arXiv subjects

Ashok K. Singal

Publications and source records attributed to Ashok K. Singal.

At least 55 records · Page 3Linked to original sources

arXiv2016

Radiation reaction and the pitch angle changes for a charge undergoing synchrotron losses

In the derivation of synchrotron radiation formulas it has been assumed that the pitch angle of a charge remains constant during the radiation process. However from the radiation reaction formula, while the component of the velocity vector perpendicular to the magnetic field reduces in magnitude due to radiative losses, the parallel component does not undergo any change during radiation. Therefore there is a change in the ratio of the two components, implying a change in the pitch angle. We derive the exact formula for the change in energy of radiating electrons by taking into account the change of the pitch angle due to radiative losses. From this we derive the characteristic decay time of synchrotron electrons over which they turn from highly relativistic into mildly relativistic ones.

astro-ph.HE↗

arXiv2015

Compatibility of Larmor's formula with radiation reaction for a radiating charge

It is shown that the well-known disparity in classical electrodynamics between the power losses calculated from the radiation reaction and that from Larmor's formula, is succinctly understood when a proper distinction is made between quantities expressed in terms of a "real time" and those expressed in terms of a retarded time. It is explicitly shown that an accelerated charge, taken to be a sphere of vanishingly small radius $r_{\rm o} $, experiences at any time a self-force proportional to the acceleration it had at a time $r_{\rm o} /c$ earlier, while the rate of work done on the charge is obtained by a scalar product of the self-force with the instantaneous (present) value of its velocity. Now if the retarded value of acceleration is expressed in terms of the present values of acceleration, then we get the rate of work done according to the radiation reaction equation, however if we instead express the present value of velocity in terms of its time-retarded value, then we get back the familiar Larmor's radiation formula. From this simple relation between the two we show that they differ because Larmor's formula, in contrast with the radiation reaction, is written not in terms of the real-time values of quantities specifying the charge motion but is instead expressed in terms of the time-retarded values. Moreover, it is explicitly shown that the difference in the two formulas for radiative power loss exactly matches the difference in the temporal rate of the change of energy in the self-fields between the retarded and real times. From this it becomes obvious that the ad hoc introduction of an acceleration-dependent energy term, usually referred to in the prevalent literature as Schott-term, in order to make the two formulas comply with each other, is redundant.

physics.class-ph↗

arXiv2015

The Paradox of Power Loss in a Lossless Infinite Transmission Line

We discuss here the famous paradox of a continuous power drainage from the source at the input of an otherwise lossless infinite transmission line. The solution of the paradox lies in the realization that in an open-circuit finite transmission line/ladder network, there is an incident as well as a reflected wave and the input impedence is determined by the superposition of both waves. It is explicitly shown that the reactive input impedance of even a single block, comprising say a simple LC circuit, is determined at all driving frequencies from the superposition of incident and reflected waves, and that the input impedance remains reactive in nature (i.e., an imaginary value) even when additional blocks are added indefinitely. However in a ladder network or transmission line, taken to be {\em infinite right from the beginning}, there is no reflected wave (assuming the circuit to be ideal with no discontinuities en route). Thus the source while continuously supplying power in the forward direction, does not retrieve it from a reflected wave and unlike in the case of a finite line, there is a net power loss. This apparently lost energy ultimately appears in the electromagnetic fields in the reactive elements (capacitances and inductances which to begin with had no such stored energy), further down the line as the incident wave advances forward. It is also shown that radiation plays absolutely no role in resolving this intriguing paradox.

physics.class-ph↗

arXiv2015

The Paradox of Two Charged Capacitors -- A New Perspective

It is shown that the famous paradox of two charged capacitors is successfully resolved if all the energy changes in the system are properly considered when some of the charges are transferred from one capacitor to the other. It happens so even when the connecting wire has an identically zero resistance, giving rise to no Ohmic losses in the wire. It is shown that in such a case the "missing energy" goes into the kinetic energy of conducting charges. It is shown that radiation plays no significant role in resolving the paradox. The problem can be formulated and successfully resolved in a novel form, where the capacitance of the system is increased by stretching the plates of the original capacitor, without involving any connecting wires in a circuit. There is an outward self-force due to mutual repulsion among charges stored within each capacitor plate, and the work done by these self-forces during an expansion is indeed equal to the missing energy of the capacitor system.

