1,200 research outputs found
On the Apparent Nulls and Extreme Variability of PSR J1107-5907
We present an analysis of the emission behaviour of PSR J1107-5907, a source
known to exhibit separate modes of emission, using observations obtained over
approximately 10 yr. We find that the object exhibits two distinct modes of
emission; a strong mode with a broad profile and a weak mode with a narrow
profile. During the strong mode of emission, the pulsar typically radiates very
energetic emission over sequences of ~200-6000 pulses (~60 s-24 min), with
apparent nulls over time-scales of up to a few pulses at a time. Emission
during the weak mode is observed outside of these strong-mode sequences and
manifests as occasional bursts of up to a few clearly detectable pulses at a
time, as well as low-level underlying emission which is only detected through
profile integration. This implies that the previously described null mode may
in fact be representative of the bottom-end of the pulse intensity distribution
for the source. This is supported by the dramatic pulse-to-pulse intensity
modulation and rarity of exceptionally bright pulses observed during both modes
of emission. Coupled with the fact that the source could be interpreted as a
rotating radio transient (RRAT)-like object for the vast majority of the time,
if placed at a further distance, we advance that this object likely represents
a bridge between RRATs and extreme moding pulsars. Further to these emission
properties, we also show that the source is consistent with being a
near-aligned rotator and that it does not exhibit any measurable spin-down rate
variation. These results suggest that nulls observed in other intermittent
objects may in fact be representative of very weak emission without the need
for complete cessation. As such, we argue that longer (> 1 h) observations of
pulsars are required to discern their true modulation properties.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Neutron star glitches have a substantial minimum size
Glitches are sudden spin-up events that punctuate the steady spin down of
pulsars and are thought to be due to the presence of a superfluid component
within neutron stars. The precise glitch mechanism and its trigger, however,
remain unknown. The size of glitches is a key diagnostic for models of the
underlying physics. While the largest glitches have long been taken into
account by theoretical models, it has always been assumed that the minimum size
lay below the detectability limit of the measurements. In this paper we define
general glitch detectability limits and use them on 29 years of daily
observations of the Crab pulsar, carried out at Jodrell Bank Observatory. We
find that all glitches lie well above the detectability limits and by using an
automated method to search for small events we are able to uncover the full
glitch size distribution, with no biases. Contrary to the prediction of most
models, the distribution presents a rapid decrease of the number of glitches
below ~0.05 Hz. This substantial minimum size indicates that a glitch must
involve the motion of at least several billion superfluid vortices and provides
an extra observable which can greatly help the identification of the trigger
mechanism. Our study also shows that glitches are clearly separated from all
the other rotation irregularities. This supports the idea that the origin of
glitches is different to that of timing noise, which comprises the unmodelled
random fluctuations in the rotation rates of pulsars.Comment: 8 pages; 4 figures. Accepted for publication in MNRA
Very Long Baseline Interferometry Measured Proper Motion and Parallax of the -ray Millisecond Pulsar PSR J0218+4232
PSR J02184232 is a millisecond pulsar (MSP) with a flux density 0.9
mJy at 1.4 GHz. It is very bright in the high-energy X-ray and -ray
domains. We conducted an astrometric program using the European VLBI Network
(EVN) at 1.6 GHz to measure its proper motion and parallax. A model-independent
distance would also help constrain its -ray luminosity. We achieved a
detection of signal-to-noise ratio S/N > 37 for the weak pulsar in all five
epochs. Using an extragalactic radio source lying 20 arcmin away from the
pulsar, we estimate the pulsar's proper motion to be
mas yr and mas yr, and a parallax of mas. The very long
baseline interferometry (VLBI) proper motion has significantly improved upon
the estimates from long-term pulsar timing observations. The VLBI parallax
provides the first model-independent distance constraints:
kpc, with a corresponding lower-limit of
kpc. This is the first pulsar trigonometric parallax measurement based
solely on EVN observations. Using the derived distance, we believe that PSR
J02184232 is the most energetic -ray MSP known to date. The
luminosity based on even our 3 lower-limit distance is high enough to
pose challenges to the conventional outer gap and slot gap models.Comment: 5 pages, 2 figures, 2 tables; published in the Astrophysical Journal
Letters on 2014 Feb. 1
Are all fast radio bursts repeating sources?
