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
X-ray Variability from the Compact Source in the Supernova Remnant RCW 103
A new ASCA observation of 1E 161348-5055, the central compact X-ray source in
the supernova remnant RCW 103, reveals an order-of-magnitude decrease in its 3
- 10 keV flux since the previous ASCA measurement four years earlier. This
result is hard to reconcile with suggestions that the bulk of the emission is
simple quasi-blackbody, cooling radiation from an isolated neutron star.
Furthermore, archived EINSTEIN and ROSAT datasets spanning 18 years confirm
that this source manifests long-term variability, to a lesser degree. This
provides a natural explanation for difficulties encountered in reproducing the
original EINSTEIN detection of 1E 161348-5055. Spectra from the new data are
consistent with no significant spectral change despite the decline in
luminosity. We find no evidence for a pulsed component in any of the data sets,
with a best upper limit on the pulsed modulation of 13 percent. We discuss the
phenomenology of this remarkable source.Comment: 5 pages with 2 embedded figures, LaTex, emulateapj.sty. To appear in
the Astrophysical Journal Letter
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