803 research outputs found
A Search for Pulsars in Quiescent Soft X-Ray Transients. I
We have carried out a deep search at 1.4 GHz for radio pulsed emission from
six soft X-ray transient sources observed during their X-ray quiescent phase.
The commonly accepted model for the formation of the millisecond radio pulsars
predicts the presence of a rapidly rotating, weakly magnetized neutron star in
the core of these systems. The sudden drop in accretion rate associated with
the end of an X-ray outburst causes the Alfv\`en surface to move outside the
light cylinder, allowing the pulsar emission process to operate. No pulsed
signal was detected from the sources in our sample. We discuss several
mechanisms that could hamper the detection and suggest that free-free
absorption from material ejected from the system by the pulsar radiation
pressure could explain our null result.Comment: accepted by Ap
Spin-down evolution and radio disappearance of the magnetar PSR J16224950
We report on 2.4 yr of radio timing measurements of the magnetar PSR
J16224950 using the Parkes telescope, between 2011 November and 2014 March.
During this period the torque on the neutron star (inferred from the rotational
frequency derivative) varied greatly, though much less erratically than in the
2 yr following its discovery in 2009. During the last year of our measurements
the frequency derivative decreased in magnitude monotonically by 20\%, to a
value of s, a factor of 8 smaller than when
discovered. The flux density continued to vary greatly during our monitoring
through 2014 March, reaching a relatively steady low level after late 2012. The
pulse profile varied secularly on a similar timescale as the flux density and
torque. A relatively rapid transition in all three properties is evident in
early 2013. After PSR J16224950 was detected in all of our 87 observations
up to 2014 March, we did not detect the magnetar in our resumed monitoring
starting in 2015 January and have not detected it in any of the 30 observations
done through 2016 September.Comment: 8 pages, 5 figures, submitted to Ap
Discovery of short-period binary millisecond pulsars in four globular clusters
We report the discovery using the Parkes radio telescope of binary
millisecond pulsars in four clusters for which no associated pulsars were
previously known. The four pulsars have pulse periods lying between 3 and 6 ms.
All are in circular orbits with low-mass companions and have orbital periods of
a few days or less. One is in a 1.7-hour orbit with a companion of planetary
mass. Another is eclipsed by a wind from its companion for 40% of the binary
period despite being in a relatively wide orbit. These discoveries result from
the use of improved technologies and prove that many millisecond pulsars remain
to be found in globular clusters.Comment: 4 pages, 2 figs, 1 table - Accepted by Astrophysical Journal Letter
Simultaneous Multi-band Radio & X-ray Observations of the Galactic Center Magnetar SGR 17452900
We report on multi-frequency, wideband radio observations of the Galactic
Center magnetar (SGR 17452900) with the Green Bank Telescope for 100
days immediately following its initial X-ray outburst in April 2013. We made
multiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us to
examine the magnetar's flux evolution, radio spectrum, and interstellar medium
parameters (such as the dispersion measure (DM), the scattering timescale and
its index). During two epochs, we have simultaneous observations from the
Chandra X-ray Observatory, which permitted the absolute alignment of the radio
and X-ray profiles. As with the two other radio magnetars with published
alignments, the radio profile lies within the broad peak of the X-ray profile,
preceding the X-ray profile maximum by 0.2 rotations. We also find that
the radio spectral index is significantly negative between 2 and
9 GHz; during the final 30 days of our observations ,
which is typical of canonical pulsars. The radio flux has not decreased during
this outburst, whereas the long-term trends in the other radio magnetars show
concomitant fading of the radio and X-ray fluxes. Finally, our wideband
measurements of the DMs taken in adjacent frequency bands in tandem are
stochastically inconsistent with one another. Based on recent theoretical
predictions, we consider the possibility that the dispersion measure is
frequency-dependent. Despite having several properties in common with the other
radio magnetars, such as , an
increase in the radio flux during the X-ray flux decay has not been observed
thus far in other systems.Comment: 15 pages, 9 figures, 3 tables; accepted to Ap
Discovery of 59ms Pulsations from 1RXS J141256.0+792204 (Calvera)
We report on a multi-wavelength study of the compact object candidate 1RXS
J141256.0+792204 (Calvera). Calvera was observed in the X-rays with XMM/EPIC
twice for a total exposure time of ~50 ks. The source spectrum is thermal and
well reproduced by a two component model composed of either two hydrogen
atmosphere models, or two blackbodies (kT_1~ 55/150 eV, kT_2~ 80/250 eV,
respectively, as measured at infinity). Evidence was found for an absorption
feature at ~0.65 keV; no power-law high-energy tail is statistically required.
