arXiv subjects

Sarira Sahu

Publications and source records attributed to Sarira Sahu.

At least 37 records · Page 2Linked to original sources

arXiv2021

Neutrino decoherence in an electron and nucleon background

We consider the decoherence effects in the propagation of active neutrinos due to the non-forward neutrino scattering processes in a matter background composed of electrons and nucleons. We calculate the contribution to the imaginary part of the neutrino self-energy arising from such processes. Since the initial neutrino state is depleted but does not actually disappear (the initial neutrino transitions into a neutrino of a different flavor but does not decay) those processes should be associated with decoherence effects that cannot be described in terms of the coherent evolution of the state vector. Based on the formalism developed in our previous work for treating the non-forward scattering processes using the notion of the stochastic evolution of the state, we identify the jump operators, as used in the context of the master or Lindblad equation, in terms of the results of the the calculation of the non-forward neutrino scattering contribution to the imaginary part of the neutrino self-energy. As a guide to estimating the decoherence effects in situations of practical interest we give explicit formulas for the decoherence terms for different background conditions, and point out some of the salient features in particular the neutrino energy dependence. To establish contact wih previous works in which the decoherence terms are treated as phenomenological parameters, we consider the solution to the evolution equation in the two-generation case. We give formulas that are useful for estimating the effects of the decoherence terms under various conditions and environments, including the typical conditions applicable to long baseline experiments, where matter effects are important. In those contexts the effects appear to be small, and indicative that if significant decoherence effects were to be found they would be due to non-standard contributions to the decoherence terms.

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arXiv2021

Taming the pinch singularities in the two-loop neutrino self-energy in a medium

We consider the calculation of the thermal self-energy of a neutrino that propagates in a medium composed of fermions and scalars interacting via a Yukawa-type coupling, in the case that the neutri no energy is much larger than the fermion and scalar masses, as well as the temperature and chemical potentials of the background. In this kinematic regime the one-loop contribution to the imaginary part of the self-energy is negligible. We consider the two-loop contribution and we encounter the so-called pinch singularities which are known to arise in higher loop self-energy calculations in Thermal Field Theory. With a judicious use of the properties and parametrizations of the thermal propagators the singularities are treated effectively and actually disappear. From the imaginary part of the self-energy, we obtain a precise formula for the damping matrix expressed in terms of integrals over the background particle distributions. The formulas predict a specific dependence of the damping terms on the neutrino energy, depending on the background conditions. For guidance to estimating the effects in specific contexts, we compute the damping terms for several limiting cases of the momentum distribution functions of the background particles. We discuss briefly the connection between the results of our calculations for the damping matrix and the decoherence effects described in terms of the Lindblad equation.

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arXiv2021

Extreme HBL-like behavior of Markarian 421 and its two-zone photohadronic interpretation

Markarian 421 is the nearest high-energy peaked blazar and also is the first extragalactic source to be detected in multi-TeV $γ$-rays. It has been observed in multiwavelength for exceptionally long period of time with dense monitoring and several major outbursts have been detected from this source. In March 2010, the source was in a high state of activity and was observed in multiwavelength by various telescopes for 13 consecutive days. During this period the position of the synchrotron peak was found to be above $10^{17}$ Hz and also the position of the second peak was shifted towards higher energy, a signature of extreme HBL-like behavior. We observed that the standard photohadronic model is inadequate to explain the observed spectra. However, a recently proposed two-zone photohadronic model explains very well the GeV-TeV flaring events observed by both MAGIC and VERITAS telescopes. From the observation of the highest energy $γ$-ray event on MJD 55266 we also estimated the minimum bulk Lorentz factor.

