63 research outputs found
A Search for Pulsars in Steep Spectrum Radio Sources
We report on a time-domain search for pulsars in 44 steep spectrum radio
sources originally identified from recent imaging surveys. The time-domain
search was conducted at 327 MHz using the Ooty radio telescope, and utilized a
semi-coherent dedispersion scheme retaining the sensitivity even for
sub-millisecond periods up to reasonably high dispersion measures. No new
pulsars were found. We discuss the nature of these steep spectrum sources and
argue that majority of the sources in our sample should either be pulsars or a
new category of Galactic sources. Several possibilities that could hinder
detection of these sources as pulsars, including anomalously high scattering or
alignment of the rotation and magnetic axes, are discussed in detail, and we
suggest unconventional search methods to further probe these possibilities.Comment: Accepted for publication in Ap
X-ray and Near-IR Variability of the Anomalous X-ray Pulsar 1E 1048.1-5937: From Quiescence Back to Activity
(Abridged) We report on new and archival X-ray and near-infrared observations
of the anomalous X-ray pulsar 1E 1048.1-5937 performed between 2001-2007 with
RXTE, CXO, Swift, HST, and VLT. During its ~2001-2004 active period, 1E
1048.-5937 exhibited two large, long-term X-ray pulsed-flux flares as well as
short bursts, and large (>10x) torque changes. Monitoring with RXTE revealed
that the source entered a phase of timing stability in 2004; at the same time,
a series of four simultaneous observations with CXO and HST in 2006 showed that
its X-ray flux and spectrum and near-IR flux, all variable prior to 2005,
stabilized. The near-IR flux, when detected by HST (H~22.7 mag) and VLT
(K_S~21.0 mag), was considerably fainter than previously measured. Recently, in
2007 March, this newfound quiescence was interrupted by a sudden flux
enhancement, X-ray spectral changes and a pulse morphology change, simultaneous
with a large spin-up glitch and near-IR enhancement. Our RXTE observations
revealed a sudden pulsed flux increase by a factor of ~3 in the 2-10 keV band.
In observations with CXO and Swift, we found that the total X-ray flux
increased much more than the pulsed flux, reaching a peak value of >7 times the
quiescent value (2-10 keV). With these recent data, we find a strong
anti-correlation between X-ray flux and pulsed fraction, and a correlation
between X-ray spectral hardness and flux. Simultaneously with the radiative and
timing changes, we observed a significant X-ray pulse morphology change such
that the profile went from nearly sinusoidal to having multiple peaks. We
compare these remarkable events with other AXP outbursts and discuss
implications in the context of the magnetar model and other models of AXP
emission.Comment: 13 pages (6 figures) in emulateapj style. Accepted for publication in
ApJ. New version includes referee's corrections; split Figure 1 into 2
figures; modified Figs. 4b and 6b; rearranged and renumbered of some figures
and sections; added an X-ray dataset; improved analysis of pulse morphology
and pulsed fraction; added paragraph to sec. 3.2.
Observational evidence for the origin of X-ray sources in globular clusters
Low-mass X-ray binaries, recycled pulsars, cataclysmic variables and
magnetically active binaries are observed as X-ray sources in globular
clusters. We discuss the classification of these systems, and find that some
presumed active binaries are brighter than expected. We discuss a new
statistical method to determine from observations how the formation of X-ray
sources depends on the number of stellar encounters and/or on the cluster mass.
We show that cluster mass is not a proxy for the encounter number, and that
optical identifications are essential in proving the presence of primordial
binaries among the low-luminosity X-ray sources.Comment: 10 pages, 7 figures, to appear in IAUS 246, Dynamical evolution of
dense stellar systems, ed. E. Vesperin
Single-pulse classifier for the LOFAR Tied-Array All-sky Survey
Searches for millisecond-duration, dispersed single pulses have become a standard tool used during radio pulsar surveys in the last decade. They have enabled the discovery of two new classes of sources: rotating radio transients and fast radio bursts. However, we are now in a regime where the sensitivity to single pulses in radio surveys is often limited more by the strong background of radio frequency interference (RFI, which can greatly increase the false-positive rate) than by the sensitivity of the telescope itself. To mitigate this problem, we introduce the Single-pulse Searcher (SPS). This is a new machine-learning classifier designed to identify astrophysical signals in a strong RFI environment, and optimized to process the large data volumes produced by the new generation of aperture array telescopes. It has been specifically developed for the LOFAR Tied-Array All-Sky Survey (LOTAAS), an ongoing survey for pulsars and fast radio transients in the northern hemisphere. During its development, SPS discovered seven new pulsars and blindly identified ˜80 known sources. The modular design of the software offers the possibility to easily adapt it to other studies with different instruments and characteristics. Indeed, SPS has already been used in other projects, e.g. to identify pulses from the fast radio burst source FRB 121102. The software development is complete and SPS is now being used to re-process all LOTAAS data collected to date
The lofar tied-array all-sky survey: Timing of 35 radio pulsars and an overview of the properties of the lofar pulsar discoveries
The LOFAR Tied-Array All-Sky Survey (LOTAAS) is the most sensitive untargeted radio pulsar survey performed at low radio
frequencies (119−151 MHz) to date and has discovered 76 new radio pulsars, including the 23.5-s pulsar J0250+5854, which up until
recently was the slowest spinning radio pulsar known. In this paper, we report on the timing solutions of 35 pulsars discovered by
LOTAAS, which include a nulling pulsar and a mildly recycled pulsar, and thereby complete the full timing analysis of the LOTAAS
