1,153 research outputs found
Condensed Surfaces of Magnetic Neutron Stars, Thermal Surface Emission, and Particle Acceleration Above Pulsar Polar Caps
For sufficiently strong magnetic fields and/or low temperatures, the neutron
star surface may be in a condensed state with little gas or plasma above it.
Such surface condensation can significantly affect the thermal emission from
isolated neutron stars, and may lead to the formation of a charge-depleted
acceleration zone ("vacuum gap") in the magnetosphere above the stellar polar
cap. Using the latest results on the cohesive property of magnetic condensed
matter, we quantitatively determine the conditions for surface condensation and
vacuum gap formation in magnetic neutron stars. We find that condensation can
occur if the thermal energy kT of the neutron star surface is less than about
8% of its cohesive energy Q_s, and that a vacuum gap can form if the neutron
star's rotation axis and magnetic moment point in opposite directions and kT is
less than about 4% of Q_s. Thus, vacuum gap accelerators may exist for some
neutron stars. Motivated by this result, we also study the physics of pair
cascades in the vacuum gap model for photon emission by accelerating electrons
and positrons due to both curvature radiation and resonant/nonresonant inverse
Compton scattering. Our calculations of the condition of cascade-induced vacuum
breakdown and the related pulsar death line/boundary generalize previous works
to the superstrong field regime. We find that inverse Compton scatterings do
not produce a sufficient number of high energy photons in the gap and thus do
not lead to pair cascades for most neutron star parameters. We discuss the
implications of our results for the recent observations of neutron star thermal
radiation as well as for the detection/non-detection of radio emission from
high-B pulsars and magnetars.Comment: 25 pages, 11 figures. Minor changes. MNRAS in pres
Cohesive property of magnetized neutron star surfaces: Computations and implications
The cohesive energy of condensed matter in strong magnetic fields is a
fundamental quantity characterizing magnetized neutron star surfaces. The
cohesive energy refers to the energy required to pull an atom out of the bulk
condensed matter at zero pressure. Theoretical models of pulsar and magnetar
magnetospheres depend on the cohesive properties of the surface matter in
strong magnetic fields. For example, depending on the cohesive energy of the
surface matter, an acceleration zone ("polar gap") above the polar cap of a
pulsar may or may not form. Also, condensation of the neutron star surface, if
it occurs, can significantly affect thermal emission from isolated neutron
stars. We describe our calculations of the cohesive property of matter in
strong magnetic fields, and discuss the implications of our results to the
recent observations of neutron star surface emission as well as to the
detection/non-detection of radio emission from magnetars.Comment: 12 pages, 4 figures. Minor changes to Author/Comments fields. To
appear in Advances in Space Researc
Timing the Nearby Isolated Neutron Star RX J1856.5-3754
RX J1856.5-3754 is the X-ray brightest among the nearby isolated neutron
stars. Its X-ray spectrum is thermal, and is reproduced remarkably well by a
black-body, but its interpretation has remained puzzling. One reason is that
the source did not exhibit pulsations, and hence a magnetic field
strength--vital input to atmosphere models--could not be estimated. Recently,
however, very weak pulsations were discovered. Here, we analyze these in
detail, using all available data from the XMM-Newton and Chandra X-ray
observatories. From frequency measurements, we set a 2-sigma upper limit to the
frequency derivative of \dot\nu<1.3e-14 Hz/s. Trying possible phase-connected
timing solutions, we find that one solution is far more likely than the others,
and we infer a most probable value of \dot\nu=(-5.98+/-0.14)e-16 Hz/s. The
inferred magnetic field strength is 1.5e13 G, comparable to what was found for
similar neutron stars. From models, the field seems too strong to be consistent
with the absence of spectral features for non-condensed atmospheres. It is
sufficiently strong, however, that the surface could be condensed, but only if
it is consists of heavy elements like iron. Our measurements imply a
characteristic age of about 4 Myr. This is longer than the cooling and
kinematic ages, as was found for similar objects, but at almost a factor ten,
the discrepancy is more extreme. A puzzle raised by our measurement is that the
implied rotational energy loss rate of about 3e30 erg/s is orders of magnitude
smaller than what was inferred from the H-alpha nebula surrounding the source.Comment: 4 pages, 2 figures, 2 tables; accepted for publication in
Astrophysical Journal (Letters
Radiative transitions of the helium atom in highly magnetized neutron star atmospheres
Recent observations of thermally emitting isolated neutron stars revealed
spectral features that could be interpreted as radiative transitions of He in a
magnetized neutron star atmosphere. We present Hartree-Fock calculations of the
polarization-dependent photoionization cross sections of the He atom in strong
magnetic fields ranging from 10^12 G to 10^14 G. Convenient fitting formulae
for the cross sections are given as well as related oscillator strengths for
various bound-bound transitions. The effects of finite nucleus mass on the
radiative absorption cross sections are examined using perturbation theory.Comment: 14 pages, 7 figures. Minor changes. MNRAS in pres
Electrodynamics of Magnetars III: Pair Creation Processes in an Ultrastrong Magnetic Field and Particle Heating in a Dynamic Magnetosphere
We consider the details of the QED processes that create electron-positron
pairs in magnetic fields approaching and exceeding 10^{14} G. The formation of
free and bound pairs is addressed, and the importance of positronium
dissociation by thermal X-rays is noted. We calculate the collision cross
section between an X-ray and a gamma ray, and point out a resonance in the
cross section when the gamma ray is close to the threshold for pair conversion.