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arXiv2020

A two-zone photohadronic interpretation of the EHBL-like behavior of the 2016 multi-TeV flares of 1ES 1959+650

The high-energy-peaked blazar 1ES 1959+650 is a well-known and well studied nearby blazar that has undergone several episodes of multi-TeV flaring. In 2002 for the first time an orphan TeV flare was observed from this blazar. During a multiwavelength campaign between 29th April to 21st November 2016, MAGIC telescopes observed multi-TeV flarings during the nights of 13th, 14th June and 1st July 2016 when the position of the synchrotron peak was found to be above $10^{17}$ Hz. Also observed was that the second peak of the spectral energy distribution shifted towards higher energy, and exhibiting extreme HBL-like behavior. The photohadronic model which is very successful in explaning the multi-TeV flaring from many high energy blazars including 1ES 1959+650 is applied to study the flaring events of 2016. It is observed that the photohadronic model is unable to explain the observed spectra. Here, we use a two-zone photohadronic model to explain the observed spectra. We clearly demonstrate that the low energy regime (zone-1) of the spectra corresponds to the standard flaring events of the high energy blazar and the high energy regime (zone-2) of the spectra are solely due to the extreme nature of the flaring events. Our two-zone photohadronic model explains very well the multi-TeV flaring events observed by MAGIC telescopes.

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arXiv2020

A two-zone photohadronic scenario for EHBL-like behavior of Mrk 501

Major outbursts have been observed from the well-known high-energy peaked blazar Markarian 501 since its discovery in 1996. Especially, two episodes of very high energy gamma-ray flaring events during May-July 2005 and June 2012 are of special significance, when the source exhibited extreme HBL-like behavior. The successful standard photohadronic model seems inadequate to explain these extraneous behaviors. We propose a two-zone photohadronic scenario to overcome this problem. In this picture, the low energy regime (zone-1) of the spectrum follows the standard photohadronic interpretation, while the high energy regime (zone-2) of the spectrum is new, with a spectral index $δ_2\geq 3.1$, which is solely due to the extreme nature of the flaring event. We also estimate the bulk Lorentz factor corresponding to these extreme flaring events. By analyzing many flaring events before and after these extreme events we argue that the extreme HBL-like events are transient and may repeat in future.

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arXiv2020

Photohadronic model for the neutrino and $γ$-ray emission from TXS 0506+056

The detection of a high energy muon neutrino on 22 September 2017 by IceCube neutrino detector coincides with the multiwavelength flaring from the BL Lac object TXS 0506+056, most likely confirming AGN as a source of high energy cosmic rays and neutrinos. Using the photohadronic model, we have explained the very high energy $γ$-rays observed by MAGIC telescopes few days after the neutrino event and extend the model to calculate the neutrino flux at different windows consistent with the flaring period of TXS 0506+056 and compared with the IceCube and MAGIC estimates. We also use this model to estimate the neutrino flux from the flaring of FSRQ PKS B1424-418 which is believed to be associated with the 2 PeV neutrino event observed by IceCube.

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arXiv2020

Origin of sub-TeV afterglow emission from gamma-ray bursts GRB 190114C and GRB 180720B

The detection of GRB 180722B and GRB 190114C in sub-TeV gamma-rays has opened up a new window to study gamma-ray bursts in high energy gamma-rays. Recently it is shown that the synchrotron and inverse Compton processes are responsible for the production of these high energy gamma-rays during the afterglow. Here, for the first time we demonstrate that, the photohadronic scenario which is successful in explaining the multi-TeV flaring in high energy blazars is also applicable for gamma-ray bursts. We show that the sub-TeV spectra of GRB 190114C and GRB 180720B are due to the interaction of high energy protons with the background photons in the synchrotron self-Compton region and synchrotron region respectively. The nature of the background photon distributions help us to constraint their bulk Lorentz factors.

