Synchrotron Emission Of Electrons Research Articles

Synthetic measurements of runaway electron synchrotron emission in the SPARC tokamak.

With plasma currents up to 8.7 MA, the SPARC tokamak runs the risk of forming multi-MA beams of relativistic "runaway" electrons (REs), which could damage plasma facing components if unmitigated. The infrared (IR) and visible imaging and visible spectroscopy systems in SPARC are designed with measurements of synchrotron emission from REs in mind. Synchrotron radiation is emitted by REs along their direction of motion, opposite the plasma current. Matched clockwise and counterclockwise wide views are proposed to detect synchrotron and background radiation, allowing observation of RE synchrotron emission in both plasma current configurations. Due to SPARC's high toroidal magnetic field strength, 12.2T on axis, the synchrotron light spectrum is expected to peak in the visible-IR wavelength range. The synthetic diagnostic tool, Synchrotron Orbit-Following Toolkit, is used to model synchrotron images and spectra for three scenarios, with appropriate magnetic equilibria for each: REs generated during plasma current ramp-up, steady-state flat-top (although unlikely, but serving as a reference), and disruptions. Required time resolutions, achievable spatial coverage, and appropriate spectral ranges for various RE energies are assessed.

Direct Measurements of Synchrotron-emitting Electrons at Near-Sun Shocks

In this study, we present the first-ever direct measurements of synchrotron-emitting heliospheric traveling shocks, intercepted by the Parker Solar Probe (PSP) during its close encounters. Given that much of our understanding of powerful astrophysical shocks is derived from synchrotron radiation, these observations by PSP provide an unprecedented opportunity to explore how shocks accelerate relativistic electrons and the conditions under which they emit radiation. The probe’s unparalleled capabilities to measure both electromagnetic fields and energetic particles with high precision in the near-Sun environment has allowed us to directly correlate the distribution of relativistic electrons with the resulting photon emissions. Our findings reveal that strong quasi-parallel shocks emit radiation at significantly higher intensities than quasi-perpendicular shocks due to the efficient acceleration of ultrarelativistic electrons. These experimental results are consistent with theory and recent observations of supernova remnant shocks and advance our understanding of shock physics across diverse space environments.

Structured Jet Model for Multiwavelength Observations of the Jetted Tidal Disruption Event AT 2022cmc

AT 2022cmc is a recently documented tidal disruption event that exhibits a luminous jet, accompanied by fast-declining X-ray and long-lasting radio and millimeter emission. Motivated by the distinct spectral and temporal signatures between the X-ray and radio observations, we propose a multizone model involving relativistic jets with different Lorentz factors. We systematically study the evolution of faster and slower jets in an external density profile, considering the continuous energy injection rate associated with time-dependent accretion rates before and after the mass fallback time. We investigate time-dependent multiwavelength emission from both the forward shock (FS) and reverse shock (RS) regions of the fast and slow jets, in a self-consistent manner. Our analysis demonstrates that the energy injection rate can significantly impact the jet evolution and subsequently influence the lightcurves. We find that the X-ray spectra and lightcurves could be described by electron synchrotron emission from the RS of the faster jet, in which the late-time X-ray upper limits, extending to 400 days after the disruption, could be interpreted as a jet break. Meanwhile, the radio observations can be interpreted as a result of synchrotron emission from the FS region of the slower jet. We also discuss prospects for testing the model with current and future observations.

Multiple Emission Regions in Jets of the Low-Luminosity Active Galactic Nucleus in NGC 4278

The Large High Altitude Air Shower Observatory (LHAASO) has detected very-high-energy gamma rays from the low-ionization nuclear emission-line region galaxy NGC 4278, which has a low-luminosity active galactic nucleus (LLAGN) and symmetric, mildly relativistic S-shaped twin jets detected by radio observations. Few LLAGNs have been detected in gamma rays due to their faintness. Earlier, several radio-emitting components were detected in the jets of NGC 4278. We model their radio emission with synchrotron emission of ultra-relativistic electrons to estimate the strength of the magnetic field inside these components within a time-dependent framework after including the ages of the different components. We show that the synchrotron and synchrotron self-Compton emission by these components cannot explain the Swift X-ray data and the LHAASO gamma-ray data from NGC 4278. We suggest that a separate component in one of the jets is responsible for the high-energy emission, whose age, size, magnetic field, and the spectrum of the ultra-relativistic electrons inside it have been estimated after fitting the multiwavelength data of NGC 4278 with the sum of the spectral energy distributions from the radio components and the high-energy component. We note that the radio components of NGC 4278 are larger than the high-energy component, which has also been observed in several high-luminosity active galactic nuclei.