We also discuss how the pair creation rate in the open-field circuit and the
outer magnetosphere can be strongly enhanced by instabilities near the light
cylinder. When the current has a strong fluctuating component, a cascade
develops. We examine the details of particle heating, and show that a high rate
of pair creation can be sustained close to the star, but only if the spin
period is shorter than several seconds. The dissipation rate in this turbulent
state can easily accommodate the observed radio output of the transient
radio-emitting magnetars, and even their infrared emission. Finally, we outline
how a very high rate of pair creation on the open magnetic field lines can help
to stabilize a static twist in the closed magnetosphere and to regulate the
loss of magnetic helicity by reconnection at the light cylinder.Comment: 25 pages, submitted to the Astrophysical Journa
XMM-Newton Observations of Radio Pulsars B0834+06 and B0826-34 and Implications for Pulsar Inner Accelerator
We report the X-ray observations of two radio pulsars with drifting
subpulses: B0834 + 06 and B0826 - 34 using \xmm\. PSR B0834 + 06 was detected
with a total of 70 counts from the three EPIC instruments over 50 ks exposure
time. Its spectrum was best described as that of a blackbody (BB) with
temperature K and bolometric luminosity
of erg s. As it is typical in
pulsars with BB thermal components in their X-ray spectra, the hot spot surface
area is much smaller than that of the canonical polar cap, implying a
non-dipolar surface magnetic field much stronger than the dipolar component
derived from the pulsar spin-down (in this case about 50 times smaller and
stronger, respectively). The second pulsar PSR B0826 - 34 was not detected over
50 ks exposure time, giving an upper limit for the bolometric luminosity erg s. We use these data as well as the radio
emission data concerned with drifting subpulses to test the Partially Screened
Gap (PSG) model of the inner accelerator in pulsars.Comment: Accepted for publication by The Astrophysical Journa
The Development of the Puerto Rico Lightning Detection Network for Meteorological Research
A land-based Puerto Rico Lightning Detection Network (PR-LDN) dedicated to the academic research of meteorological phenomena has being developed. Five Boltek StormTracker PCI-Receivers with LTS-2 Timestamp Cards with GPS and lightning detectors were integrated to Pentium III PC-workstations running the CentOS linux operating system. The Boltek detector linux driver was compiled under CentOS, modified, and thoroughly tested. These PC-workstations with integrated lightning detectors were installed at five of the University of Puerto Rico (UPR) campuses distributed around the island of PR. The PC-workstations are left on permanently in order to monitor lightning activity at all times. Each is networked to their campus network-backbone permitting quasi-instantaneous data transfer to a central server at the UPR-Bayam n campus. Information generated by each lightning detector is managed by a C-program developed by us called the LDN-client. The LDN-client maintains an open connection to the central server operating the LDN-server program where data is sent real-time for analysis and archival. The LDN-client also manages the storing of data on the PC-workstation hard disk. The LDN-server software (also an in-house effort) analyses the data from each client and performs event triangulations. Time-of-arrival (TOA) and related hybrid algorithms, lightning-type and event discriminating routines are also implemented in the LDN-server software. We also have developed software to visually monitor lightning events in real-time from all clients and the triangulated events. We are currently monitoring and studying the spatial, temporal, and type distribution of lightning strikes associated with electrical storms and tropical cyclones in the vicinity of Puerto Rico
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