physics.class-ph↗

arXiv2015

Cosmological Parallax--Distance Formula

The standard cosmological parallax--distance formula, as found in the literature, including text-books and reference books on cosmology, requires a correction. This correction stems from the fact that in the standard text-book derivation it has been ignored that any chosen baseline in a gravitationally bound system does not partake in the cosmological expansion. Though the correction is available in the literature for some time, the text-books still continue to use the older, incorrect formula, and its full implications are not yet fully realized. Apart from providing an alternate correct, closed-form expression that is more suitable and convenient for computations for certain limiting cases of FRW ($Λ=0$) world models, we also demonstrate how one can compute parallax distance for the currently favored flat-space accelerating-universe ($Λ>0, k=0$) cosmologies. Further, we show that the correction in parallax distance at large redshifts could amount to a factor of three or even more. Moreover, even in an infinite universe the parallax distance does not increase indefinitely with redshift and that even the farthest possible observable point may have a finite parallax angle, a factor that needs to be carefully taken into account when using distant objects as the background field against which the parallax of a foreground object is to be measured. Some other complications that could arise in parallax measurements of a distant source, like that due to the deflection of incoming light by the gravitation field of the Sun and other planetary bodies in the solar system, are pointed out.

astro-ph.CO↗

arXiv2014

A large anisotropy in the sky distribution of 3CRR quasars and other radio galaxies

We report the presence of large anisotropies in the sky distributions of powerful extended quasars as well as some other sub-classes of radio galaxies in the 3CRR survey, the most reliable and most intensively studied complete sample of strong steep-spectrum radio sources. The anisotropies lie about a plane passing through the equinoxes and the north celestial pole. Out of a total of 48 quasars in the sample, 33 of them lie in one half of the observed sky and the remaining 15 in the other half. The probability that in a random distribution of 3CRR quasars in the sky, statistical fluctuations could give rise to an asymmetry in observed numbers up to this level is only $\sim 1\%$. Also only about 1/4th of Fanaroff-Riley 1 (FR1) type of radio galaxies lie in the first half of the observed sky and the remainder in the second half. If we include all the observed asymmetries in the sky distributions of quasars and radio galaxies in the 3CRR sample, the probability of their occurrence by a chance combination reduces to $\sim 2 \times 10^{-5}$. Two pertinent but disturbing questions that could be raised here are -- firstly why should there be such large anisotropies present in the sky distribution of some of the strongest and most distant discrete sources, implying inhomogeneities in the universe at very large scales (covering a fraction of the universe)? Secondly why should such anisotropies lie about a great circle decided purely by the orientation of earth's rotation axis and/or the axis of its revolution around the sun? Are these alignments a mere coincidence or do they imply that these axes have a preferential placement in the larger scheme of things, implying an apparent breakdown of the Copernican principle or its more generalization, cosmological principle, upon which the standard cosmological model is based upon?

astro-ph.CO↗

arXiv2014

Energy conservation for a radiating charge in classical electrodynamics

It is shown that the well-known disparity in classical electrodynamics between the power radiated in electromagnetic fields and the power-loss, as calculated from the radiation reaction on a charge undergoing a non-uniform motion, is successfully resolved when a proper distinction is made between quantities expressed in terms of a "real time" and those expressed in terms of a retarded time. It is shown that the expression for the real-time radiative power loss from a charged particle is somewhat different from the familiar Larmor's formula, or in a relativistic case, from Liénard's formula.

physics.class-ph↗

arXiv2014

Comment on "Cosmic radio dipole from NVSS and WENSS"

We show that a recent purported correction to the effects of Doppler boosting of flux density in an erstwhile published formula for calculating the observer's motion from the cosmic radio dipole in sky brightness is erroneous. The thereby computed downward correction in the estimated magnitude for the observer's motion needs to be scrapped and the results derived therefore need to be reverted back to their erstwhile values.

astro-ph.CO↗

arXiv2014

Dependence of Fanaroff--Riley break of radio galaxies on luminosity and redshift

We investigate the dependence of the Fanaroff-Riley (FR) 1/2 dichotomy of radio galaxies on their luminosities and redshifts. Because of a very strong redshift-luminosity correlation (Malmquist bias) in a flux-limited sample, any redshift-dependent effect could appear as a luminosity related effect and vice versa. A question could then arise - do all the morphological differences seen in the two classes (FR 1 and 2 types) of sources, usually attributed to the differences in their luminosities, could as well be primarily a redshift-dependent effect? A sharp break in luminosity, seen among the two classes, could after all reflect a sharp redshift-dependence due to a rather critical ambient density value at some cosmic epoch. A doubt on these lines does not seem to have been raised in past and things have never been examined with this particular aspect in mind. We want to ascertain the customary prevalent view in the literature that the systematic differences in the two broad morphology types of FR 1 and 2 radio galaxies are indeed due to the differences in their luminosities, and not due to a change in redshift. Here we investigate the dependence of FR 1/2 dichotomy of radio galaxies on luminosity and redshift by using the 3CR sample, where the FR 1/2 dichotomy was first seen, supplemented by data from an additional sample (MRC), that goes about a factor of 5 or more deeper in flux-density than the original 3CR sample. This lets us compare sources with similar luminosities but at different redshifts as well as examine sources at similar redshifts but with different luminosities, thereby allowing us a successful separation of the otherwise two intricately entangled effects. We find that the morphology type is not directly related to redshift and the break between the two types of morphologies seems to depend only upon the radio luminosity.

