arXiv subjects

P. Goswami

Publications and source records attributed to P. Goswami.

30 records · Page 2Linked to original sources

arXiv2023

Constraints on the intergalactic magnetic field using Fermi-LAT and H.E.S.S. blazar observations

Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron-positron pairs, which can subsequently initiate electromagnetic cascades. The $γ$-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of $B > 7.1\times10^{-16}$ G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of $10^4$ ($10^7$) yr, IGMF strengths below $1.8\times10^{-14}$ G ($3.9\times10^{-14}$ G) are excluded, which rules out specific models for IGMF generation in the early universe.

astro-ph.HE↗

arXiv2023

Constraining the cosmic-ray pressure in the inner Virgo Cluster using H.E.S.S. observations of M 87

The origin of the gamma-ray emission from M87 is currently a matter of debate. This work aims to localize the VHE (100 GeV-100 TeV) gamma-ray emission from M87 and probe a potential extended hadronic emission component in the inner Virgo Cluster. The search for a steady and extended gamma-ray signal around M87 can constrain the cosmic-ray energy density and the pressure exerted by the cosmic rays onto the intra-cluster medium, and allow us to investigate the role of the cosmic rays in the active galactic nucleus feedback as a heating mechanism in the Virgo Cluster. H.E.S.S. telescopes are sensitive to VHE gamma rays and have been utilized to observe M87 since 2004. We utilized a Bayesian block analysis to identify M87 emission states with H.E.S.S. observations from 2004 until 2021, dividing them into low, intermediate, and high states. Because of the causality argument, an extended ($\gtrsim$kpc) signal is allowed only in steady emission states. Hence, we fitted the morphology of the 120h low state data and found no significant gamma-ray extension. Therefore, we derived for the low state an upper limit of 58"(corresponding to $\approx$4.6kpc) in the extension of a single-component morphological model described by a rotationally symmetric 2D Gaussian model at 99.7% confidence level. Our results exclude the radio lobes ($\approx$30 kpc) as the principal component of the VHE gamma-ray emission from the low state of M87. The gamma-ray emission is compatible with a single emission region at the radio core of M87. These results, with the help of two multiple-component models, constrain the maximum cosmic-ray to thermal pressure ratio $X_{CR,max.}$$\lesssim$$0.32$ and the total energy in cosmic-ray protons (CRp) to $U_{CR}$$\lesssim$5$\times10^{58}$ erg in the inner 20kpc of the Virgo Cluster for an assumed CRp power-law distribution in momentum with spectral index $α_{p}$=2.1.

astro-ph.HE↗

arXiv2023

Detection of extended gamma-ray emission around the Geminga pulsar with H.E.S.S

Geminga is an enigmatic radio-quiet gamma-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy gamma-ray emission around the pulsar was discovered by Milagro and later confirmed by HAWC, which are both water Cherenkov detector-based experiments. However, evidence for the Geminga pulsar wind nebula in gamma rays has long evaded detection by imaging atmospheric Cherenkov telescopes (IACTs) despite targeted observations. The detection of gamma-ray emission on angular scales > 2 deg poses a considerable challenge for the background estimation in IACT data analysis. With recent developments in understanding the complementary background estimation techniques of water Cherenkov and atmospheric Cherenkov instruments, the H.E.S.S. IACT array can now confirm the detection of highly extended gamma-ray emission around the Geminga pulsar with a radius of at least 3 deg in the energy range 0.5-40 TeV. We find no indications for statistically significant asymmetries or energy-dependent morphology. A flux normalisation of $(2.8\pm0.7)\times10^{-12}$ cm$^{-2}$s$^{-1}$TeV$^{-1}$ at 1 TeV is obtained within a 1 deg radius region around the pulsar. To investigate the particle transport within the halo of energetic leptons around the pulsar, we fitted an electron diffusion model to the data. The normalisation of the diffusion coefficient obtained of $D_0 = 7.6^{+1.5}_{-1.2} \times 10^{27}$ cm$^2$s$^{-1}$, at an electron energy of 100 TeV, is compatible with values previously reported for the pulsar halo around Geminga, which is considerably below the Galactic average.

astro-ph.HE↗

arXiv2023

H.E.S.S. follow-up observations of GRB221009A

GRB221009A is the brightest gamma-ray burst ever detected. To probe the very-high-energy (VHE, $>$\!100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hours after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of $Φ_\mathrm{UL}^{95\%} = 9.7 \times 10^{-12}~\mathrm{erg\,cm^{-2}\,s^{-1}}$ above $E_\mathrm{thr} = 650$ GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the SED occurring above the X-ray band. Compared to the VHE-bright GRB190829A, the upper limits for GRB221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB221009A, effectively ruling out an IC dominated scenario.

astro-ph.HE↗

arXiv2023

HESS J1809$-$193: a halo of escaped electrons around a pulsar wind nebula?