Minimalist model of the W50/SS433 extended X-ray jet: Anisotropic wind with recollimation shocks

W50 is a radio nebula around the hyperaccreting Galactic microquasar SS433. We focus on one peculiar feature of W50, that is, on a pair of so-called extended X-ray jets (EXJs). These jets have a size of ∼20 pc and a sharp inner boundary, and their spectra are well represented by a featureless X-ray continuum. We argue that EXJs could be an outcome of a powerful anisotropic wind produced by a supercritical accretion disk. In the simplest version of this model, the wind itself consists of two components. The first component is a nearly isotropic outflow that subtends most of the solid angle as seen from the compact source and creates the quasi-spherical part of the W50 nebula. The second component is a more collimated wind that is aligned with the rotation axis of the binary system (polar wind). The isotropic outflow passes through the termination shock, and its increased thermal pressure creates a sequence of recollimation shocks in the polar wind, giving it the appearance of an extended X-ray structure. In this model, the EXJ continuum spectrum is due to synchrotron emission of electrons that are accelerated at the shocks that arise in the polar wind. At variance with many other studies, the EXJ structures in this model are not directly related to the highly collimated and precessing 0.26 c baryonic jets. Instead, the EXJ and the ears of W50 are produced by the part of the wind whose Eddington-level kinetic luminosity is confined to a half-opening angle of 5–10 degrees. This is not necessarily a recollimated version of the 0.26 c jets.

HESS J1809-193: Gamma-Ray Emission by Cosmic Rays from a Past Explosion

The very-high-energy γ-ray source HESS J1809-193 has been detected by the LHAASO and HAWC observatory beyond 100 TeV energy. It is an interesting candidate for exploring the underlying mechanisms of γ-ray production due to the presence of supernova remnants, pulsars, and molecular clouds close to it. We have considered the injection of the energetic cosmic rays from a past explosion, whose reminiscent may be SNR G011.0-00.0, which is located within the extended γ-ray source HESS J1809-193. We explain the multiwavelength data from the region of HESS J1809-193 with synchrotron, inverse Compton, and bremsstrahlung emission of cosmic-ray electrons and secondary γ-ray production in interactions of cosmic-ray protons with the cold protons in the local molecular clouds within a time-dependent framework including the diffusion loss of cosmic rays. The observational data have been modeled with the secondary photons produced by the time-evolved cosmic-ray spectrum assuming the age of the explosion is 4500 yr.

THE STUDY OF X-RAY SPECTRUM OF COMA GALAXY CLUSTER

The X-ray spectrum of the Coma galaxy cluster was studied using the data from the XMM-Newton observatory. We combined 7 observations performed with the MOS camera of XMM-Newton in the 40′ × 40′ region centered at the Coma cluster. The analyzed observations were performed in 2000–2005 and have a total duration of 196 ksec. We focus on the analysis of the MOS camera spectra due to their lower affection by strong instrumental line-like background. The obtained spectrum was fitted with a model including contributions from the Solar system/Milky Way hot plasma and a power law X-ray background. The contribution of the instrumental background was modeled as a power law (not convolved with the effective area) and a number of Gaussian lines. The contribution from the Coma cluster was modeled with a single-temperature hot plasma emission. In addition, we searched for possible non-thermal radiation present in the vicinity of the center of the Coma cluster, originating e.g. from synchrotron emission of relativistic electrons on a turbulent magnetic field. We compared the results with previous works by other authors and spectra obtained from other instruments that operate in the similar energy range of 1−10 keV. Careful and detailed spectrum analysis shall be a necessary contribution to our future work – searching for axion-like dark matter particles’ manifestations in the Coma cluster.

