431 research outputs found
X-ray observations of the high magnetic field radio pulsar PSR J1814-1744
PSR J1814-1744 is a 4 s radio pulsar with surface dipole magnetic field
strength 5.5*10^13 G, inferred assuming simple magnetic dipole braking. This
pulsar's spin parameters are very similar to those of anomalous X-ray pulsars
(AXPs), suggesting that this may be a transition object between the radio
pulsar and AXP population, if AXPs are isolated, high magnetic field neutron
stars as has recently been hypothesized. We present archival X-ray observations
of PSR J1814-1744 made with ROSAT and ASCA. X-ray emission is not detected from
the position of the radio pulsar. The derived upper flux limit implies an X-ray
luminosity significantly smaller than those of all known AXPs. This conclusion
is insensitive to the possibility that X-ray emission from PSR J1814-1744 is
beamed or that it undergoes modest variability. When interpreted in the context
of the magnetar mechanism, these results argue that X-ray emission from AXPs
must depend on more than merely the inferred surface magnetic field strength.
This suggests distinct evolutionary paths for radio pulsars and AXP, despite
their proximity in period--period derivative phase space.Comment: 11 pages, including 2 embedded figures. Accepted by Ap
Chandra and RXTE Observations of 1E 1547.0-5408: Comparing the 2008 and 2009 Outbursts
We present results from observations of the magnetar 1E 1547.0-5408 (SGR
J1550-5418) taken with the Chandra X-ray Observatory and the Rossi X-ray Timing
Explorer (RXTE) following the source's outbursts in 2008 October and 2009
January. During the time span of the Chandra observations, which covers days 4
through 23 and days 2 through 16 after the 2008 and 2009 events, respectively,
the source spectral shape remained stable, while the pulsar's spin-down rate in
the same span in 2008 increased by a factor of 2.2 as measured by RXTE. The
lack of spectral variation suggests decoupling between magnetar spin-down and
radiative changes, hence between the spin-down-inferred magnetic field strength
and that inferred spectrally. We also found a strong anti-correlation between
the phase-averaged flux and the pulsed fraction in the 2008 and 2009 Chandra
data, but not in the pre-2008 measurements. We discuss these results in the
context of the magnetar model.Comment: 4 figures, accepted for publication in Ap
On the Extended Emission Around the Anomalous X-ray Pulsar 1E 1547.0-5408
We present an analysis of the extended emission around the anomalous X-ray
pulsar 1E 1547.0-5408 using four XMM-Newton observations taken with the source
in varying states of outburst as well as in quiescence. We find that the
extended emission flux is highly variable and strongly correlated with the flux
of the magnetar. Based on this result, as well as on spectral and energetic
considerations, we conclude that the extended emission is dominated by a
dust-scattering halo and not a pulsar wind nebula (PWN), as has been previously
argued. We obtain an upper limit on the 2-10 keV flux of a possible PWN of
4.7e-14 erg/s/cm^2, three times less than the previously claimed value,
implying an efficiency for conversion of spin-down energy into nebular
luminosity of <9e-4 (assuming a distance of 4 kpc). We do, however, find strong
evidence for X-ray emission from the supernova remnant shell surrounding the
pulsar, as previously reported.Comment: 16 pages, 3 tables, 4 figures, published in the Astrophysical Journa
Shapiro Effect as a Possible Cause of the Low-Frequency Pulsar Timing Noise in Globular Clusters
A prolonged timing of millisecond pulsars has revealed low-frequency
uncorrelated noise, presumably of astrophysical origin, in the pulse arrival
time (PAT) residuals for some of them. In most cases, pulsars in globular
clusters show a low-frequency modulation of their rotational phase and spin
rate. The relativistic time delay of the pulsar signal in the curved space time
of randomly distributed and moving globular cluster stars (the Shapiro effect)
is suggested as a possible cause of this modulation.