Context. HESS J1809$-$193 is an unassociated very-high-energy $γ$-ray source located on the Galactic plane. While it has been connected to the nebula of the energetic pulsar PSR J1809$-$1917, supernova remnants and molecular clouds present in the vicinity also constitute possible associations. Recently, the detection of $γ$-ray emission up to energies of $\sim$100 TeV with the HAWC observatory has led to renewed interest in HESS J1809$-$193. Aims. We aim to understand the origin of the $γ$-ray emission of HESS J1809$-$193. Methods. We analysed 93.2 h of data taken on HESS J1809$-$193 above 0.27 TeV with the High Energy Stereoscopic System (H.E.S.S.), using a multi-component, three-dimensional likelihood analysis. In addition, we provide a new analysis of 12.5 yr of Fermi-LAT data above 1 GeV within the region of HESS J1809$-$193. The obtained results are interpreted in a time-dependent modelling framework. Results. For the first time, we were able to resolve the emission detected with H.E.S.S. into two components: an extended component that exhibits a spectral cut-off at $\sim$13 TeV, and a compact component that is located close to PSR J1809$-$1917 and shows no clear spectral cut-off. The Fermi-LAT analysis also revealed extended $γ$-ray emission, on scales similar to that of the extended H.E.S.S. component. Conclusions. Our modelling indicates that based on its spectrum and spatial extent, the extended H.E.S.S. component is likely caused by inverse Compton emission from old electrons that form a halo around the pulsar wind nebula. The compact component could be connected to either the pulsar wind nebula or the supernova remnant and molecular clouds. Due to its comparatively steep spectrum, modelling the Fermi-LAT emission together with the H.E.S.S. components is not straightforward. (abridged)

astro-ph.HE↗

arXiv2022

A deep spectromorphological study of the $γ$-ray emission surrounding the young massive stellar cluster Westerlund 1

Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible for a significant fraction of cosmic rays (CRs) accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy is a prime candidate for studying this hypothesis. While the very-high-energy $γ$-ray source HESS J1646-458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified. We aim to identify the physical processes responsible for the $γ$-ray emission around Westerlund 1 and thus to better understand the role of massive stellar clusters in the acceleration of Galactic CRs. Using 164 hours of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the $γ$-ray emission of HESS J1646-458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs. We detected large-scale ($\sim 2^\circ$ diameter) $γ$-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with $γ$-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and is uniform across the entire source region. We did not find a clear correlation of the $γ$-ray emission with gas clouds as identified through H I and CO observations. We conclude that, of the known objects within the region, only Westerlund 1 can explain the bulk of the $γ$-ray emission. Several CR acceleration sites and mechanisms are conceivable, and discussed in detail. (abridged)

astro-ph.HE↗

arXiv2017

Evidence for Magnetic Weyl Fermions in a Correlated Metal

Recent discovery of both gapped and gapless topological phases in weakly correlated electron systems has introduced various relativistic particles and a number of exotic phenomena in condensed matter physics. The Weyl fermion is a prominent example of three dimensional (3D), gapless topological excitation, which has been experimentally identified in inversion symmetry breaking semimetals. However, their realization in spontaneously time reversal symmetry (TRS) breaking magnetically ordered states of correlated materials has so far remained hypothetical. Here, we report a set of experimental evidence for elusive magnetic Weyl fermions in Mn$_3$Sn, a non-collinear antiferromagnet that exhibits a large anomalous Hall effect even at room temperature. Detailed comparison between our angle resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations reveals significant bandwidth renormalization and damping effects due to the strong correlation among Mn 3$d$ electrons. Moreover, our transport measurements have unveiled strong evidence for the chiral anomaly of Weyl fermions, namely, the emergence of positive magnetoconductance only in the presence of parallel electric and magnetic fields. The magnetic Weyl fermions of Mn$_3$Sn have a significant technological potential, since a weak field ($\sim$ 10 mT) is adequate for controlling the distribution of Weyl points and the large fictitious field ($\sim$ a few 100 T) in the momentum space. Our discovery thus lays the foundation for a new field of science and technology involving the magnetic Weyl excitations of strongly correlated electron systems.

cond-mat.str-el↗

arXiv2016

Inter-planar coupling dependent magnetoresistivity in high purity layered metals

The magnetic field-induced changes in the conductivity of metals are the subject of intense interest, both for revealing new phenomena and as a valuable tool for determining their Fermi surface. Here, we report a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one. This effect is correlated with the inter-layer coupling disappearing for fields applied along the so-called Yamaji angles where the inter-layer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from the axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral- (e.g. Weyl) fermions. Its observation in PdCoO$_2$, PtCoO$_2$ and Sr$_2$RuO$_4$ suggests that the anomaly affects the transport of clean conductors, particularly near the quantum limit.

