477 research outputs found
Timing Properties of Magnetars
We study the pulse morphologies and pulse amplitudes of thermally emitting
neutron stars with ultrastrong magnetic fields. The beaming of the radiation
emerging from a magnetar was recently shown to be predominantly non-radial,
with a small pencil and a broad fan component. We show that the combination of
this radiation pattern with the effects of strong lensing in the gravitational
field of the neutron star yields pulse profiles that show a qualitatively
different behavior compared to that of the radially-peaked beaming patterns
explored previously. Specifically, we find that: (i) the pulse profiles of
magnetars with a single hot emission region on their surface exhibit 1-2 peaks,
whereas those with an antipodal emission geometry have 1-4 peaks, depending on
the neutron star compactness, the observer's viewing angle, and the size of the
hot regions; (ii) the energy dependence of the beaming pattern may give rise to
weakly or strongly energy-dependent pulse profiles and may introduce phase lags
between different energy bands; (iii) the non-radial beaming pattern can give
rise to high pulsed fractions even for very relativistic neutron stars; (iv)
the pulsed fraction may not vary monotonically with neutron star compactness;
(v) the pulsed fraction does not decrease monotonically with the size of the
emitting region; (vi) the pulsed fraction from a neutron star with a single hot
pole has, in general, a very weak energy dependence, in contrast to the case of
an antipodal geometry. Comparison of these results to the observed properties
of anomalous X-ray pulsars strongly suggests that they are neutron stars with a
single hot region of ultrastrong magnetic field.Comment: 22 pages, 13 color figures, ApJ in pres
Bumpy Spin-Down of Anomalous X-Ray Pulsars: The Link with Magnetars
The two anomalous X-ray pulsars (AXPs) with well-sampled timing histories, 1E
1048.1-5937 and 1E 2259+586, are known to spin down irregularly, with `bumps'
superimposed on an overall linear trend. Here we show that if AXPs are
non-accreting magnetars, i.e. isolated neutron stars with surface magnetic
fields B_0 > 10^{10} T, then they spin down electromagnetically in exactly the
manner observed, due to an effect called `radiative precession'. Internal
hydromagnetic stresses deform the star, creating a fractional difference
epsilon=(I_3-I_1)/I_1 ~ 10^{-8} between the principal moments of inertia I_1
and I_3; the resulting Eulerian precession couples to an oscillating component
of the electromagnetic torque associated with the near-zone radiation fields,
and the star executes an anharmonic wobble with period tau_pr ~ 2 pi / epsilon
Omega(t) ~ 10 yr, where Omega(t) is the rotation frequency as a function of
time t. We solve Euler's equations for a biaxial magnet rotating in vacuo; show
that the computed Omega(t) matches the measured timing histories of 1E
1048.1-5937 and 1E 2259+586; predict Omega(t) for the next 20 years for both
objects; predict a statistical relation between and tau_pr, to be
tested as the population of known AXPs grows; and hypothesize that radiative
precession will be observed in future X-ray timing of soft gamma-ray repeaters
(SGRs).Comment: 9 pages, 2 figures, to be published in The Astrophysical Journal
Letter
Broad-band X-ray measurements of the black hole candidate XTE J1908+094
XTE J1908+094 is an X-ray transient that went into outburst in February 2002.
After two months it reached a 2-250 keV peak flux of 1 to 2 X 10-8 erg/s/cm2.
Circumstantial evidence points to an accreting galactic black hole as the
origin of the the X-radiation: pulsations nor thermonuclear flashes were
detected that would identify a neutron star and the spectrum was unusually hard
for a neutron star at the outburst onset. We report on BeppoSAX and RXTE All
Sky Monitor observations of the broad-band spectrum of XTE J1908+094. The
spectrum is consistent with a model consisting of a Comptonization component by
a ~40 keV plasma (between 2 and 60 keV this component can be approximated by a
power law with a photon index of 1.9 to 2.1), a multicolor accretion disk
blackbody component with a temperature just below 1 keV and a broad emission
line at about 6 keV. The spectrum is heavily absorbed by cold interstellar
matter with an equivalent hydrogen column density of 2.5 X 10+22 cm-2, which
makes it difficult to study the black body component in detail. The black body
component exhibits strong evolution about 6 weeks into the outburst. Two weeks
later this is followed by a swift decay of the power law component. The
broadness of the 6 keV feature may be due to relativistic broadening or Compton
scattering of a narrow Fe-K line.Comment: Accepted for publication in Astronomy & Astrophysic
Radio Quiet Pulsars with Ultra-Strong Magnetic Fields
The notable absence of radio pulsars having measured magnetic dipole surface
field strengths above Gauss naturally raises the
question of whether this forms an upper limit to pulsar magnetization. Recently
there has been increasing evidence that neutron stars possessing higher dipole
spin-down fields do in fact exist, including a growing list of anomalous X-ray
pulsars (AXPs) with long periods and spinning down with high period
derivatives, implying surface fields of -- Gauss.
Furthermore, the recently reported X-ray period and period derivative for the
Soft Gamma-ray Repeater (SGR) source SGR1806-20 suggest a surface field around
Gauss. None of these high-field pulsars have yet been detected as
radio pulsars. We propose that high-field pulsars should be radio-quiet because
electron-positron pair production in their magnetospheres, thought to be
essential for radio emission, is efficiently suppressed in ultra-strong fields
( Gauss) by the action of photon splitting, a
quantum electrodynamical process in which a photon splits into two. Our
computed radio quiescence boundary in the radio pulsar diagram,
where photon splitting overtakes pair creation, is located just above the
boundary of the known radio pulsar population, neatly dividing them from the
AXPs. We thus identify a physical mechanism that defines a new class of
high-field radio-quiet neutron stars that should be detectable by their pulsed
emission at X-ray and perhaps -ray energies.Comment: 4 pages, including one figure and one table, in AASTeX emulatapj
format, Astrophysical Journal Letters, in pres
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