energy is extremely insensitive to viewing perspective for equatorial emission, arguing in favour of such a site for the origin of SGR activity
Hard X-ray Quiescent Emission in Magnetars via Resonant Compton Upscattering
Non-thermal quiescent X-ray emission extending between 10 keV and around 150
keV has been seen in about 10 magnetars by RXTE, INTEGRAL, Suzaku, NuSTAR and
Fermi-GBM. For inner magnetospheric models of such hard X-ray signals, inverse
Compton scattering is anticipated to be the most efficient process for
generating the continuum radiation, because the scattering cross section is
resonant at the cyclotron frequency. We present hard X-ray upscattering spectra
for uncooled monoenergetic relativistic electrons injected in inner regions of
pulsar magnetospheres. These model spectra are integrated over bundles of
closed field lines and obtained for different observing perspectives. The
spectral turnover energies are critically dependent on the observer viewing
angles and electron Lorentz factor. We find that electrons with energies less
than around 15 MeV will emit most of their radiation below 250 keV, consistent
with the turnovers inferred in magnetar hard X-ray tails. Electrons of higher
energy still emit most of the radiation below around 1 MeV, except for
quasi-equatorial emission locales for select pulse phases. Our spectral
computations use a new state-of-the-art, spin-dependent formalism for the QED
Compton scattering cross section in strong magnetic fields.Comment: 5 pages, 2 figures, to appear in Proc. "Physics of Neutron Stars -
2017," Journal of Physics: Conference Series, eds. G. G. Pavlov, et al., held
in Saint Petersburg, Russia, 10-14 July, 201
High Energy Neutrinos and Photons from Curvature Pions in Magnetars
We discuss the relevance of the curvature radiation of pions in strongly
magnetized pulsars or magnetars, and their implications for the production of
TeV energy neutrinos detectable by cubic kilometer scale detectors, as well as
high energy photons.Comment: 19 pages, 4 figures, to appear in JCA
Compton Scattering in Ultra-Strong Magnetic Fields: Numerical and Analytical Behavior in the Relativistic Regime
This paper explores the effects of strong magnetic fields on the Compton
scattering of relativistic electrons. Recent studies of upscattering and energy
loss by relativistic electrons that have used the non-relativistic, magnetic
Thomson cross section for resonant scattering or the Klein-Nishina cross
section for non-resonant scattering do not account for the relativistic quantum
effects of strong fields ( G). We have derived a
simplified expression for the exact QED scattering cross section for the
broadly-applicable case where relativistic electrons move along the magnetic
field. To facilitate applications to astrophysical models, we have also
developed compact approximate expressions for both the differential and total
polarization-dependent cross sections, with the latter representing well the
exact total QED cross section even at the high fields believed to be present in
environments near the stellar surfaces of Soft Gamma-Ray Repeaters and
Anomalous X-Ray Pulsars. We find that strong magnetic fields significantly
lower the Compton scattering cross section below and at the resonance, when the
incident photon energy exceeds in the electron rest frame. The cross
section is strongly dependent on the polarization of the final scattered
photon. Below the cyclotron fundamental, mostly photons of perpendicular
polarization are produced in scatterings, a situation that also arises above
this resonance for sub-critical fields. However, an interesting discovery is
that for super-critical fields, a preponderance of photons of parallel
polarization results from scatterings above the cyclotron fundamental. This
characteristic is both a relativistic and magnetic effect not present in the
Thomson or Klein-Nishina limits.Comment: AASTeX format, 31 pages included 7 embedded figures, accepted for
publication in The Astrophysical Journa
Magnetic Photon Splitting: the S-Matrix Formulation in the Landau Representation
Calculations of reaction rates for the third-order QED process of photon
splitting in strong magnetic fields traditionally have employed either the
effective Lagrangian method or variants of Schwinger's proper-time technique.
Recently, Mentzel, Berg and Wunner (1994) presented an alternative derivation
via an S-matrix formulation in the Landau representation. Advantages of such a
formulation include the ability to compute rates near pair resonances above
pair threshold. This paper presents new developments of the Landau
representation formalism as applied to photon splitting, providing significant
advances beyond the work of Mentzel et al. by summing over the spin quantum
numbers of the electron propagators, and analytically integrating over the
component of momentum of the intermediate states that is parallel to field. The
ensuing tractable expressions for the scattering amplitudes are satisfyingly
compact, and of an appearance familiar to S-matrix theory applications. Such
developments can facilitate numerical computations of splitting considerably
both below and above pair threshold. Specializations to two regimes of interest
are obtained, namely the limit of highly supercritical fields and the domain
where photon energies are far inferior to that for the threshold of
single-photon pair creation. In particular, for the first time the
low-frequency amplitudes are simply expressed in terms of the Gamma function,
its integral and its derivatives. In addition, the equivalence of the
asymptotic forms in these two domains to extant results from effective
Lagrangian/proper-time formulations is demonstrated.Comment: 19 pages, 3 figures, REVTeX; accepted for publication in Phys. Rev.