Proton Synchrotron Origin of the Very-high-energy Emission of GRB 190114C

We consider here a proton-synchrotron model to explain the MAGIC observation of GRB 190114C afterglow in the energy band of 0.2–1 TeV, while the X-ray spectra are explained by electron-synchrotron emission. Given the uncertainty of the particle acceleration process, we consider several variations of the model, and show that they all match the data very well. We find that the values of the uncertain model parameters are reasonable: isotropic explosion energy ∼1054.5 erg, ambient density ∼10–100 cm−3, and the fraction of electrons/protons accelerated to a high-energy power law is of a few percent. All these values are directly derived from the observed teraelectronvolt and X-ray fluxes. The model also requires that protons be accelerated to observed energies as high as a few 1020 eV. Further, assuming that the jet break takes place after 106 s gives the beaming-corrected energy of the burst to be ≈1053 erg, which is one to two orders of magnitude higher than usually inferred, because of the small fraction of electrons accelerated. Our modeling is consistent with both late time data at all bands, from optical to X-rays, and with numerical models of particle acceleration. Our results thus demonstrate the relevance of proton-synchrotron emission to the high-energy observations of gamma-ray bursts during their afterglow phase.

UHECR echoes from the Council of Giants

ABSTRACT Recent anisotropy studies of UHECR data at energies ≳40 EeV have disclosed a correlation of their angular distribution with the extragalactic local structure, specifically with either local starburst galaxies or AGN. Using Monte Carlo simulations taking into account photodisintegration processes, we further explore a framework in which these UHECRs were accelerated by Centaurus A in a recent powerful outburst before being scattered by magnetic fields associated with local, Council of Giant, extragalactic structure. We find that the observed intermediate-scale anisotropies can be accounted for by the Council of Giant structure imposing a response function on the initial outburst of UHECRs from a single source located at Centaurus A’s position. The presence of these local structures create ‘echoes’ of UHECRs after the initial impulse and focusing effects. The strongest echo wave has a lag of ∼20 Myr, comparable to the age of synchrotron-emitting electrons in the giant Centaurus A lobes. Through consideration of the composition of both the direct and echo wave components, we find that the distribution of the light (1 < ln A < 1.5) component across the sky offers exciting prospects for testing the echo model using future facilities such as Auger prime. Our results demonstrate the potential that UHECR nuclei offer, as ‘composition clocks’, for probing propagation scenarios from local sources.

X-Ray Flares in the Long-term Light Curve of Low-luminosity Active Galactic Nucleus M81*

Most active galactic nuclei at the center of the nearby galaxies have supermassive black holes accreting at sub-Eddington rates through hot accretion flows or radiatively inefficient accretion flows, which efficiently produce jets. The association of radio and X-ray flares with the knot ejection from M81* inspires us to model its multiwavelength spectral energy distribution (SED) during these flares to constrain the physical parameters of the jet. Moreover, we construct a long-term light curve in X-rays to identify the flares in the available data and constrain the jet parameters during those periods. The jet activity may vary on short as well as long timescales, which may produce flares in different frequency bands. The SEDs from radio to X-ray during the quiescent as well as flaring states are found to be satisfactorily explained by synchrotron emission of relativistic electrons from a single zone. The variation in the values of the jet parameters during the different states is shown and compared with high-synchrotron peaked blazars.

Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157

Context. Several new ultrahigh-energy (UHE) γ-ray sources have recently been discovered by the Large High Altitude Air Shower Observatory (LHAASO) collaboration. These represent a step forward in the search for the so-called Galactic PeVatrons, the enigmatic sources of the Galactic cosmic rays up to PeV energies. However, it has been shown that multi-TeV γ-ray emission does not necessarily prove the existence of a hadronic accelerator in the source; indeed this emission could also be explained as inverse Compton scattering from electrons in a radiation-dominated environment. A clear distinction between the two major emission mechanisms would only be made possible by taking into account multi-wavelength data and detailed morphology of the source. Aims. We aim to understand the nature of the unidentified source LHAASO J2108+5157, which is one of the few known UHE sources with no very high-energy (VHE) counterpart. Methods. We observed LHAASO J2108+5157 in the X-ray band with XMM-Newton in 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good-quality data. In addition, we analyzed 12 years of Fermi-LAT data, to better constrain emission of its high-energy (HE) counterpart 4FGL J2108.0+5155. We used naima and jetset software packages to examine the leptonic and hadronic scenario of the multi-wavelength emission of the source. Results. We found an excess (3.7σ) in the LST-1 data at energies E > 3 TeV. Further analysis of the whole LST-1 energy range, assuming a point-like source, resulted in a hint (2.2σ) of hard emission, which can be described with a single power law with a photon index of Γ = 1.6 ± 0.2 the range of 0.3 − 100 TeV. We did not find any significant extended emission that could be related to a supernova remnant (SNR) or pulsar wind nebula (PWN) in the XMM-Newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. We revealed a new potential hard source in Fermi-LAT data with a significance of 4σ and a photon index of Γ = 1.9 ± 0.2, which is not spatially correlated with LHAASO J2108+5157, but including it in the source model we were able to improve spectral representation of the HE counterpart 4FGL J2108.0+5155. Conclusions. The LST-1 and LHAASO observations can be explained as inverse Compton-dominated leptonic emission of relativistic electrons with a cutoff energy of 100−30+70 TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a PWN or a TeV halo. Furthermore, the spectral properties of the HE counterpart are consistent with a Geminga-like pulsar, which would be able to power the VHE-UHE emission. Nevertheless, the lack of a pulsar in the neighborhood of the UHE source is a challenge to the PWN/TeV-halo scenario. The UHE γ rays can also be explained as π0 decay-dominated hadronic emission due to interaction of relativistic protons with one of the two known molecular clouds in the direction of the source. Indeed, the hard spectrum in the LST-1 band is compatible with protons escaping a shock around a middle-aged SNR because of their high low-energy cut-off, but the origin of the HE γ-ray emission remains an open question.

NuSTAR discovery of the hard X-ray emission and a wide-band X-ray spectrum from the Pictor A western hotspot

Abstract Utilizing Chandra, XMM-Newton, and NuSTAR, a wide-band X-ray spectrum from 0.2 to 20 keV is reported from the western hotspot of Pictor A. In particular, the X-ray emission is significantly detected in the 3 to 20 keV band at 30σ by NuSTAR. This is the first detection of hard X-rays with energies above 10 keV from a jet termination hotspot of active galactic nuclei. The hard X-ray spectrum is well described with a power-law model with a photon index of Γ = 1.8 ± 0.2, and the flux is obtained to be (4.5 ± 0.4) × 10−13 erg s−1 cm−2 in the 3 to 20 keV band. The obtained spectrum is smoothly connected with those soft X-ray spectra observed by Chandra and XMM-Newton. The wide-band spectrum shows a single power-law spectrum with a photon index of Γ = 2.07 ± 0.03, excluding any cut-off/break features. Assuming the X-rays to be synchrotron radiation of the electrons, the energy index of the electrons is estimated as p = 2Γ − 1 = 3.14 ± 0.06 from the wide-band spectrum. Given that the X-ray synchrotron-emitting electrons quickly lose their initial energies via synchrotron radiation, the energy index of electrons at acceleration sites is estimated as pacc = p − 1 = 2.14 ± 0.06. This is consistent with the prediction of the diffusive shock acceleration. Since the spectrum has no cut-off feature up to 20 keV, the maximum electron energy is estimated to be no less than 40 TeV.

Thermal Electrons in Mildly Relativistic Synchrotron Blast Waves

Abstract Numerical models of collisionless shocks robustly predict an electron distribution composed of both thermal and nonthermal electrons. Here, we explore in detail the effect of thermal electrons on the emergent synchrotron emission from subrelativistic shocks. We present a complete “thermal + nonthermal” synchrotron model and derive properties of the resulting spectrum and light curves. Using these results, we delineate the relative importance of thermal and nonthermal electrons for subrelativistic shock-powered synchrotron transients. We find that thermal electrons are naturally expected to contribute significantly to the peak emission if the shock velocity is ≳0.2c, but would be mostly undetectable in nonrelativistic shocks. This helps explain the dichotomy between typical radio supernovae and the emerging class of “AT2018cow-like” events. The signpost of thermal electron synchrotron emission is a steep optically-thin spectral index and a ν 2 optically-thick spectrum. These spectral features are also predicted to correlate with a steep postpeak light-curve decline rate, broadly consistent with observed AT2018cow-like events. We expect that thermal electrons may be observable in other contexts where mildly relativistic shocks are present and briefly estimate this effect for gamma-ray burst afterglows and binary–neutron-star mergers. Our model can be used to fit spectra and light curves of events and accounts for both thermal and nonthermal electron populations with no additional physical degrees of freedom.

