Rapid Temporal Variability Research Articles

The internal Faraday screen of Sagittarius A*

We report on 85−101 GHz light curves of the Galactic Center supermassive black hole, Sagittarius A* (Sgr A*), observed in April 2017 with the Atacama Large Millimeter/submillimeter Array (ALMA). This study of high-cadence full-Stokes data provides new measurements of the fractional linear polarization at a 1−2% level resolved in 4 s time segments, and stringent upper limits on the fractional circular polarization at 0.3%. We compare these findings to ALMA light curves of Sgr A* at 212−230 GHz observed three days later, characterizing a steep depolarization of the source at frequencies below about 150 GHz. We obtain time-dependent rotation measure (RM) values, with the mean RM at 85−101 GHz being a factor of two lower than that at 212−230 GHz. Together with the rapid temporal variability of the RM and its different statistical characteristics in both frequency bands, these results indicate that the Faraday screen in Sgr A* is largely internal, with about half of the Faraday rotation taking place inside the inner 10 gravitational radii, contrary to the common external Faraday screen assumption. We then demonstrate how this observation can be reconciled with theoretical models of radiatively inefficient accretion flows for a reasonable set of physical parameters. Comparisons with numerical general relativistic magnetohydrodynamic simulations suggest that the innermost part of the accretion flow in Sgr A* is much less variable than these models predict; in particular, the observed magnetic field structure appears to be coherent and persistent.

New Insights Into the Role of Atmospheric Transport and Mixing on Column and Surface Concentrations of NO2 at a Coastal Urban Site

AbstractWe use a multi‐year record of Pandora‐derived NO2 total column abundance in Boston to examine the influence of atmospheric transport on column NO2 and its surface concentrations during the warm season in a coastal urban environment. We derive tropospheric NO2 estimates from the total column with a measurement‐model fusion approach using near‐real‐time estimates of stratospheric NO2 from NASA's Goddard Earth Observing System Composition Forecast model system and find the average influence of stratospheric NO2 at this urban site can be 30%–70% depending on season and time of day. Sea breeze days tend to exhibit rapid temporal variability in the column that which can go in the opposite direction of changes in surface NO2 concentrations. By comparing tropospheric NO2 with surface concentrations, we constrain the role of boundary layer entrainment processes in the evolution of surface NO2 concentrations, while highlighting the value of column measurements in identifying sea breeze frontal dynamics. We estimate an apparent equal mixing layer height of NO2 and infer that surface NOx emissions remain concentrated near the surface regardless of atmospheric stability regime. When comparing the Pandora‐ to TROPOMI‐derived column NO2 measurements, we find that sea breeze days present a unique challenge likely due to higher spatial heterogeneity in NO2 and the meteorology involved that is not well represented in retrieval inputs. Our observations provide new insights into column and surface variability of NO2 which will be relevant to interpreting geostationary observations, especially in coastal urban locations.

High-temporal-resolution of lithium isotopes in Yangtze River headwater: Hydrological control on weathering in high-relief catchments

How climate change regulates silicate weathering in tectonically active areas remains clear. To evaluate the roles of temperature and hydrology in continental-scale silicate weathering in high-relief catchments, we applied a high temporal resolution of lithium isotopes in the Yalong River, which drains the high-relief borders of the eastern Tibetan Plateau. The dissolved δ7Li values range from +12.2‰ to +13.7‰ in the non-monsoon season and are higher and significantly vary from +13.5‰ to +19.4‰ in the monsoon season. The negative correlation between dissolved δ7Li and the Li/Na ratio is attributed to the formation of various proportions of δ7Li-low secondary minerals during weathering. From non-monsoon to monsoon season, the weathering intensity decreases with increasing secondary minerals formation and the weathering transforms from a supply limited to a kinetically limited weathering regime, indicated by a negative correlation between dissolved δ7Li value and SWR/D ratio (SWR = silicate weathering rate and D = total denudation rate). No correlations between temperature and dissolved δ7Li values were observed, and SWR suggested that temperature is not the direct control factor of silicate weathering in high-relief areas. The dissolved δ7Li values display positive correlations with discharge, physical erosion rates (PERs), and SWR. This positive correlations was attributed to an increase in the PER which caused the formation of more secondary minerals with increasing discharge. These results indicate the rapid temporal variability of riverine Li isotopes and chemical weathering process in response to changes in hydrology rather than temperature. Combined with the compiled PER, SWR, and Li isotopes at various altitudes, we further suggest that weathering in high-altitude catchments is more sensitive to hydrological changes than weathering in low-altitude catchments. These results highlight the key role of the hydrologic cycle (runoff and discharge) and the geomorphic regime in controlling global silicate weathering.