Photon Splitting Cascades in Gamma-Ray Pulsars and the Spectrum of PSR1509-58
Magnetic photon splitting, a QED process that becomes important only in
magnetic fields approaching the quantum critical value, B_cr = 4.41 X 10^13
Gauss, is investigated as a mechanism for attenuation of gamma-rays emitted
near the surface of strongly-magnetized pulsars. We model photon splitting
attenuation and subsequent splitting cascades in gamma-ray pulsars, including
the dipole field and curved spacetime geometry of the neutron star
magnetosphere. We focus specifically on PSR1509-58, which has the highest
surface magnetic field of all the gamma-ray pulsars (B_0 = 3 X 10^13 Gauss). We
find that splitting will not be important for most gamma-ray pulsars, i.e.
those with B_0 <~ 0.2 B_cr, but will be important for gamma-ray pulsars having
B_0 >~ 0.3 B_cr, where the splitting attenuation lengths and escape energies
become comparable to or less than those for pair production. We compute Monte
Carlo spectral models for PSR1509-58. We find that photon splitting, or
combined splitting and pair production, can explain the unusually low cutoff
energy (between 2 and 30 MeV) of PSR1509-58, and that the model cascade
spectra, which display strong polarization, are consistent with the observed
spectral points and upper limits for polar cap emission at a range of magnetic
colatitudes up to ~ 25 degrees.Comment: 39 pages, 14 embedded figures, AASTEX To appear in ApJ, January 20,
199
Full polar cap cascade scenario: -ray and X-ray luminosities from spin-powered pulsars
We modify polar cap cascade picture to include the ICS of the higher
generation pairs. In such a ``full-cascade'' scenario, not only the
perpendicular portion of the energy of the pairs goes to high energy radiation
via SR, but the parallel portion of the energy of the pairs can also contribute
to high energy emission via ICS with the soft thermal photons from either the
full neutron star surface or the hot polar cap. An important output of such a
scenario is that the soft tail of the ICS spectrum can naturally result in a
non-thermal X-ray component which can contribute to the luminosities observed
by ROSAT and ASCA. Here we present an analytic description of such a full polar
cap cascade scenario within the framework of Harding & Muslimov acceleration
model. We present the theoretical predictions of the -ray luminosities,
the thermal and non-thermal X-ray luminosities for the known spin-powered X-ray
pulsars. Our results show that the observed different dependences of the high
energy luminosities on the pulsar spin-down luminosities, i.e., and , are well
reproduced. Our model predicts that the {\em pulsed} soft X-rays in the ROSAT
band from most of the millisecond pulsars might be of thermal origin if there
is no strong multipole field components near the surfaces of these pulsars.Comment: 23 pages, emulateapj style, final version to appear in the
Astrophysical Journa
Photon Splitting and Pair Creation in Highly Magnetized Pulsars
The absence of radio pulsars with long periods has lead to the popular notion
of a high P ``death line.'' In the standard picture, beyond this boundary,
pulsars with low spin rates cannot accelerate particles above the stellar
surface to high enough energies to initiate pair cascades, and the pair
creation needed for radio emission is strongly suppressed. In this paper we
explore the possibility of another pulsar ``death line'' in the context of
polar cap models, corresponding to high magnetic fields B in the upper portion
of the period-period derivative diagram, a domain where few radio pulsars are
observed. The origin of this high B boundary, which may occur when B becomes
comparable to or exceeds Gauss, is also due
to the suppression of magnetic pair creation, but primarily because of
ineffective competition with magnetic photon splitting. Threshold pair creation
also plays a prominent role in the suppression of cascades. We present Monte
Carlo calculations of the pair yields in photon splitting/pair cascades which
show that, in the absence of scattering effects, pair production is effectively
suppressed, but only if all three modes of photon splitting allowed by QED are
operating in high fields. This paper describes the probable shape and position
of the new ``death line,'' above which pulsars are expected to be radio quiet,
but perhaps still X-ray and gamma-ray bright. The hypothesized existence of
radio-quiet sources finds dramatic support in the recent discovery of
ultra-strong fields in Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars.
Guidelines for moderate to high B pulsar searches at radio wavelengths and also
in the soft and hard gamma-ray bands are presented.Comment: 19 pages, including 1 table and 9 figures, AASTeX apjgalley format,
To appear in The Astrophysical Journal, Vol 547, February 1, 2001 issu
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