441 research outputs found
Low-Frequency Radio Transients in the Galactic Center
We report the detection of a new radio transient source, GCRT J1746-2757,
located only 1.1 degrees north of the Galactic center. Consistent with other
radio transients toward the Galactic center, this source brightened and faded
on a time scale of a few months. No X-ray counterpart was detected. We also
report new 0.33 GHz measurements of the radio counterpart to the X-ray
transient source, XTE J1748-288, previously detected and monitored at higher
radio frequencies. We show that the spectrum of XTE J1748-288 steepened
considerably during a period of a few months after its peak. We also discuss
the need for a more efficient means of finding additional radio transients
GCRT J1742-3001: A New Radio Transient Towards the Galactic Center
We report the detection of a new transient radio source, GCRT J1742-3001,
located ~1 degree from the Galactic center. The source was detected ten times
from late 2006 to 2007 May in our 235 MHz transient monitoring program with the
Giant Metrewave Radio Telescope (GMRT). The radio emission brightened in about
one month, reaching a peak observed flux density of ~100 mJy on 2007 January
28, and decaying to ~50 mJy by 2007 May when our last monitoring observation
was made. Two additional faint, isolated 235 MHz detections were made in
mid-2006, also with the GMRT. GCRT J1742-3001 is unresolved at each epoch, with
typical resolutions of ~20 arcsec x 10 arcsec. No polarization information is
available from the observations. Based on nondetections in observations
obtained simultaneously at 610 MHz, we deduce that the spectrum of GCRT
J1742-3001 is very steep, with a spectral index less than about -2. Follow-up
radio observations in 2007 September at 330 MHz and 1.4 GHz, and in 2008
February at 235 MHz yielded no detections. No X-ray counterpart is detected in
a serendipitous observation obtained with the X-ray telescope aboard the Swift
satellite during the peak of the radio emission in early 2007. We consider the
possibilities that GCRT J1742-3001 is either a new member of an existing class
of radio transients, or is representative of a new class having no associated
X-ray emission.Comment: 19 pages, 3 figures, submitted to Ap
A powerful bursting radio source towards the Galactic Centre
Transient astronomical sources are typically powered by compact objects and
usually signify highly explosive or dynamic events. While radio astronomy has
an impressive record of obtaining high time resolution observations, usually it
is achieved in quite narrow fields-of-view. Consequently, the dynamic radio sky
is poorly sampled, in contrast to the situation in the X- and gamma-ray bands
in which wide-field instruments routinely detect transient sources. Here we
report a new transient source, GCRT J1745-3009, detected in 2002 during a
moderately wide-field radio transient monitoring program of the Galactic center
(GC) region at 0.33 GHz. The characteristics of its bursts are unlike those
known for any other class of radio transient. If located in or near the GC, its
brightness temperature (~10^16 K) and the implied energy density within GCRT
J1745-3009 vastly exceeds that observed in most other classes of radio
astronomical sources, and is consistent with coherent emission processes rarely
observed. We conclude that GCRT J1745-3009 is the first member of a new class
of radio transient sources, the first of possibly many new classes to be
identified through current and upcoming radio surveys.Comment: 16 pages including 3 figures. Appears in Nature, 3 March 200
Self-trapping transition for nonlinear impurities embedded in a Cayley tree
The self-trapping transition due to a single and a dimer nonlinear impurity
embedded in a Cayley tree is studied. In particular, the effect of a perfectly
nonlinear Cayley tree is considered. A sharp self-trapping transition is
observed in each case. It is also observed that the transition is much sharper
compared to the case of one-dimensional lattices. For each system, the critical
values of for the self-trapping transitions are found to obey a
power-law behavior as a function of the connectivity of the Cayley tree.Comment: 6 pages, 7 fig
High Resolution, High Sensitivity Imaging of the Galactic Center at 330 MHz
We present results derived from a wide field, sub-arcminute resolution VLA image of the Galactic Center region at 330 MHz (λ = 90 cm). With a resolution of ∼7″ × 12″ and an rms noise of 1.6 mJy beam−1, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency (eg. LaRosa et al. 2000). The improvement in sensitivity has significantly increased the census of small diameter sources in the region, resulted in the detection of two new Non-Thermal Filaments (NTFs) and 18 new NTF candidates, and resulted in the lowest frequency (tentative) detection of Sgr A*
Time evolution of models described by one-dimensional discrete nonlinear Schr\"odinger equation
The dynamics of models described by a one-dimensional discrete nonlinear
Schr\"odinger equation is studied. The nonlinearity in these models appears due
to the coupling of the electronic motion to optical oscillators which are
treated in adiabatic approximation. First, various sizes of nonlinear cluster
embedded in an infinite linear chain are considered. The initial excitation is
applied either at the end-site or at the middle-site of the cluster. In both
the cases we obtain two kinds of transition: (i) a cluster-trapping transition