We present Monte-Carlo simulations of a cosmological population of repeating
fast radio burst (FRB) sources whose comoving density follows the cosmic star
formation rate history. We assume a power-law model for the intrinsic energy
distribution for each repeating FRB source located at a randomly chosen
position in the sky and simulate their dispersion measures (DMs) and
propagation effects along the chosen lines-of-sight to various telescopes. In
one scenario, an exponential distribution for the intrinsic wait times between
pulses is chosen, and in a second scenario we model the observed pulse arrival
times to follow a Weibull distribution. For both models we determine whether
the FRB source would be deemed a repeater based on the telescope sensitivity
and time spent on follow-up observations. We are unable to rule out the
existence of a single FRB population based on comparisons of our simulations
with the longest FRB follow-up observations performed. We however rule out the
possibility of FRBs 171020 and 010724 repeating with the same rate statistics
as FRB 121102 and also constrain the slope of a power-law fit to the FRB energy
distribution to be . All-sky simulations of repeating FRB
sources imply that the detection of singular events correspond to the bright
tail-end of the adopted energy distribution due to the combination of the
increase in volume probed with distance, and the position of the burst in the
telescope beam.Comment: 10 pages, 4 figures, accepted for publication in MNRA
A deep search for pulsar wind nebulae using pulsar gating
Using the Australia Telescope Compact Array (ATCA) we have imaged the fields
around five promising pulsar candidates to search for radio pulsar wind nebulae
(PWNe). We have used the ATCA in its pulsar gating mode; this enables an image
to be formed containing only off-pulse visibilities, thereby dramatically
improving the sensitivity to any underlying PWN. Data from the Molonglo
Observatory Synthesis Telescope were also used to provide sensitivity on larger
spatial scales. This survey found a faint new PWN around PSR B0906-49; here we
report on non-detections of PWNe towards PSRs B1046-58, B1055-52, B1610-50 and
J1105-6107. Our radio observations of the field around PSR B1055-52 argue
against previous claims of an extended X-ray and radio PWNe associated with the
pulsar. If these pulsars power unseen, compact radio PWN, upper limits on the
radio flux indicate that less than 1e-6 of their spin-down energy is used to
power this emission. Alternatively PSR B1046-58 and PSR B1610-50 may have
relativistic winds similar to other young pulsars and the unseen PWN is
resolved and fainter than our surface brightness sensitivity threshold. We can
then determine upper limits on the local ISM density of 2.2e-3 cm^-3 and 1e-2
cm^-3, respectively. Furthermore we constrain the spatial velocities of these
pulsars to be less than ~450 km/s and thus rule out the association of PSR
B1610-50 with SNR G332.4+00.1 (Kes 32). Strong limits on the ratio of unpulsed
to pulsed emission are also determined for three pulsars.Comment: 10 pages, 5 figures, MNRAS in pres
The identification of the optical companion to the binary millisecond pulsar J0610-2100 in the Galactic field
We have used deep V and R images acquired at the ESO Very Large Telescope to
identify the optical companion to the binary pulsar PSR J0610-2100, one of the
black-widow millisecond pulsars recently detected by the Fermi Gamma-ray
Telescope in the Galactic plane. We found a faint star (V~26.7) nearly
coincident (\delta r ~0".28) with the pulsar nominal position. This star is
visible only in half of the available images, while it disappears in the
deepest ones (those acquired under the best seeing conditions), thus indicating
that it is variable. Although our observations do not sample the entire orbital
period (P=0.28 d) of the pulsar, we found that the optical modulation of the
variable star nicely correlates with the pulsar orbital period and describes a
well defined peak (R~25.6) at \Phi=0.75, suggesting a modulation due to the
pulsar heating. We tentatively conclude that the companion to PSR J0610-2100 is
a heavily ablated very low mass star (~ 0.02Msun) that completely filled its
Roche Lobe.Comment: 17 pages, 5 figures - Accepted for pubblication in Ap
X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary
We present XMM-Newton and Chandra observations of the low-mass X-ray binary
XSS J12270--4859, which experienced a dramatic decline in optical/X-ray
brightness at the end of 2012, indicative of the disappearance of its accretion
disk. In this new state, the system exhibits previously absent
orbital-phase-dependent, large-amplitude X-ray modulations with a decline in
flux at superior conjunction. The X-ray emission remains predominantly
non-thermal but with an order of magnitude lower mean luminosity and
significantly harder spectrum relative to the previous high flux state. This
phenomenology is identical to the behavior of the radio millisecond pulsar
binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray
emission is produced in an intra-binary shock driven by the pulsar wind. This
further demonstrates that XSS J12270-4859 no longer has an accretion disk and
has transformed to a full-fledged eclipsing "redback" system that hosts an
active rotation-powered millisecond pulsar. There is no evidence for diffuse
X-ray emission associated with the binary that may arise due to outflows or a
wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely
to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical
Journa
X-ray Observations of High-B Radio Pulsars
The study of high-magnetic-field pulsars is important for examining the
relationships between radio pulsars, magnetars, and X-ray-isolated neutron
stars (XINSs). Here we report on X-ray observations of three such
high-magnetic-field radio pulsars. We first present the results of a deep
XMM-Newton observation of PSR J1734-3333, taken to follow up on its initial
detection in 2009. The pulsar's spectrum is well fit by a blackbody with a
temperature of 300 +/- 60 eV, with bolometric luminosity L_bb = 2.0(+2.2
-0.7)e+32 erg/s = 0.0036E_dot for a distance of 6.1 kpc. We detect no X-ray
pulsations from the source, setting a 1 sigma upper limit on the pulsed
fraction of 60% in the 0.5-3 keV band. We compare PSR J1734-3333 to other
rotation-powered pulsars of similar age and find that it is significantly
hotter, supporting the hypothesis that the magnetic field affects the observed
thermal properties of pulsars. We also report on XMM-Newton and Chandra
observations of PSRs B1845-19 and J1001-5939. We do not detect either pulsar,
setting 3 sigma upper limits on their blackbody temperatures of 48 and 56 eV,
respectively. Despite the similarities in rotational properties, these sources
are significantly cooler than all but one of the XINSs, which we attribute to
the two groups having been born with different magnetic fields and hence
evolving differently.Comment: 18 pages, 2 tables, 5 figures, accepted for publication in the
Astrophysical Journa
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