scholarly journals Astronomical masers and Dicke’s superradiance

2020 ◽  
Vol 494 (4) ◽  
pp. 5194-5206 ◽  
Author(s):  
Fereshteh Rajabi ◽  
Martin Houde
Keyword(s):  
Population Inversion ◽  
Column Density ◽  
Critical Level ◽  
Temporal Evolution ◽  
Transient Regime ◽  
Critical Threshold ◽  
Radiation Process ◽  
Radiation Properties ◽  
Coherence Level ◽  
Maser Action

ABSTRACT We consider the radiation properties and processes of a gas with a population inversion using the formalism based on the Maxwell–Bloch equations. We focus on the maser action and Dicke’s superradiance to establish their relationship in the overall radiation process during the temporal evolution of the system as a function of position. We show that the maser action and superradiance are not competing phenomena but are rather complementary and define two distinct limits for the intensity of radiation. Masers characterize the quasi-steady state limit, when the population inversion density and the polarization amplitude vary on time-scales longer than those of non-coherent processes affecting their evolution (e.g. collisions), while superradiance defines the fast transient regime taking place when these conditions are reversed. We show how a transition from a maser regime to superradiance will take place whenever a critical threshold for the column density of the population inversion is reached, at which point a strong level of coherence is established in the system and a powerful burst of radiation can ensue during the transient regime. This critical level also determines the spatial region where a transition from the unsaturated to the saturated maser regimes will take place; superradiance can thus be seen as the intermediary between the two. We also quantify the gain in radiation intensity attained during the superradiance phase relative to the two maser regimes and show how the strong coherence level during superradiance is well suited to explain observations that reveal intense and fast radiation flares in maser-hosting regions.

scholarly journals Star Formation Thresholds in Low Surface Brightness Dwarf Galaxies

1999 ◽  
Vol 171 ◽  
pp. 274-281
Author(s):  
Liese van Zee
Keyword(s):  
Star Formation ◽  
Comparative Study ◽  
Column Density ◽  
Surface Brightness ◽  
Dwarf Galaxies ◽  
Critical Threshold ◽  
Active Star ◽  
Comparison Sample ◽  
Active Star Formation ◽  
Low Surface Brightness

AbstractThe results of a comparative study of star formation thresholds in gas-rich, low surface brightness, dwarf galaxies are presented. Approximately half the galaxies in the study were “high MH/LB” galaxies, which appear to have inefficient star formation properties. The comparison sample comprised of otherwise “normal” dwarf galaxies, with moderate current star formation rates. In all systems, sites of active star formation were associated with local peaks in the HI column density. For both types of galaxies, the azimuthally averaged gas column density is low. Similar to other LSB systems, the global gas densities are well below the critical threshold for star formation throughout the system. Thus, star formation is inhibited globally, but occurs locally in these gas-rich dwarf galaxies.

scholarly journals Λ-Doublet Population Inversion in Collisions of OH, OD, CH, CD and NH+

1980 ◽  
Vol 87 ◽  
pp. 583-587 ◽  
Author(s):  
R. N. Dixon ◽  
D. Field
Keyword(s):  
Qualitative Agreement ◽  
Scattering Theory ◽  
Population Inversion ◽  
Closed Shell ◽  
Inelastic Collisions ◽  
The Other ◽  
New Approach ◽  
Maser Action

The results of a new approach to the problem of the collisional step in the pumping cycle for OH and CH masers are reported. Rotationally inelastic collisions of OH and CH with both open and closed shell collision partners are considered using an expression derived from scattering theory. It is shown how Λ-doublet population inversion may arise in OH and CH. H-atoms and H2 may show opposite behaviour with CH, one partner cooling where collisions with the other lead to inversion. Implications for maser action are discussed and reveal excellent qualitative agreement with observation. Λ-doublet population inversion in OD, CD and NH+ is also considered.

