224 research outputs found
The first Suzaku observation of SGR 1806-20
The soft gamma-ray repeater SGR 1806-20 has been attracting a lot of
attention owing to the fact that in December 2004 it emitted the most powerful
giant flare ever observed. Here we present the results of the first Suzaku
observation of SGR 1806-20, that seems to have reached a state characterized by
a flux close to the pre-flare level and by a relatively soft spectrum. Despite
this, the source remained quite active, as testified by several short bursts
observed by Suzaku. We discuss the broadband spectral properties of SGR 1806-20
in the context of the magnetar model, considering its recent theoretical
developments.Comment: 3 pages, 1 figure. Proceedings of the conference "40 Years of
Pulsars, Millisecond Pulsars, Magnetars and More", Montreal, August 12-17
2007. AIP, in pres
On the nature of the intermittent pulsar PSR B1931+24
PSR B1931+24 is the first intermittent radio pulsar discovered to date,
characterized by a 0.8 s pulsation which turns on and off quasi-periodically
every ~35 days, with a duty cycle of ~10%. We present here X-ray and optical
observations of PSR B1931+24 performed with the Chandra X-ray Observatory and
Isaac Newton Telescope, respectively. Simultaneous monitoring from the Jodrell
Bank Observatory showed that this intermittent pulsar was in the radio-on phase
during our observations. We do not find any X-ray or optical counterpart to PSR
B1931+24 translating into an upper limit of ~2x10^{31} erg/s on the X-ray
luminosity, and of g' > 22.6 on the optical magnitude. If the pulsar is
isolated, these limits cannot constrain the dim X-ray and optical emission
expected for a pulsar of that age (~1.6 Myr). We discuss the possibility that
the quasi-periodic intermittent behavior of PSR B1931+24 is due to the presence
of a low mass companion star or gaseous planet, tight with the pulsar in an
eccentric orbit. In order to constrain the parameters of this putative binary
system we re-analysed the pulsar radio timing residuals and we found that (if
indeed hosted in a binary system), PSR B1931+24 should have a very low mass
companion and an orbit of low inclination.Comment: 6 pages; accepted for publication in MNRA
Transient Phenomena in Anomalous X-ray Pulsars
In 2003 a previously unpulsed Einstein and ROSAT source cataloged as soft and
dim (Lx of few 10^33 ergs) thermal emitting object, namely XTE J1810-197, was
identified as the first unambiguous transient Anomalous X-ray Pulsar. Two years
later this source was also found to be a bright highly polarized transient
radio pulsar, a unique property among both AXPs and radio pulsars. In September
2006 the Swift Burst Alert Telescope (BAT) detected an intense burst from the
candidate AXP CXOU J164710.2-455216, which entered in an outburst state
reaching a peak emission of at least a factor of 300 higher than quiescence.
Here, we briefly outline the recent results concerning the outburst phenomena
observed in these two AXPs. In particular, XTE J1810-197 has probed to be a
unique laboratory to monitor the timing and spectral properties of a
cooling/fading AXP, while new important information have been inferred from
X-ray and radio band simultaneous observations. CXOU J164710.2-455216 has been
monitored in X-rays and radio bands since the very beginning of its outbursting
state allowing us to cover the first phases of the outburst and to study the
timing and spectral behavior during the first months.Comment: 5 pages, 3 figures, proceedings of the conference "40 Years of
Pulsars, Millisecond Pulsars, Magnetars and More", Montreal, August 12-17
2007. To be published by AIP
GBT Discovery of Two Binary Millisecond Pulsars in the Globular Cluster M30
We report the discovery of two binary millisecond pulsars in the
core-collapsed globular cluster M30 using the Green Bank Telescope (GBT) at 20
cm. PSR J2140-2310A (M30A) is an eclipsing 11-ms pulsar in a 4-hr circular
orbit and PSR J2140-23B (M30B) is a 13-ms pulsar in an as yet undetermined but
most likely highly eccentric (e>0.5) and relativistic orbit. Timing
observations of M30A with a 20-month baseline have provided precise
determinations of the pulsar's position (within 4" of the optical centroid of
the cluster), and spin and orbital parameters, which constrain the mass of the
companion star to be m_2 >~ 0.1Msun. The position of M30A is coincident with a
possible thermal X-ray point source found in archival Chandra data which is
most likely due to emission from hot polar caps on the neutron star. In
addition, there is a faint (V_555 ~ 23.8) star visible in archival HST F555W
data that may be the companion to the pulsar. Eclipses of the pulsed radio
emission from M30A by the ionized wind from the compact companion star show a
frequency dependent duration (\propto\nu^{-\alpha} with \alpha ~ 0.4-0.5) and
delay the pulse arrival times near eclipse ingress and egress by up to 2-3 ms.
Future observations of M30 may allow both the measurement of post-Keplerian
orbital parameters from M30B and the detection of new pulsars due to the
effects of strong diffractive scintillation.Comment: 10 pages, 6 figures, Submitted to ApJ. This version includes many
recommended modifications, an improved structure, a new author, and a
completely redone optical analysi
LOFAR discovery of the fastest-spinning millisecond pulsar in the Galactic field
We report the discovery of PSR J09520607, a 707-Hz binary millisecond
pulsar which is now the fastest-spinning neutron star known in the Galactic
field (i.e., outside of a globular cluster). PSR J09520607 was found using
LOFAR at a central observing frequency of 135 MHz, well below the 300 MHz to 3
GHz frequencies typically used in pulsar searches. The discovery is part of an
ongoing LOFAR survey targeting unassociated Fermi Large Area Telescope
-ray sources. PSR J09520607 is in a 6.42-hr orbit around a very
low-mass companion ( M) and we identify a
strongly variable optical source, modulated at the orbital period of the
pulsar, as the binary companion. The light curve of the companion varies by 1.6
mag from at maximum to , indicating that it is
irradiated by the pulsar wind. Swift observations place a 3- upper
limit on the keV X-ray luminosity of erg
s (using the 0.97 kpc distance inferred from the dispersion measure).