Given the smallness of the aberration corrections that arise from the
nonstationarity of the gravitational field of the randomly distributed ensemble
of stars under consideration, a formula is derived for the Shapiro effect for a
pulsar in a globular cluster. The derived formula is used to calculate the
autocorrelation function of the low-frequency pulsar noise, the slope of its
power spectrum, and the behavior of the statistic that characterizes
the spectral properties of this noise in the form of a time function. The
Shapiro effect under discussion is shown to manifest itself for large impact
parameters as a low-frequency noise of the pulsar spin rate with a spectral
index of n=-1.8 that depends weakly on the specific model distribution of stars
in the globular cluster. For small impact parameters, the spectral index of the
noise is n=-1.5.Comment: 23 pages, 6 figure
Search for Periodic Gravitational Wave Sources with the Explorer Detector
We have developped a procedure for the search of periodic signals in the data
of gravitational wave detectors. We report here the analysis of one year of
data from the resonant detector Explorer, searching for pulsars located in the
Galactic Center (GC). No signals with amplitude greater than , in the range 921.32-921.38 Hz, were observed using data
collected over a time period of 95.7 days, for a source located at
hours and degrees. Our
procedure can be extended for any assumed position in the sky and for a more
general all-sky search, even with a frequency correction at the source due to
the spin-down and Doppler effects.Comment: One zipped file (Latex+eps figures). 33 pages, 14 figures. This and
related material also at http://grwav3.roma1.infn.it
PSR J1847-0130: A Radio Pulsar with Magnetar Spin Characteristics
We report the discovery of PSR J1847-0130, a radio pulsar with a 6.7-s spin
period, in the Parkes multibeam survey of the Galactic plane. The slowdown rate
for the pulsar, 1.3x10^{-12} s/s, is high and implies a surface dipole magnetic
field strength of 9.4x10^{13} G. This inferred dipolar magnetic field strength
is the highest by far among all known radio pulsars and over twice the
``quantum critical field'' above which some models predict radio emission
should not occur. The inferred dipolar magnetic field strength and period of
this pulsar are in the same range as those of the anomalous X-ray pulsars,
which have been identified as being "magnetars" whose luminous X-ray emission
is powered by their large magnetic fields. We have examined archival ASCA data
and place an upper limit on the X-ray luminosity of J1847-0130 which is lower
than the luminosities of all but one AXP. The properties of this pulsar prove
that inferred dipolar magnetic field strength and period cannot alone be
responsible for the unusual high-energy properties of the magnetars and creates
new challenges for understanding the possible relationship between these two
manifestations of young neutron stars.Comment: Accepted by ApJ Letter
SGR J1550-5418 bursts detected with the Fermi Gamma-ray Burst Monitor during its most prolific activity
We have performed detailed temporal and time-integrated spectral analysis of
286 bursts from SGR J1550-5418 detected with the Fermi Gamma-ray Burst Monitor
(GBM) in January 2009, resulting in the largest uniform sample of temporal and
spectral properties of SGR J1550-5418 bursts. We have used the combination of
broadband and high time-resolution data provided with GBM to perform
statistical studies for the source properties. We determine the durations,
emission times, duty cycles and rise times for all bursts, and find that they
are typical of SGR bursts. We explore various models in our spectral analysis,
and conclude that the spectra of SGR J1550-5418 bursts in the 8-200 keV band
are equally well described by optically thin thermal bremsstrahlung (OTTB), a
power law with an exponential cutoff (Comptonized model), and two black-body
functions (BB+BB). In the spectral fits with the Comptonized model we find a
mean power-law index of -0.92, close to the OTTB index of -1. We show that
there is an anti-correlation between the Comptonized Epeak and the burst
fluence and average flux. For the BB+BB fits we find that the fluences and
emission areas of the two blackbody functions are correlated. The
low-temperature BB has an emission area comparable to the neutron star surface
area, independent of the temperature, while the high-temperature blackbody has
a much smaller area and shows an anti-correlation between emission area and
temperature. We compare the properties of these bursts with bursts observed
from other SGR sources during extreme activations, and discuss the implications
of our results in the context of magnetar burst models.Comment: 13 pages, 10 figures, 2 tables; minor changes, ApJ in pres