Polarized imaging of visible synchrotron emission from runaway electron plateaus in DIII-D

A new approach to estimate the pitch angle of highly energetic postdisruption runaway electrons is presented by imaging the polarized runaway electron synchrotron emission. The vertical to horizontal polarization ratio is found to range between 3 and 14 in image-average studies looking at repeated shots as well as the polarization in a single discharge. Numerical studies with guiding center and full orbit simulations performed with the SOFT and KORC codes are given and compared to the experiment. The pitch angle evolution analyzed using a 1D impurity diffusion model in combination with kinetic simulations is also presented. These studies find agreement in the magnitude of the pitch angle, but not in the temporal rate of change of pitch angle. The measured pitch angle evolution timescale of order 50 ms falls between a modeled timescale of 20 ms in an equilibrium model and ≫100 ms in a nonequilibrium model.

A Search for Rapid Mid-infrared Variability in Gamma-Ray-emitting Narrow-line Seyfert 1 Galaxies

Abstract Fermi-LAT’s detection of γ-rays from narrow-line Seyfert 1 galaxies (NLS1s) has received increasing attention. Understanding these γ-NLS1s is of interest because they have some properties similar to blazars, which are known to show rapid and large-amplitude variability. Based on the largest sample of γ-NLS1s (25 sources), we carried out a systematic search for rapid mid-infrared (MIR, 3.4 and 4.6 μm) variability using the multiepoch data of the Wide-field Infrared Survey Explorer (WISE). We also compared a few variability properties between γ-NLS1s and γ-ray blazars. Our main results are as follows. (1) Thirteen γ-NLS1s showed significant (>3σ) rapid variability in at least one of the two MIR bands. The MIR emission of these sources is dominated by the synchrotron emission of relativistic electrons in the jet. (2) The γ-NLS1s with flat radio spectra are more variable than those not detected in γ-rays. (3) The γ-NLS1s tend to show smaller amplitude of variability as well as lower duty cycle relative to γ-ray blazars. (4) The γ-NLS1s tend to show a trend of bluer-when-brighter on both intraday and long timescales, similar to γ-ray blazars. (5) The γ-NLS1s that are more variable on long timescales have larger amplitudes of variability and higher duty cycles on intraday timescales. (6) In the majority of cases, the γ-NLS1s fall in the WISE Gamma-ray Blazar Strip (WGS). However, we noted migrations outside of the WGS due to significant variability.

Intraday Mid-infrared Variability of PMN J0948+0022 on 2020 April 26

Abstract We report a new period mid-infrared variability of PMN J0948+0022 on 2020 April 26, observed by the Wide-field Infrared Survey Explore (WISE). This source faded consistently over a period of one day. The amplitude of variability is about 0.21 and 0.18 mag in the W1 (3.4 μm) and W2 (4.6 μm) bands, respectively. The intraday variability constrains the size of the mid-infrared emission region to about 5 × 10−4 pc, which is rather smaller than that of the dust torus but consistent with the base of a jet. The mid-infrared color showed a significant bluer-when-brighter trend, similar to γ-ray emitting blazars. These characteristics suggest that the mid-infrared emission was dominated by the synchrotron emission of relativistic electrons in the jet.

Multiwavelength monitoring of NGC 1275 over a decade: evidence of a shift in synchrotron peak frequency and long-term multiband flux increase

ABSTRACT We carried out a detailed study of the temporal and broad-band spectral behaviour of one of the brightest misaligned active galaxies in γ-rays, NGC 1275, utilizing 11 years of Fermi data and the available Swift and AstroSat observations. Based on the cumulative flux distribution of the γ-ray light curve, we identified four distinct activity states and noticed an increase in the baseline flux during the first three states. A similar nature of increase in the average flux was also noticed in the X-ray and UV bands. Strong flaring activity in γ-rays was noticed in the fourth state. The source was observed twice by AstroSat for shorter intervals (∼days) during longer observing (∼years) states 3 and 4. During the AstroSat observing periods, the source γ-ray flux was higher than the average flux observed during longer duration states. The increase in average baseline flux from state 1 to state 3 can be explained by considering a corresponding increase of jet particle normalization. The inverse Comptonization of synchrotron photons explained the average X-ray and γ-ray emission by jet electrons during the first three longer duration states. However, during the shorter duration AstroSat observing periods, a shift of synchrotron peak frequency was noticed, and the synchrotron emission of jet electrons explained the observed X-ray flux well.