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arXiv2020

Neutrino decoherence in a fermion and scalar background

We consider the decoherence effects in the propagation of neutrinos in a background composed of a scalar particle and a fermion due to the non-forward neutrino scattering processes. Using a simple model for the coupling of the form $\bar f_Rν_Lϕ$ we calculate the contribution to the imaginary part of the neutrino self-energy arising from the non-forward neutrino scattering processes in such backgrounds, from which the damping terms are determined. In the case we are considering, in which the initial neutrino state is depleted but does not actually disappear (the initial neutrino transitions into a neutrino of a different flavor but does not decay into a $fϕ$ pair, for example), we associate the damping terms with decoherence effects. For this purpose we give a precise prescription to identify the decoherence terms, as used in the context of the master or Linblad equation, in terms of the damping terms we have obtained from the calculation of the imaginary part of the neutrino self-energy from the non-forward neutrino scattering processes. The results can be directly useful in the context of Dark Matter-neutrino interaction models in which the scalar and/or fermion constitute the dark-matter, and can also serve to guide the generalizations to other models and/or situations in which the decoherence effects in the propagation of neutrinos originate from the non-forward scattering processes may be important. As a guide to estimating such decoherence effects, the contributions to the absorptive part of the self-energy and the corresponding damping terms are computed explicitly in the context of the model we consider, for several limiting cases of the momentum distribution functions of the background particles.

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arXiv2019

The VHE SED modelling of Markarian 501 in 2009

The high energy blazar, Markarian 501 was observed as a part of multi-instrument and multiwavelength campaign spanning the whole electromagnetic spectrum for 4.5 months during March 15 to August 1, 2009. On May 1, Whipple 10m telescope observed a very strong $γ$-ray flare in a time interval of about 0.5 h. Apart from this flare, high state and low state emissions were also observed by Whipple, VERITAS and MAGIC telescopes. Using the photohadronic model and accounting for the absorption of the extragalactic background light (EBL) to these very high energy $γ$-rays, excellent fits are obtained for the observed spectra. We have shown that the intrinsic spectrum for low state emission is flat, however, for high and very high states this is a power-law with slowly increasing exponent.

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arXiv2019

Multi-TeV flaring from high energy blazars: An evidence of the photohadronic process

The high energy peaked blazars are known to undergo episodes of flaring in GeV-TeV gamma-rays involving different time scales and the flaring mechanism is not well understood despite long term simultaneous multiwavelength observations. These gamma-rays en route to Earth undergo attenuation by the extra galactic background light. Using the photohadronic model, where the seed photons follow a power-law spectrum and a template extragalactic background light model, we derive a simple relation between the observed multi-TeV gamma-ray flux and the intrinsic flux with a single parameter. We study 42 flaring epochs of 23 blazars and excellent fit to most of the observed spectra are obtained, strengthening the photohadronic origin of multi-TeV gamma-rays. We can also constrain the power spectrum of the seed photons during the flaring period. The blazars of unknown redshifts, whose multi-TeV flaring spectra are known, stringent bounds on the former can be placed using the photohadronic model.

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arXiv2019

Multi-TeV flaring in nearby High Energy Blazars: A photohadronic scenario

Blazars are a subclass of AGN and flaring in multi-TeV gamma-ray seems to be the major activity in high energy blazars a subgrup of blazars. Flaring is also unpredictable and switches between quiescent and active states involving different time scales and fluxes. While in some high energy blazars a strong temporal correlation between X-ray and multi-TeV gamma-ray has been observed, outbursts in some other have no low energy counterparts and explanation of such extreme activity needs to be addressed through different mechanisms as it is not understood well. The extragalactic background light (EBL) plays an important role in the observation of these high energy gamma-rays as it attenuates through pair production of electron-positron and also changes the spectral shape of the high energy photons. In the context of the photohadronic model and taking EBL correction into account, flaring can be explained very well. In a series of papers we have developed this model to explain multi-TeV flaring events form many blazars. Here in this review, the photohadronic model is discussed and applied to explain the multi-TeV flaring from nearby high energy blazars: Markarian 421, Markarian 501 and 1ES1959+650.