Using pn and MOS data we discovered pulsations in the X-ray emission at a
period P=59.2 ms. The detection is highly significant (> 11 sigma), and
unambiguously confirms the neutron star nature of Calvera. The pulse profile is
nearly sinusoidal, with a pulsed fraction of ~18%. We looked for the timing
signature of Calvera in the Fermi Large Area Telescope (LAT) database and found
a significant (~5 sigma) pulsed signal at a period coincident with the X-ray
value. The gamma-ray timing analysis yielded a tight upper limit on the period
derivative, dP/dt < 5E-18 s/s (dE_rot/dt <1E33 erg/s, B<5E10 G for magneto-
dipolar spin-down). Radio searches at 1.36 GHz with the 100-m Effelsberg radio
telescope yielded negative results, with a deep upper limit on the pulsed flux
of 0.05 mJy. Diffuse, soft (< 1 keV) X-ray emission about 13' west of the
Calvera position is present both in our pointed observations and in archive
ROSAT all-sky survey images, but is unlikely associated with the X-ray pulsar.
Its spectrum is compatible with an old supernova remnant (SNR); no evidence for
diffuse emission in the radio and optical bands was found. The most likely
interpretations are that Calvera is either a central compact object escaped
from a SNR or a mildly recycled pulsar; in both cases the source would be the
first ever member of the class detected at gamma-ray energies.Comment: 20 pages, 15 figures and 4 tables. Accepted for publication in MNRA
X-ray and radio observations of the magnetar Swift J1834.9-0846 and its dust-scattering halo
We present a long-term study of the 2011 outburst of the magnetar Swift
J1834.9-0846 carried out using new Chandra observations, as well as all the
available Swift, RXTE, and XMM-Newton data. The last observation was performed
on 2011 November 12, about 100 days after the onset of the bursting activity
that had led to the discovery of the source on 2011 August 07. This long time
span enabled us to refine the rotational ephemeris and observe a downturn in
the decay of the X-ray flux. Assuming a broken power law for the long-term
light curve, the break was at ~46 d after the outburst onset, when the decay
index changed from alpha ~ 0.4 to ~4.5. The flux decreased by a factor ~2 in
the first ~50 d and then by a factor ~40 until November 2011 (overall, by a
factor ~70 in ~100 d). At the same time, the spectrum, which was well described
by an absorbed blackbody all along the outburst, softened, the temperature
dropping from ~1 to ~0.6 keV. Diffuse X-ray emission extending up to 20" from
the source was clearly detected in all Chandra observations. Its spatial and
spectral properties, as well as its time evolution, are consistent with a
dust-scattering halo due to a single cloud located at a distance of
200 pc from Swift J1834.9-0846, which should be in turn located at a
distance of ~5 kpc. Considering the time delay of the scattered photons, the
same dust cloud might also be responsible for the more extended emission
detected in XMM-Newton data taken in September 2011. We searched for the radio
signature of Swift J1834.9-0846 at radio frequencies using the Green Bank Radio
Telescope and in archival data collected at Parkes from 1998 to 2003. No
evidence for radio emission was found, down to a flux density of 0.05 mJy (at 2
GHz) during the outburst and ~0.2-0.3 mJy (at 1.4 GHz) in the older data.Comment: 11 pages, 9 figures and 4 tables, accepted for publication in MNRA
The glitches of the Anomalous X-ray Pulsar 1RXS J170849.0--400910
We report on a timing analysis of archival observations of the Anomalous
X-ray Pulsar 1RXSJ170849.0-400910 made with the RXTE Proportional Counter
Array. We detect a new large glitch (Delta nu / nu ~ 3 x 10^-6) which occurred
between 2001 March 27 and 2001 May 6, with an associated large increase in the
spin-down rate (Delta nudot/nudot ~ 0.3). The short time (1.5 yrs) elapsed from
the previously detected glitch and the large amplitude of the new spin-up place
this source among the most frequent glitchers, with large average glitch
amplitudes, similar to those of the Vela pulsar. The source shows different
recoveries after the glitches: in the first one it is well described by a long
term linear trend similar to those seen in Vela-like glitches; in the second
case the recovery is considerably faster and is better described by an
exponential plus a fractional change in the long-term spin-down rate of the
order of 1%. No recovery of the latter is detected but additional observations
are necessary to confirm this result. Observed glitch properties are compared
to those of radio pulsar glitches; current models are discussed in light of our
results. It appears that glitches may represent yet another peculiarity of
AXPs. Starquake-based models appear to be prefered on qualitative grounds.
Alternative models can be applied to individual glitches but fail in explaining
both. Thus the two events may as well arise from two different mechanisms.Comment: Accepted for publication in ApJ Main Journal on August 20, minor
changes after referee's report. 27 pages and 6 figure
Timing of Millisecond Pulsars in NGC 6752: Evidence for a High Mass-to-Light Ratio in the Cluster Core
Using pulse timing observations we have obtained precise parameters,
including positions with about 20 mas accuracy, of five millisecond pulsars in
NGC 6752. Three of them, located relatively close to the cluster center, have
line-of-sight accelerations larger than the maximum value predicted by the
central mass density derived from optical observation, providing dynamical
evidence for a central mass-to-light ratio >~ 10, much higher than for any
other globular cluster. It is likely that the other two millisecond pulsars
have been ejected out of the core to their present locations at 1.4 and 3.3
half-mass radii, respectively, suggesting unusual non-thermal dynamics in the
cluster core.Comment: Accepted by ApJ Letter. 5 pages, 2 figures, 1 tabl
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