pulsar discoveries. We give an overview of the findings from the full LOTAAS sample of 76 pulsars, discussing their pulse profiles,
radio spectra, and timing parameters. We found that the pulse profiles of some of the pulsars show profile variations in time or
frequency, and while some pulsars show signs of scattering, a large majority display no pulse broadening. The LOTAAS discoveries
have on average steeper radio spectra and longer spin periods (1.4×), as well as lower spin-down rates (3.1×) compared to the known
pulsar population. We discuss the cause of these differences and attribute them to a combination of selection effects of the LOTAAS
survey as well as previous pulsar surveys, though we cannot rule out that older pulsars tend to have steeper radio spectra
The MODEST questions: challenges and future directions in stellar cluster research
We present a review of some of the current major challenges in stellar
cluster research, including young clusters, globular clusters, and galactic
nuclei. Topics considered include: primordial mass segregation and runaway
mergers, expulsion of gas from clusters, the production of stellar exotica seen
in some clusters (eg blue stragglers and extreme horizontal--branch stars),
binary populations within clusters, the black--hole population within stellar
clusters, the final parsec problem, stellar dynamics around a massive black
hole, and stellar collisions. The Modest Questions posed here are the outcome
of discussions which took place at the Modest-6A workshop held in Lund, Sweden,
in December, 2005. Modest-6A was organised as part of the activities of the
Modest Collaboration (see www.manybody.org for further details)Comment: 24 pages, no figures, accepted for publication in New Astronom
Limits on Absorption from a 332-MHz survey for Fast Radio Bursts
Fast Radio Bursts (FRBs) are bright, extragalactic radio pulses whose origins
are still unknown. Until recently, most FRBs have been detected at frequencies
greater than 1 GHz with a few exceptions at 800 MHz. The recent discoveries of
FRBs at 400 MHz from the Canadian Hydrogen Intensity Mapping Experiment (CHIME)
telescope has opened up possibilities for new insights about the progenitors
while many other low frequency surveys in the past have failed to find any
FRBs. Here, we present results from a FRB survey recently conducted at the
Jodrell Bank Observatory at 332 MHz with the 76-m Lovell telescope for a total
of 58 days. We did not detect any FRBs in the survey and report a 90 upper
limit of 5500 FRBs per day per sky for a Euclidean Universe above a fluence
threshold of 46 Jy ms. We discuss the possibility of absorption as the main
cause of non-detections in low frequency (< 800 MHz) searches and invoke
different absorption models to explain the same. We find that Induced Compton
Scattering alone cannot account for absorption of radio emission and that our
simulations favour a combination of Induced Compton Scattering and Free-Free
Absorption to explain the non-detections. For a free-free absorption scenario,
our constraints on the electron density are consistent with those expected in
the post-shock region of the ionized ejecta in Super-Luminous SuperNovae
(SLSNe).Comment: 12 pages, 9 Figures, 2 Tables, Second revision submitted to MNRA
X-ray and Optical Study of Low Core Density Globular Clusters NGC6144 and E3
We report on the Chandra X-ray Observatory and Hubble Space Telescope
observation of two low core density globular clusters, NGC6144 and E3. By
comparing the number of X-ray sources inside the half-mass radius to those
outside, we found 6 X-ray sources within the half-mass radius of NGC6144, among
which 4 are expected to be background sources; 3 X-ray sources are also found
within the half-mass radius of E3, of which 3 is expected to be background
source. Therefore, we cannot exclude that all our sources are background
sources. However, combining the results from X-ray and optical observations, we
found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be
cataclysmic variables and that 1 source in NGC6144 is an active binary, based
on the X-ray and optical properties. The number of faint X-ray sources in
NGC6144 and E3 found with Chandra and HST is higher than a prediction based on
collision frequency, but is closer to that based on mass. Our observations
strongly suggest that the compact binary systems in NGC6144 and E3 are
primordial in origin.Comment: 28 pages, 9 figures, 6 tables, Accepted for publication in Ap
An Accurate Determination of the Optical Periodic Modulation in the X-Ray Binary SAX J1808.4-3658
We report on optical imaging of the X-ray binary SAX J1808.4-3658 with the
8-m Gemini South Telescope. The binary, containing an accretion-powered
millisecond pulsar, appears to have a large periodic modulation in its
quiescent optical emission. In order to clarify the origin of this modulation,
we obtained three time-resolved -band light curves (LCs) of the source in
five days. The LCs can be described by a sinusoid, and the long time-span
between them allows us to determine optical period P=7251.9 s and phase 0.671
at MJD 54599.0 (TDB; phase 0.0 corresponds to the ascending node of the pulsar
orbit), with uncertainties of 2.8 s and 0.008 (90 % confidence), respectively.
This periodicity is highly consistent with the X-ray orbital ephemeris. By
considering this consistency and the sinusoidal shape of the LCs, we rule out
the possibility of the modulation arising from the accretion disk. Our study
supports the previous suggestion that the X-ray pulsar becomes rotationally
powered in quiescence, with its energy output irradiating the companion star,
causing the optical modulation. While it has also been suggested that the
accretion disk would be evaporated by the pulsar, we argue that the disk exists
and gives rise to the persistent optical emission. The existence of the disk
can be verified by long-term, multi-wavelength optical monitoring of the source
in quiescence, as an increasing flux and spectral changes from the source would
be expected based on the standard disk instability model.Comment: 7 pages, 3 figures. Accepted for publication in Ap
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