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arXiv2019

Neutrino damping in a fermion and scalar background

We consider the propagation of a neutrino in a background composed of a scalar particle and a fermion using a simple model for the coupling of the form $λ\bar f_Rν_Lϕ$. In the presence of these interactions there can be damping terms in the neutrino effective potential and index of refraction. We calculate the imaginary part of the neutrino self-energy in this case, from which the damping terms are determined. The results are useful in the context of Dark Matter-neutrino interaction models in which the scalar and/or fermion constitute the dark-matter. The corresponding formulas for models in which the scalar particle couples to two neutrinos via a coupling of the form $λ^{(ννϕ)}\barν^c_Rν_Lϕ$ are then obtained as a special case, which can be important also in the context of neutrino collective oscillations in a supernova and in the Early Universe hot plasma before neutrino decoupling. A particular feature of our results is that the damping term in a $νϕ$ background is independent of the antineutrino-neutrino asymmetry in the background. Therefore, the relative importance of the damping term may be more significant if the neutrino-antineutrino asymmetry in the background is small, because the leading $Z$-exchange and $ϕ$-exchange contributions to the effective potential, which are proportional to the neutrino-antineutrino asymmetry, are suppressed in that case, while the damping term is not.

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arXiv2018

Neutrino dispersion relation in a magnetized multi-stream matter background

We study the propagation of a neutrino in a medium that consists of two or more thermal backgrounds of electrons and nucleons moving with some relative velocity, in the presence of a static and homogeneous electromagnetic field. We calculate the neutrino self-energy and dispersion relation using the linear thermal Schwinger propagator, we give the formulas for the dispersion relation and discuss general features of the results obtained, in particular the effects of the stream contributions. As a specific example we discuss in some detail the case of a magnetized two-stream electron, i.e., two electron backgrounds with a relative velocity $\vec v$ in the presence of a magnetic field. For a neutrino propagating with momentum $\vec k$, in the presence of the stream the neutrino dispersion relation acquires an anisotropic contribution of the form $\hat k\cdot\vec v$ in addition to the well known term $\hat k\cdot\vec B$, as well as an additional contribution proportional to $\vec B\cdot\vec v$. We consider the contribution from a nucleon stream background as an example of other possible stream backgrounds, and comment on possible generalizations to take into account the effects of inhomogeneous fields. We explain why a term of the form $\hat k\cdot(\vec v\times\vec B)$ does not appear in the dispersion relation in the constant field case, while a term of similar form can appear in the presence of an inhomogeneous field involving its gradient.

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arXiv2018

Neutrino propagation in an electron background with an inhomogeneous magnetic field

We study the electromagnetic coupling of a neutrino that propagates in a two-stream electron background medium. Specifically, we calculate the electromagnetic vertex function for a medium that consists of a \emph{normal} electron background plus another electron \emph{stream} background that is moving with a velocity four-vector $v^μ$ relative to the normal background. The results can be used as the basis for studying the neutrino electromagnetic properties and various processes in such a medium. As an application, we calculate the neutrino dispersion relation in the presence of an external magnetic field ($\vec B$), focused in the case in which $B$ is inhomogeneous, keeping only the terms of the lowest order in $1/m^2_W$ and linear in the $B$ and its gradient. We show that the dispersion relation contains additional anisotropic terms involving the derivatives of $\vec B$, such as the gradient of $\hat k\cdot(\vec v\times\vec B)$, which involve the stream background velocity, and a term of the form $\hat k\cdot(\nabla\times \vec B)$ that can be present in the absence of the stream background, in addition to a term of the form $\hat k\cdot\vec v$ and the well known term $\hat k\cdot\vec B$ that arises in the constant $\vec B$ case. The derivative-dependent terms are even under a $CP$ transformation. As a result, in contrast to the latter two just mentioned, they depend on the sum of the particle and antiparticle densities and therefore can be non-zero in a $CP$-symmetric medium in which the particle and antiparticle densities are equal.