astro-ph.CO↗

arXiv2014

Fanaroff-Riley dichotomy of radio galaxies and the Malmquist bias

We examine the possibility that a claimed dependence of the FR 1/2 break value in radio luminosity on the absolute magnitude of the optical host galaxy could be due to the Malmquist bias, where a redshift-luminosity correlation appears in a flux-limited sample because of an observational selection effect. In such a sample, the redshift dependence of a phenomenon could appear as a luminosity-dependent effect and may not be really representing an intrinsic property of the radio sample. We test this on the radio complete MRC (Molonglo Reference Catalog) sample, where Spearman rank correlation and Kendall rank correlation tests show that the correlations are indeed stronger between the redshift and the optical luminosity than that between the radio luminosity and the optical luminosity, suggesting that the latter correlation perhaps arises because of the Malmquist Bias. We further show that similar effects of the Malmquist bias could also be present elsewhere in other correlations claimed in the literature between the radio luminosity and other observed properties of FR1 and FR2 sources.

astro-ph.GA↗

arXiv2014

Life on a tidally-locked planet

A tidally-locked planet in its orbit around a star keeps the same face towards the star. This happens when the rotation period of the planet around its own axis becomes equal to its revolution period around the star. Many questions then arise. What gives rise to a tidal locking? Are there any tidally-locked planets somewhere? What will be the atmospheric system of a tidally-locked planet? Could life be sustained on a tidally-locked planet? What will be a sort of DAY in the life of a denizen of a tidally-locked planet? We dwell on these questions.

physics.pop-ph↗

arXiv2014

Discordance of the unified scheme with observed properties of quasars and high-excitation galaxies in the 3CRR sample

We examine the consistency of the unified scheme of FR II-type radio galaxies and quasars with their observed number and size distributions in the 3CRR sample. We separate the low-excitation galaxies from the high-excitation ones, as the former might not harbor a quasar within and thus may not be partaking in the unified scheme models. In the updated 3CRR sample, at low redshifts (z<0.5), the relative number and luminosity distributions of high-excitation galaxies and quasars do roughly match the expectations from the orientation-based unified scheme model. However, a foreshortening in the observed sizes of quasars, a must in the orientation-based model, is not seen with respect to radio galaxies even when the low-excitation galaxies are excluded. This dashes the hope that the unified scheme might still work if one includes only the high-excitation galaxies.

astro-ph.CO↗

arXiv2013

Incongruity of the unified scheme with a 3CRR-like equatorial strong-source sample

We examine the consistency of the unified scheme of the powerful extragalactic radio sources with the 408 MHz BRL sample from the equatorial sky region, selected at the same flux-density level as the 3CRR sample. We find that, unlike in the 3CRR sample, a foreshortening in the observed sizes of quasars, expected from the orientation-based unified scheme model, is not seen in the BRL sample, at least in different redshift bins up to z~1. Even the quasar fraction in individual redshift bins up to z~1 does not match with that expected from the unified scheme, where radio galaxies and quasars are supposed to belong to a common parent population at all redshifts. This not only casts strong doubts on the unified scheme, but also throws up an intriguing result that in a sample selected from the equatorial sky region, using almost the same criteria as in the 3CRR sample from the northern hemisphere, the relative distribution of radio galaxies and quasars differs qualitatively from the 3CRR sample.

astro-ph.CO↗

arXiv2013

Unification scheme of radio galaxies and quasars falsified by their observed size distributions

In the currently popular orientation-based unified scheme, a radio galaxy appears as a quasar when its principal radio-axis happens to be oriented within a certain cone opening angle around the observer's line of sight. Due to geometrical projection, the observed sizes of quasars should therefore appear smaller than those of radio galaxies. We show that this simple, unambiguous prediction of the unified scheme is not borne out by the actually observed angular sizes of radio galaxies and quasars. Except in the original 3CR sample, based on which the unified scheme was proposed, in other much larger samples no statistically significant difference is apparent in the size distributions of radio galaxies and quasars. The population of low-excitation radio galaxies with apparently no hidden quasars inside, which might explain the observed excess number of radio galaxies at low redshifts, cannot still account for the absence of any foreshortening of the sizes of quasars at large redshifts. On the other hand from infrared and X-ray studies there is evidence of hidden quasar within a dusty torus in many RGs, at $z>0.5$. It seems difficult how to reconcile this with the absence of foreshortening of quasar sizes at even these redshifts, and perhaps one has to allow that the major radio axis may not have anything to do with the optical axis of the torus. Otherwise to resolve the dichotomy of radio galaxies and quasars, a scheme quite different from the present might be required.