Though no eclipses of the radio pulsar are observed, the properties of the
system classify it as a black widow binary. The radio pulsed spectrum of PSR
J09520607, as determined through flux density measurements at 150 and 350
MHz, is extremely steep with (where ).
We discuss the growing evidence that the fastest-spinning radio pulsars have
exceptionally steep radio spectra, as well as the prospects for finding more
sources like PSR J09520607.Comment: 9 pages, 3 figures, 1 table, published in ApJ letter
Unveiling the hard X-ray spectrum from the "burst-only" source SAX J1753.5-2349 in outburst
Discovered in 1996 by BeppoSAX during a single type-I burst event, SAX
J1753.5-2349 was classified as "burst-only" source. Its persistent emission,
either in outburst or in quiescence, had never been observed before October
2008, when SAX J1753.5-2349 was observed for the first time in outburst. Based
on INTEGRAL observations,we present here the first high-energy emission study
(above 10 keV) of a so-called "burst-only". During the outburst the SAX
J1753.5-2349 flux decreased from 10 to 4 mCrab in 18-40 keV, while it was found
being in a constant low/hard spectral state. The broad-band (0.3-100 keV)
averaged spectrum obtained by combining INTEGRAL/IBIS and Swift/XRT data has
been fitted with a thermal Comptonisation model and an electron temperature >24
keV inferred. However, the observed high column density does not allow the
detection of the emission from the neutron star surface. Based on the whole set
of observations of SAX J1753.5-2349, we are able to provide a rough estimate of
the duty cycle of the system and the time-averaged mass-accretion rate. We
conclude that the low to very low luminosity of SAX J1753.5-2349 during
outburst may make it a good candidate to harbor a very compact binary system.Comment: 5 pages, 3 figures, 2 tables; accepted for publication in MNRAS
Letter
Hard X-ray variability of Magnetar's Tails observed with INTEGRAL
Magnetar's persistent emission above 10 keV was recently discovered thanks to
the imaging capabilities of the IBIS coded mask telescope on board the INTEGRAL
satellite. The only two sources that show some degree of long term variability
are SGR 1806-20 and 1RXS J170849.0-400910. We find some indications that
variability of these hard tails could be the driver of the spectral variability
measured in these sources below 10 keV. In addition we report for the first
time the detection at 2.8 sigma level of pulsations in the hard X-ray tail of
SGR 1806-20.Comment: 3 pages, 3 figures, proceedings of the conference "40 Years of
Pulsars, Millisecond Pulsars, Magnetars and More", Montreal, August 12-17
2007. To be published by AIP
An Eccentric Binary Millisecond Pulsar in the Galactic Plane
Binary pulsar systems are superb probes of stellar and binary evolution and
the physics of extreme environments. In a survey with the Arecibo telescope, we
have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 ms
in a highly eccentric (e = 0.44) 95-day orbit around a solar mass companion.
Infrared observations identify a possible main-sequence companion star.
Conventional binary stellar evolution models predict neither large orbital
eccentricities nor main-sequence companions around millisecond pulsars.
Alternative formation scenarios involve recycling a neutron star in a globular
cluster then ejecting it into the Galactic disk or membership in a hierarchical
triple system. A relativistic analysis of timing observations of the pulsar
finds its mass to be 1.74+/-0.04 Msun, an unusually high value.Comment: 28 pages, 4 figures inc Supplementary On-Line Material. Accepted for
publication in Science, published on Science Express: 10.1126/science.115758
The noise properties of 42 millisecond pulsars from the European Pulsar Timing Array and their impact on gravitational wave searches
The sensitivity of Pulsar Timing Arrays to gravitational waves depends on the
noise present in the individual pulsar timing data. Noise may be either
intrinsic or extrinsic to the pulsar. Intrinsic sources of noise will include
rotational instabilities, for example. Extrinsic sources of noise include
contributions from physical processes which are not sufficiently well modelled,
for example, dispersion and scattering effects, analysis errors and
instrumental instabilities. We present the results from a noise analysis for 42
millisecond pulsars (MSPs) observed with the European Pulsar Timing Array. For
characterising the low-frequency, stochastic and achromatic noise component, or
"timing noise", we employ two methods, based on Bayesian and frequentist
statistics. For 25 MSPs, we achieve statistically significant measurements of
their timing noise parameters and find that the two methods give consistent
results. For the remaining 17 MSPs, we place upper limits on the timing noise
amplitude at the 95% confidence level. We additionally place an upper limit on
the contribution to the pulsar noise budget from errors in the reference
terrestrial time standards (below 1%), and we find evidence for a noise
component which is present only in the data of one of the four used telescopes.
Finally, we estimate that the timing noise of individual pulsars reduces the
sensitivity of this data set to an isotropic, stochastic GW background by a
factor of >9.1 and by a factor of >2.3 for continuous GWs from resolvable,
inspiralling supermassive black-hole binaries with circular orbits.Comment: Accepted for publication by the Monthly Notices of the Royal
Astronomical Societ
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