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arXiv2018

On the non-detection of Glashow resonance in IceCube

Electron anti-neutrinos at the Glashow resonance (GR, at $E_{\bar ν_e} \sim 6.3$ PeV) have an enhanced probability to be detected. With three neutrinos detected by IceCube in the (1-2) PeV energy range at present, one would expect that about 1 to 4 GR $\barν_e$ should have been detected. The high-energy $\sim 8.7$ PeV muon neutrino detected by IceCube may not be a GR event. If so, we expect to detect 50 to 70 GR $\barν_e$, then one would have a "missing Glashow-resonance problem". This would suggest (1) that $pγ$ interaction rather than $pp$ interaction is the dominant channel to produce the observed IceCube high-energy neutrinos, (2) that multi-pion $pγ$ interactions are suppressed, and (3) that the magnetic field and photon energy density in the $pγ$ emission region is such that significant $μ^+$ cooling occurs before decaying, yet $π^+$'s essentially do not cool before decaying.

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arXiv2018

Propagation of GeV neutrinos through Earth

We have studied the Earth matter effect on the oscillation of upward going GeV neutrinos by taking into account the three active neutrino flavors. For neutrino energy in the range 3 to 12 GeV we observed three distinct resonant peaks for the oscillation process $ν_e\leftrightarrow ν_{μ,τ}$ in three \textit{distinct} densities. However, according to the most realistic density profile of the Earth, the second peak at neutrino energy 6.18 GeV corresponding to the density $6.6\,g/cm^3$ does not exist. So the resonance at this energy can not be of MSW-type. For the calculation of observed flux of these GeV neutrinos on Earth, we considered two different flux ratios at the source, the standard scenario with the flux ratio $1:2:0$ and the muon damped scenario with $0:1:0$. It is observed that at the detector while the standard scenario gives the observed flux ratio $1:1:1$, the muon damped scenario has a different ratio. For muon damped case with $E_ν < 20$ GeV, we always get observed neutrino fluxes as $Φ_{ν_e} < Φ_{ν_μ}\simeq Φ_{ν_τ}$ and for $E_ν > 20$ GeV, we get the average $Φ_{ν_e}\sim 0$ and $Φ_{ν_μ}\simeq Φ_{ν_τ}\simeq 0.45$. The upcoming PINGU will be able to shed more light on the nature of the resonance in these GeV neutrinos and hopefully will also be able to discriminate among different processes of neutrino production at the source in GeV energy range.

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arXiv2018

The nature of the intrinsic spectra from the VHE emission of H 2356-309 and 1ES 1101-232

The VHE emission from the HBLs H 2356-309 and 1ES 1101-232 were observed by HESS telescopes during 2004--2007. Particularly the observation in 2004 from H 2356-309 and during 2004--2005 from 1ES 1101-232 were analyzed to derive strong upper limits on the EBL which was found to be consistent with the lower limits from the integrated light of resolved galaxies. Here we have used the photohadronic model corroborated by two template EBL models to fit the observed VHE gamma-ray data from these two HBLs and to predict their intrinsic spectra. We obtain very good fit to the VHE spectra of these two HBLs. However, the predicted intrinsic spectra are different for each EBL model. For the HBL H 2356-309, we obtain a flat intrinsic spectrum and for 1ES 1101-232 the spectrum is mildly harder than 2 but much softer than 1.5.

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arXiv2018

The origin of multi-TeV flares from the nearest blazar Markarian 421

Markarian 421 is a high-peaked BL Lac object and it has undergone many strong outbursts since its discovery as a TeV source in 1992. Markarian 421 has been stud- ied intensively and was observed by various Cherenkov tele- scope arrays ever since. The outbursts of April 2004 observed by the Whipple telescope and of February 2010 by the HESS telescopes are explained well in this work by using the photohadronic model. To account for the attenuation of these high- energy gamma-rays by the extragalactic background light (EBL), we use template EBL models. The intrinsic spectrum of each epoch is different even though the high-energy protons have almost the same spectral index. We observe that this difference in intrinsic spectra is due to the change in the spectral index of the low-energy tail of the synchrotron self Compton (SSC) photons during different epochs of flaring. Our results show that the contemporaneous multiwavelength observations, particularly in the low-energy tail region of the SSC emission of the source, are important in explaining the flaring phenomenon.

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