Energy Release During Disk Accretion onto a Rapidly Rotating Neutron Star
The energy release L_s on the surface of a neutron star (NS) with a weak
magnetic field and the energy release L_d in the surrounding accretion disk
depend on two independent parameters that determine its state (for example,
mass M and cyclic rotation frequency f) and is proportional to the accretion
rate. We derive simple approximation formulas illustrating the dependence of
the efficiency of energy release in an extended disk and in a boundary layer
near the NS surface on the frequency and sense of rotation for various NS
equations of state. Such formulas are obtained for the quadrupole moment of a
NS, for a gap between its surface and a marginally stable orbit, for the
rotation frequency in an equatorial Keplerian orbit and in the marginally
stable circular orbit, and for the rate of NS spinup via disk accretion. In the
case of NS and disk counterrotation, the energy release during accretion can
reach . The sense of NS rotation is a factor that strongly
affects the observed ratio of nuclear energy release during bursts to
gravitational energy release between bursts in X-ray bursters. The possible
existence of binary systems with NS and disk counterrotation in the Galaxy is
discussed. Based on the static criterion for stability, we present a method of
constructing the dependence of gravitational mass M on Kerr rotation parameter
j and on total baryon mass (rest mass) m for a rigidly rotating neutron star.
We show that all global NS characteristics can be expressed in terms of the
function M(j, m) and its derivatives.Comment: 42 pages, 12 figures, to appear in Astronomy Letters, 2000, v.26,
p.69
The characteristics of millisecond pulsar emission: II. Polarimetry
We have made polarimetric monitoring observations of millisecond pulsars
visible from the northern hemisphere at 1410 MHz. Their emission properties are
compared with those of normal pulsars. Although we demonstrated in paper I that
millisecond pulsars exhibit the same flux density spectra and similar profile
complexity, our results presented here suggest that millisecond pulsar profiles
do not comply with the predictions of classification schemes based on
``normal'' pulsars. The frequency development of a large number of millisecond
pulsar profiles is abnormal when compared with the development seen for normal
pulsars. Moreover, the polarization characteristics suggest that
millisecond-pulsar magnetospheres might not simply represent scaled versions of
the magnetospheres of normal pulsars, supporting results of paper I. However,
phenomena such as mode-changing activity in both intensity and polarization are
recognized here for the first time (e.g., J1730--2304). This suggests that
while the basic emission mechanism remains insensitive to rotational period,
the conditions that, according to the canonical pulsar model, regulate the
radio emission, might be satisfied at different regions in millisecond pulsar
magnetospheres.
At least three types of model have been proposed to describe the millisecond
pulsar magnetospheres. A comparison of the predictions of these models with the
observations suggests that individual cases are better explained by different
processes. However, we show that millisecond pulsars can be grouped according
to common emission properties, a grouping that awaits verification from future
multifrequency observations.Comment: 38 pages, 8 figures, accepted for publication in ApJ, (see
astro-ph/9801177 for paper I
X-ray Observations of the New Pulsar--Supernova Remnant System PSR J1119-6127 and SNR G292.2-0.5
PSR J1119-6127 is a recently discovered 1700-year-old radio pulsar that has a
very high inferred surface dipolar magnetic field. We present a detailed
analysis of a pointed ASCA observation and archival ROSAT data of J1119-6127
and its surroundings. Both data sets reveal extended emission coincident with
the newly-discovered radio supernova remnant G292.2-0.5, reported in a
companion paper by Crawford et al. (astro-ph/0012287).. A hard point source,
offset ~1.5' from the position of the radio pulsar, is seen with the ASCA GIS.
No pulsations are detected at the radio period with a pulsed fraction upper
limit of 61% (95% confidence). The limited statistics prevent a detailed
spectral analysis, although a power-law model with photon index (Gamma=~1-2)
describes the data well. Both the spectral model and derived X-ray luminosity
are consistent with those measured for other young radio pulsars, although the
spatial offset renders an identification of the source as the X-ray counterpart
of the pulsar uncertain.Comment: Accepted by ApJ; 12 Pages in emulated ApJ format with embedded tables
and color figure
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