Double-peaked Pulse Profile of FRB 200428: Synchrotron Maser Emission from Magnetized Shocks Encountering a Density Jump

Abstract Very recently a fast radio burst (FRB) 200428 associated with a strong X-ray burst from the Galactic magnetar SGR 1935+2154 was detected; this provides direct evidence supporting the magnetar progenitor models of FRBs. Assuming that the FRB radiation mechanism is synchrotron maser emission from magnetized shocks, we develop a specific scenario by introducing a density-jump structure of upstream medium, thus making the double-peaked character of FRB 200428 a natural outcome. The luminosity and emission frequency of two pulses can be well explained in this scenario. Furthermore, we find that the synchrotron emission of shock-accelerated electrons is in the X-ray band, and therefore can be responsible for at least a portion of observed X-ray fluence. With the proper upgrade, in the future this density-jump scenario could be applied to FRBs with multiple peaks.

Do Afterglow Synchrotron Radiations Follow the Lp,iso–Ep,z–Γ0 Relation of Gamma-Ray Bursts? The Cases of GRBs 190114C, 130427A, and 180720B

Abstract Bimodal spectral energy distributions (SEDs) of gamma-ray burst (GRB) afterglow of GRBs 190114C, 130427A, and 180720B confirm that they are originated from the synchrotron emission (Syn) and synchrotron self-Compton scattering (SSC) process of electrons accelerated in the jets. The radiation mechanism and the physics of the observed spectrum-luminosity/energy relations of GRBs remain as open questions. By extracting the Syn component through fitting their early afterglow SEDs with the Syn+SSC model, we find that their luminosity (L syn), peak energy (E p,syn,z), and the Lorentz factor of the afterglow fireball (Γ t ) follow the L p,iso–E p,z–Γ0 relation of prompt gamma rays, where L p,iso is the isotropic luminosity, E p,z is the peak energy of the spectrum in the burst frame, and Γ0 is the initial Lorentz factor of the fireball. To examine whether late afterglows are consistent with this relation, we calculate the synchrotron component at late afterglows. It is found that they also follow the same L p,iso–E p,z–Γ0 relation, albeit they are not consistent with the L p,iso–E p,z relation. Our results may imply that the L p,iso–E p,z–Γ0 relation would be a universal feature of synchrotron radiations of electrons accelerated in GRB jets throughout the prompt and afterglow phases among GRBs. Its origin is not fully understood, and possible explanations are briefly discussed.

Chandra Imaging of the Western Hotspot in the Radio Galaxy Pictor A: Image Deconvolution and Variability Analysis

Abstract Here we present an analysis of the X-ray morphology and flux variability of the particularly bright and extended western hotspot in the nearest powerful (FR II-type) radio galaxy, Pictor A, based on data obtained with the Chandra X-ray Observatory. The hotspot marks the position where the relativistic jet, which originates in the active nucleus of the system, interacts with the intergalactic medium, at hundreds-of-kiloparsec distances from the host galaxy, forming a termination shock that converts jet bulk kinetic energy to internal energy of the plasma. The hotspot is bright in X-rays due to the synchrotron emission of electrons accelerated to ultrarelativistic energies at the shock front. In our analysis, we make use of several Chandra observations targeting the hotspot over the last decades with various exposures and off-axis angles. For each pointing, we study in detail the point-spread function, which allows us to perform the image deconvolution, and to resolve the hotspot structure. In particular, the brightest segment of the X-ray hotspot is observed to be extended in the direction perpendicular to the jet, forming a thin, ∼3 kpc long, feature that we identify with the front of the reverse shock. The position of this feature agrees well with the position of the optical intensity peak of the hotspot, but is clearly offset from the position of the radio intensity peak, located ∼1 kpc further downstream. In addition, we measure the net count rate on the deconvolved images, finding a gradual flux decrease by about 30% over the 15 yr timescale of the monitoring.