The star-melt python package for emission-line analysis of YSOs

ABSTRACT We introduce the star-melt python package that we developed to facilitate the analysis of time-resolved emission-line spectroscopy of young stellar objects. star-melt automatically extracts, identifies, and fits emission lines. We summarize our analysis methods that utilizes the time domain of high-resolution stellar spectra to investigate variability in the line profiles and corresponding emitting regions. This allows us to probe the innermost disc and accretion structures of YSOs. Local temperatures and densities can be determined using Boltzmann statistics, the Saha equation, and the Sobolev large velocity gradient approximation. star-melt allows for new results to be obtained from archival data, as well as facilitating timely analysis of new data as it is obtained. We present the results of applying star-melt to three YSOs, using spectra from UVES, XSHOOTER, FEROS, HARPS, and ESPaDOnS. We demonstrate what can be achieved for data with disparate time sampling, for stars with different inclinations and variability types. For EX Lupi, we confirm the presence of a localized and stable stellar-surface hotspot associated with the footprint of the accretion column. For GQ Lupi A, we find that the maximum infall rate from an accretion column is correlated with lines produced in the lowest temperatures. For CVSO109. we investigate the rapid temporal variability of a redshifted emission wing, indicative of rotating and infalling material in the inner disc. Our results show that star-melt is a useful tool for such analysis, as well as other applications for emission lines.

Evidence of an Internal Dissipation Origin for the High-energy Prompt Emission of GRB 170214A

Abstract The origin of the prompt high-energy ( MeV) emission of gamma-ray bursts (GRBs), detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, for which both an external shock origin and internal dissipation origin have been suggested, is still under debate. In the internal dissipation scenario, the high-energy emission is expected to exhibit significant temporal variability, tracking the keV/MeV fast variable behavior. Here, we report a detailed analysis of the Fermi data of GRB 170214A, which is sufficiently bright in high energies to enable a quantitative analysis of the correlation between high-energy emission and keV/MeV emission with high statistics. Our result shows a clear temporal correlation between high-energy and keV/MeV emission in the whole prompt emission phase as well as in two decomposed short time intervals. Such a correlation is also found in some other bright LAT GRBs, e.g., GRB 080916C, 090902B, and 090926A. For these GRBs as well as GRB 090510, we also find rapid temporal variability in the high-energy emission. We thus conclude that the prompt high-energy emission in these bright LAT GRBs should be due to an internal origin.

REVISITING THE LIGHT CURVES OF GAMMA-RAY BURSTS IN THE RELATIVISTIC TURBULENCE MODEL

Rapid temporal variability has been widely observed in the light curves of gamma-ray bursts (GRBs). One possible mechanism for such variability is related to the relativistic eddies in the jet. In this paper, we include the contribution of the inter-eddy medium together with the eddies to the gamma-ray emission. We show that the gamma-ray emission can either lead or lag behind the observed synchrotron emission, where the latter originates in the inter-eddy medium and provides most of seed photons for producing gamma-ray emission through the inverse-Compton scattering. As a consequence, we argue that the lead/lag found in non-stationary short-lived light curves may not reveal the intrinsic lead/lag of different emission components. In addition, our results may explain the lead of gamma-ray emission with respect to optical emission observed in GRB 080319B.