Epidemic spreading of interacting diseases with activity of nodes reshapes the critical threshold

2017 ◽  
Vol 28 (01) ◽  
pp. 1750013 ◽  
Author(s):  
Chongjun Fan ◽  
Yang Jin ◽  
Liang-An Huo ◽  
Chen Liu ◽  
Yunpeng Yang
Keyword(s):  
Temporal Evolution ◽  
Mean Field ◽  
Critical Threshold ◽  
Epidemic Spreading ◽  
Final Size ◽  
Node Activity ◽  
Transmission Rates ◽  
Field Approach ◽  
Spreading Dynamics ◽  
Theoretical Predictions

In this paper, based on susceptible–infected–susceptible (SIS) scheme, we introduce a framework that allows us to describe the spreading dynamics of two interacting diseases with active nodes. Different from previous studies, the two different diseases, propagating concurrently on the same population, can interact with each other by modifying their transmission rates. Meanwhile, according to certain probabilities, each node on the complex networks rotates between active state and inactive state. Based on heterogeneous mean-field approach, we analyze the epidemic thresholds of the two diseases and compute the temporal evolution characterizing the spreading dynamics. In addition, we validate these theoretical predictions by numerical simulations with phase diagrams. Results show that the secondary thresholds for the two opposite scenarios (mutual enhancement scenario and mutual impairment scenario) are different. We also find that the value of critical threshold and the final size of spreading dynamics are reduced as the node activity rate decreases.

scholarly journals On the Periodic Change of the Luminosity of the Cosmic Sources with an Active Medium

2020 ◽  
Keyword(s):  
Relative Density ◽  
Active Medium ◽  
Population Inversion ◽  
Black Body ◽  
Pulse Generation ◽  
Periodic Change ◽  
Level System ◽  
Spontaneous Radiation ◽  
Active System ◽  
Radiation Process

The presence of an internal layer with an active medium in a hot radiation source is considered, which can be described by a quantum two-level system located near equilibrium. The population of the upper and lower levels is approximately equal. It is shown that during convection from deeper hot layers, which supports the inversion of the populations of the active system, generation of induced radiation pulses is possible, the intensity of which is comparable to or greater than the intensity of the background spontaneous radiation of the source. With a sufficient thickness of the surface layers due to the effects of radiation scattering in them, the emission spectrum of a completely black body may well form there. Pulse generation near a previously detected new threshold of induced radiation can lead to a periodic change in the radiation intensity of the source as a whole. This threshold is determined by the equality of the squared population inversion to the total number of states. The generation of pulses of induced radiation is considered both in Einstein's representation, on the basis of balanced equations, and using a semiclassical description for small values of population inversion and for low levels of electric field intensity, when the Rabi frequency is less than the line width. The description of the induced radiation process is reduced to a one-parameter system of equations. Periodic solutions are represented by closed trajectories on the phase plane (relative density of quanta, relative density of population inversion). A similar layer with an active medium, which can be described by a quantum two-level system located near equilibrium, can exist in stars and is most likely localized in the photosphere. If there is significant convection in the star’s atmosphere, conditions can be realized for generating pulses of induced radiation. It turns out that one can see the similarity of the obtained solutions with known observations of changes in the luminosity of Cepheid stars (Cepheus delta and the North Star).

Maser action without population inversion

1963 ◽  
Vol 51 (5) ◽  
pp. 849-850 ◽  
Author(s):  
D. Marcuse
Keyword(s):  
Population Inversion ◽  
Maser Action

scholarly journals Population Inversion and Suprathermal Excitation in Carbon Monoxide

1980 ◽  
Vol 87 ◽  
pp. 127-127
Author(s):  
J. Köppen ◽  
W. H. Kegel
Keyword(s):  
Radiative Transfer ◽  
Cross Sections ◽  
Population Inversion ◽  
Column Density ◽  
Computing Time ◽  
Kinetic Temperature ◽  
Escape Probability ◽  
Physical Conditions ◽  
Wide Range ◽  
Molecular Lines

We have investigated under which physical conditions (kinetic temperature, H2 density, CO column density) the CO molecule shows suprathermal excitation and population inversion. The computations are based on a model in which the excitation is basically due to collisions with H2 molecules. The collision cross-sections were taken from Green and Thaddeus (1976). The radiative transport in the molecular lines is treated in an on-the-spot approximation (see e.g. Kegel 1979)J = S(1 - e-τ) + Ibge-τin order to keep the computing time low enough to permit the investigation of a wide range of parameters. Our approximation of the radiative transfer is mathematically equivalent to a formalism involving an escape probability β = e-τ. This escape probability arises from the fact that the radiative transfer in the line wings is neglected.

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