and (ii) a self-trapping transition. The dynamics of the quasiparticle with the
end-site initial excitation are found to exhibit, (i) a sharp self-trapping
transition, (ii) an amplitude-transition in the site-probabilities and (iii)
propagating soliton-like waves in large clusters. Ballistic propagation is
observed in random nonlinear systems. The effect of nonlinear impurities on the
superdiffusive behavior of random-dimer model is also studied.Comment: 16 pages, REVTEX, 9 figures available upon request, To appear in
Physical Review
Long-period Radio Pulsars: Population Study in the Neutron Star and White Dwarf Rotating Dipole Scenarios
© 2024 The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/The nature of two recently discovered radio emitters with unusually long periods of 18min (GLEAM-X J1627-52) and 21min (GPM J1839-10) is highly debated. Their bright radio emission resembles that of radio magnetars, but their long periodicities and lack of detection at other wavelengths challenge the neutron-star interpretation. In contrast, long rotational periods are common in white dwarfs but, although predicted, dipolar radio emission from isolated magnetic white dwarfs has never been unambiguously observed. In this work, we investigate these long-period objects as potential isolated neutron-star or white-dwarf dipolar radio emitters and find that both scenarios pose significant challenges to our understanding of radio emission via pair production in dipolar magnetospheres. We also perform population-synthesis simulations based on dipolar spin-down in both pictures, assuming different initial-period distributions, masses, radii, beaming fractions, and magnetic-field prescriptions, to assess their impact on the ultra-long pulsar population. In the neutron-star scenario, we do not expect a large number of ultra-long period pulsars under any physically motivated (or even extreme) assumptions for the period evolution. On the other hand, in the white-dwarf scenario, we can easily accommodate a large population of long-period radio emitters. However, no mechanism can easily explain the production of such bright coherent radio emission in either scenarios.Peer reviewe
A long-period radio transient active for three decades: population study in the neutron star and white dwarf rotating dipole scenarios
The nature of two recently discovered radio emitters with unusually long
periods of 18min (GLEAM-X J1627-52) and 21min (GPM J1839-10) is highly debated.
Their bright radio emission resembles that of radio magnetars, but their long
periodicities and lack of detection at other wavelengths challenge the
neutron-star interpretation. In contrast, long rotational periods are common in
white dwarfs but, although predicted, dipolar radio emission from isolated
magnetic white dwarfs has never been unambiguously observed. In this work, we
investigate these long-period objects as potential isolated neutron-star or
white-dwarf dipolar radio emitters and find that both scenarios pose
significant challenges to our understanding of radio emission via pair
production in dipolar magnetospheres. We also perform population-synthesis
simulations based on dipolar spin-down in both pictures, assuming different
initial-period distributions, masses, radii, beaming fractions, and
magnetic-field prescriptions, to assess their impact on the ultra-long pulsar
population. In the neutron-star scenario, we cannot reproduce the large number
of expected ultra-long period pulsars under any physically motivated (or even
extreme) assumptions. Thus, if GLEAM-X J1627-52 and GPM J1839-10 are confirmed
as neutron-star pulsars (even if they are magnetars), this would necessarily
call for a significant revision of our understanding of birth parameters at the
population level. On the other hand, in the white-dwarf scenario, no mechanism
can explain the production of such a bright coherent radio emission in isolated
magnetic white dwarf systems (binaries with low mass companions are still
viable), although we can easily accommodate a large population of long-period
radio emitters.Comment: 8 pages, 4 figures; ApJ Letters submitte
A Study of The Formation of Stationary Localized States Due to Nonlinear Impurities Using The Discrete Nonlinear Schr\"odinger Equation
The Discrete Nonlinear Schrdinger Equation is used to study the
formation of stationary localized states due to a single nonlinear impurity in
a Caley tree and a dimeric nonlinear impurity in the one dimensional system.
The rotational nonlinear impurity and the impurity of the form where is arbitrary and is the nonlinearity
parameter are considered. Furthermore, represents the absolute
value of the amplitude. Altogether four cases are studies. The usual Greens
function approach and the ansatz approach are coherently blended to obtain
phase diagrams showing regions of different number of states in the parameter
space. Equations of critical lines separating various regions in phase diagrams
are derived analytically. For the dimeric problem with the impurity , three values of , namely, , at and and
for are obtained. Last two values are lower than the
existing values. Energy of the states as a function of parameters is also
obtained. A model derivation for the impurities is presented. The implication
of our results in relation to disordered systems comprising of nonlinear
impurities and perfect sites is discussed.Comment: 10 figures available on reques
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