Upper limits for H 2SO 4 in the mesosphere of Venus

Rapid temporal variability of SO 2 and SO in the Venus 85–100 km mesosphere ( Sandor, B.J., Clancy, R.T., Moriarty-Schieven G.H. [2007]. Bull. Am. Astron. Soc. 39, 503; Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60) requires in situ sources and sinks for these molecules. While many loss mechanisms are recognized, no process for in situ production is known. Observational investigations to find, or constrain other potential sulfur reservoirs offer one method toward understanding the applicable photochemistry. Here, we report upper limits for gas-phase H 2SO 4 (sulfuric acid) abundances in Venus’ 85–100 km upper mesosphere, derived from 16 ground-based sub-mm spectroscopic observations in the period 2004–2008. Unlike the ubiquitous sulfuric acid solid/liquid aerosol, the gas phase would be photochemically active, potentially both source and sink for SO and SO 2. H 2SO 4 is retrieved from sub-mm lines located in the same bandpass as the SO 2 and SO lines described by Sandor et al. (Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60). H 2SO 4 upper limits reported here are thus simultaneous and spatially coincident with measurements of SO 2 and SO, providing for analysis of the three sulfur species collectively. The average H 2SO 4 abundance over 16 observations is 1 ± 2 ppb (i.e. <3 ppb). Upper limits for individual observations range from 3 to 44 ppb, where quality of the observing weather is the dominant constraint on measurement precision. The sum of H 2SO 4, SO 2 and SO varies widely. In one comparison, the sum [H 2SO 4 + SO 2 + SO] measured on one date differs by 10- σ from the sum measured 2 months later. We conclude that upper mesospheric sulfur atoms are not conserved among the three molecules, that H 2SO 4 is not a significant sulfur reservoir for balancing the observed variations of [SO 2 + SO], and is not relevant to the (still unknown) photochemistry responsible for observed behavior of SO 2 and SO. Having ruled out H 2SO 4, we infer that elemental sulfur is the most probable candidate for the needed third reservoir.

A turbulent model of gamma-ray burst variability

Abstract A popular paradigm to explain the rapid temporal variability observed in gamma-ray burst (GRB) light curves is the internal shock model. We propose an alternative model in which the radiating fluid in the GRB shell is relativistically turbulent with a typical eddy Lorentz factor γt. In this model, all pulses in the gamma-ray light curve are produced at roughly the same distance R from the centre of the explosion. The burst duration is ∼R/cΓ2, where Γ is the bulk Lorentz factor of the expanding shell, and the duration of individual pulses in the light curve is ∼R/cΓ2γ2t. The model naturally produces highly variable light curves with ∼γ2t individual pulses. Even though the model assumes highly inhomogeneous conditions, nevertheless the efficiency for converting jet energy to radiation is high.

Pre-maximum spectro-imaging of the Mira star T Leporis with AMBER/VLTI

Context. Diffuse envelopes around Mira variables are among the most important sources influencing the chemical evolution of galaxies. However they represent an observational challenge because of their complex spectral features and their rapid temporal variability. Aims. We aimed at constraining the exact brightness distribution of the Mira star T Lep with a model-independent analysis. Methods. We obtained single-epoch interferometric observations with a dataset continuous in the spectral domain (λ = 1.5−2.4 μm) and in the spatial domain (interferometric baselines ranging from 11 to 96 m). We performed a model independent image reconstruction for each spectral bin using the MIRA software. We completed the analysis by modeling the data with a simple star+layer model inspired from the images. Results. Reconstructed images confirm the general picture of a central star partially obscured by the surrounding molecular shell of changing opacity. At 1.7 μm, the shell becomes optically thin, with corresponding emission appearing as a ring circling the star. This is the first direct evidence of the spherical morphology of the molecular shell. Model fitting confirmed a spherical layer of constant size and changing opacity over the wavelengths. Rough modeling points to a continuum opacity within the shell, in addition to the CO and H2O features. Accordingly, it appeared impossible to model the data by a photosphere alone in any of the spectral bins.