astro-ph.CO↗

arXiv2012

Comment on "Exact Expression for Radiation of an Accelerated Charge in Classical Electrodynamics"

It is shown that a newly derived "exact expression" for radiation of an accelerated charge in the recent literature is simply incorrect, having arisen because of a wrong relativistic transformation of the distance parameter. The ensuing claim that the newly derived expression alone that satisfies the energy conservation for the electromagnetic radiation, is based on a wrong reasoning where a proper distinction between the time during which the radiation is received and the time for emission (retarded time of the charge) was not maintained.

physics.class-ph↗

arXiv2012

Giant pulse emission from PSR B0950+08

We present here the detection of giant-pulse emission from PSR B0950+08, a normal-period pulsar. The observations, made at 103 MHz and lasting for about ten months, have shown on a number of days the frequency of occurrence of giant pulses to be the highest among the known pulsars. The flux--density level of successive giant pulses fluctuates rapidly and their occurrence rates within a day's observations as well as between neighboring days show large variations. While on some days PSR B0950+08 shows a large number of giant pulses, there are other days when it shows only "quasi-nulls" with no detectable emission in the power spectrum or in the folded pulse data. The cumulative intensity distribution of these giant pulses appears to follow a power law, with index -2.2. After eliminating instrumental, ionospheric, interplanetary and interstellar diffractive and refractive scintillation effects as the cause, it appears that these intensity variations are intrinsic to the pulsar. We suggest that the giant pulse emission and nulling may be opposite manifestations of the same physical process, in the former case an enhanced number of charges partaking in the coherent radiation process giving rise to an extremely high intensity while in the latter case the coherence could be minimal.

astro-ph.HE↗

arXiv2012

Gyro-orbit size, brightness temperature limit and implausibility of coherent emission by bunching in synchrotron radio sources

We show that an upper limit on the maximum brightness temperature for a self-absorbed incoherent synchrotron radio source is obtained from the size of its gyro orbits, which in turn must lie well within the confines of the total source extent. These temperature limits are obtained without recourse to inverse Compton effects or the condition of equipartition of energy between magnetic fields and relativistic particles. For radio variables, the intra-day variability (IDV) implies brightness temperatures $\sim 10^{19}$ K in the co-moving rest frame of the source. This, if interpreted purely due to an incoherent synchrotron emission, would imply gyro radii $>10^{28}$ cm, the size of the universe, while from the causality arguments the inferred maximum size of the source in such a case is $\stackrel{<}{_{\sim}} 10^{15}$ cm. Such high brightness temperatures are sometimes modeled in the literature as some coherent emission process where bunches of non-thermal particles are somehow formed that radiate in phase. We show that, unlike in case of curvature radiation models proposed in pulsars, in the synchrotron radiation mechanism the oppositely charged particles would contribute together to the coherent phenomenon without the need to form separate bunches of the opposite charges. At the same time we show that bunches would disperse over dimensions larger than a wavelength in time shorter than the gyro orbital period ($\stackrel{<}{_{\sim}}0.1$ sec). Therefore a coherent emission by bunches cannot be a plausible explanation of the high brightness temperatures inferred in extragalactic radio sources showing variability over a few hours or longer.

astro-ph.HE↗

arXiv2011

Large peculiar motion of the solar system from the dipole anisotropy in sky brightness due to distant radio sources

According to the cosmological principle, the Universe should appear isotropic, without any preferred directions, to an observer whom we may consider to be fixed in the co-moving co-ordinate system of the expanding Universe. Such an observer is stationary with respect to the average distribution of the matter in the Universe and the sky brightness at any frequency should appear uniform in all directions to such an observer. However a peculiar motion of such an observer, due to a combined effect of Doppler boosting and aberration, will introduce a dipole anisotropy in the observed sky brightness; in reverse an observed dipole anisotropy in the sky brightness could be used to infer the peculiar velocity of the observer with respect to the average Universe. We determine the peculiar velocity of the solar system relative to the frame of distant radio sources, by studying the anisotropy in the sky brightness from discrete radio sources, i.e., an integrated emission from discrete sources per unit solid angle. Our results give a direction of the velocity vector in agreement with the Cosmic Microwave Background Radiation (CMBR) value, but the magnitude ($\sim 1600\pm 400$ km/s) is $\sim 4$ times the CMBR value ($369\pm 1$ km/s) at a statistically significant ($\sim 3σ$) level. A genuine difference between the two dipoles would imply anisotropic Universe, with the anisotropy changing with the epoch. This would violate the cosmological principle where the isotropy of the Universe is assumed for all epochs, and on which the whole modern cosmology is based upon.

astro-ph.CO↗