cond-mat.mes-hall↗

arXiv2013

Bulk evidence for a time reversal symmetry broken superconducting state in URu2Si2

URu$_2$Si$_2$ is claimed to be a chiral \emph{d}-wave superconductor with a $k_z (k_x \pm ik_y)$ time-reversal symmetry broken orbital component for the Cooper pair wave-function, which contains both nodal points and lines of nodes \cite{kasahara, kasahara2}. To study the magnetic response of such an unconventional state through a bulk, thermodynamic probe, we measured the magnetic torque $τ$ in very high-quality, well-characterized URu$_2$Si$_2$ single-crystals \cite{altarawneh,altarawneh2} at high magnetic-fields $H$ and at very low temperatures $T$. The magnetization $M(H) \propto τ(H) /H$ of URu$_2$Si$_2$, in its superconducting state and for angles within $15^{\circ}$ from the \emph{ab}-plane, reveals a change in its sign for $H$ approaching $H_{c2}$: from a clear diamagnetic response dominated by the pinning of vortices to a state with a smaller but "paramagnetic-like" hysteretic response which \emph{disappears} at $H_{c2}$, thus implying that it is intrinsically related to the superconducting state. We argue that this anomalous, angular-dependent behavior is evidence for a time-reversal symmetry broken superconducting state in URu$_2$Si$_2$, although not necessarily for the $k_z (k_x \pm ik_y)$ state.

cond-mat.supr-con↗

arXiv2013

Small and nearly isotropic hole-like Fermi surfaces in LiFeAs detected through de Haas van Alphen-effect

LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which and a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin uctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase-diagram is indeed observed in the isostructural NaFeAs compound. Previous angle resolved photoemission (ARPES) experiments revealed the existence of both hole and electron-like surfaces, but with rather distinct cross-sectional areas and an absence of the nesting that is thought to underpin both magnetic order and superconductivity in the pnictide family of superconductors. These ARPES observations were challenged by subsequent de Haas van Alphen (dHvA) measurements which detected a few, electron like Fermi surface sheets in rough agreement with the original band calculations. Here, we show a detailed dHvA study unveiling additional, small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which ought to correspond to hole-like orbits, as previously observed by ARPES. Therefore, our results conciliate the apparent discrepancy between ARPES and the previous dHvA results5. The small size of these Fermi surface pockets suggests a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor a s-wave pairing symmetry for LiFeAs. But similar electron-like Fermi surfaces combined with very different hole pockets between LiFeAs and LiFeP, suggest that the nodes in the gap function of LiFeP might be located on the hole-pockets.

cond-mat.supr-con↗

arXiv2013

Anomalous hysteresis as an evidence for a magnetic field-induced chiral superconducting state in LiFeAs

Magnetometry measurements in high quality LiFeAs single-crystals reveal a change in the sign of the magnetic hysteresis in the vicinity of the upper critical field $H_{c2}$, from a clear diamagnetic response dominated by the pinning of vortices, to a considerably smaller net hysteretic response of opposite sign, which \emph{disappears} at $H_{c2}$. If the diamagnetic response at high fields results from pinned vortices and associated screening super-currents, this sign change must result from currents circulating in the opposite sense, which give rise to a small field-dependent magnetic moment \emph{below} $H_{c2}$. This behavior seems to be extremely sensitive to the sample quality or stoichiometry, as we have observed it only in a few fresh crystals, which also display the de Haas van Alphen-effect. We provide arguments against the surface superconductivity, the flux compression, and the random $π$ junction scenarios, which have been previously put forward to explain a paramagnetic Meissner effect, below the lower critical field $H_{c1}$. The observed anomalous hysteresis at high fields will be compatible with the existence of chiral gap wave-functions, which possess a field dependent magnetic moment. Within a Landau-Ginzburg framework, we demonstrate how a $(d_{x^2 - y^2} + id_{xy})$ or a $(p_x+ip_y)$ chiral superconducting component can be stabilized in the mixed state of $s_{\pm}$ superconductor, due to the combined effects of the magnetic field and the presence of competing pairing channels. The realization of a particular chiral pairing depends on the microscopic details of the strengths of the competing pairing channels.

cond-mat.supr-con↗

arXiv2005

A Landau fluid model for warm collisionless plasmas

A Landau fluid model for a collisionless electron-proton magnetized plasma, that accurately reproduces the dispersion relation and the Landau damping rate of all the magnetohydrodynamic waves, is presented. It is obtained by an accurate closure of the hydrodynamic hierarchy at the level of the fourth order moments, based on linear kinetic theory. It retains non-gyrotropic corrections to the pressure and heat flux tensors up to the second order in the ratio between the considered frequencies and the ion cyclotron frequency.

physics.plasm-ph↗