Falling atmospheric pressure as a trigger for methane ebullition from peatland

Peatlands are widely regarded as a significant source of atmospheric CH4, a potent greenhouse gas. At present, most of the information on environmental emissions of CH4 comes from infrequent, temporally discontinuous ground‐based flux measurements. Enormous efforts have been made to extrapolate measured emission rates to establish seasonal or annual averages using relevant biogeochemical factors, such as water table positions or peat temperatures, by assuming that the flux was stationary during a substantial nonsampling period. However, this assumption has not been explicitly verified, and little is known about the continuous variation of the CH4 flux in a timescale of individual flux measurement. In this study, we show an abrupt change in the CH4 emission rate associated with falling atmospheric pressure. We found that the CH4 flux can change by 2 orders of magnitude within a matter of tens of minutes owing to the release of free‐phase CH4 triggered by a drop in air pressure. The contribution of the ebullition to the total CH4 flux during the measurements was significant (50–64%). These results clearly indicated that field campaigns must be designed to cover this rapid temporal variability caused by ebullition, which may be especially important in intemperate weather. Process‐based CH4 emission models should also be modified to include air pressure as a key factor for the control of ebullient CH4 release from peatland.

Mobile Monitoring of Particulate Black Carbon Concentrations in an Urban Area

P-653 Introduction: Interest in chronic exposure to air pollution has led to an increased focus on the spatial distribution of traffic related air pollutants within urban environments. Measuring this distribution has usually involved using numerous fixed-site passive NO2/NOx badges simultaneously deployed for a week or more. Here we describe a relatively simple mobile monitoring method for particulate black carbon intended to complement the fixed-site NOx sampler approach. Methods: A particle soot absorption photometer (psap) and accompanying pump were placed inside a conventional gasoline powered vehicle and powered with a marine battery connected through an inverter. The instrument sampled air from the driver side rear window and the readings were recorded each second with a serial datalogger. The vehicle was driven continuously along one of six routes. These routes were driven during peak afternoon traffic periods (∼4 to 7 PM) between July and August, 2005 and were chosen to include 36 roadway intersections previously sampled for NO2 and NOx using a simultaneous, fixed-site sampler array. Initial attempts at sampling during morning rush hour resulted in less stable measurements due to rapid temporal variability in the mixing height. One centrally located intersection was visited during each afternoon mobile sampling period. From 5 to 8 additional locations could also be sampled during any given sampling period. The vehicle traced a cloverleaf pattern at each intersection, circling the four adjacent blocks surrounding the intersection and thus passing through the actual intersection four times. The average psap readings taken during this cloverleaf traverse were used in subsequent analyses. The ratio of the average reading at each site to that at the central site was computed for each run and then averaged across all sample periods. These average site ratios were then compared with the 5 day average NOx concentraions measured previously at 36 separate locations. Results: We found a high correlation between the mobile Black Carbon and fixed site NOx measurements (r = 0.77). Discussion and Conclusions: We were able to identify the relative impact of traffic emissions at numerous locations using a simple mobile sampling strategy. The strong correlation between this method and the passive sampling network data is encouraging, given that we only sampled at any given intersection for about 10 minutes, that we were measuring black carbon rather than NOx, and that NOx and black carbon measurements were not made simultaneously nor adjusted for seasonal variability.

Rapid Temporal Variability of Faculae: High‐Resolution Observations and Modeling

We present high-resolution G-band observations (obtained with the Swedish 1 m Solar Telescope) of the rapid temporal variability of faculae, which occurs on granular timescales. By combining these observations with magnetoconvection simulations of a plage region, we show that much of this variability is not intrinsic to the magnetic field concentrations that are associated with faculae, but rather a phenomenon associated with the normal evolution and splitting of granules. We also show examples of facular variability caused by changes in the magnetic field, with movies of dynamic behavior of the striations that dominate much of the facular appearance at 01 resolution. Examples of these dynamics include merging, splitting, rapid motion, apparent fluting, and possibly swaying.

Rapid variability, dying pulse trains and black holes

After reviewing the general model and arguments by which rapid temporal variability and quasi-periodicities are considered indicative of a compact source's possible black hole character, the paper presents a scenario for 'dying pulse trains'. These originate inside the inner edge of accretion disks encircling black holes from accreting flares or other self-luminous entities executing their final few revolutions before reaching the event horizon. Confirmed detection of such phenomena with time scales in the range 0.01 (M/solar mass) to 0.5 (M/solar mass)ms, where M is the mass of the compact source, would provide much better support for its black hole candidacy. Variability on time-scales larger than this by itself places few constraints on the nature of the compact object.