Evolution Of Spectral Index Research Articles

Localising pulsations in the hard X-ray and microwave emission of an X-class flare

Aims. The aim of this work is to identify the mechanism driving pulsations in hard X-ray (HXR) and microwave emission during solar flares. Using combined HXR and microwave observations from Solar Orbiter/STIX and EOVSA, we investigate an X1.3 GOES class flare, 2022-03-30T17:21:00, which displays pulsations on timescales evolving from ∼7 s in the impulsive phase to ∼35 s later in the flare. Methods. We analysed the temporal, spatial, and spectral evolution of the HXR and microwave pulsations during the impulsive phase of the flare. We reconstructed images for individual peaks in the impulsive phase and performed spectral fitting at high cadence throughout the first phase of pulsations. Results. Our imaging analysis demonstrates that the HXR and microwave emission originates from multiple sites along the flare ribbons. The brightest sources and the location of the emission change in time. Through HXR spectral analysis, the electron spectral index is found to be anti-correlated with the HXR flux, showing a “soft-hard-soft” spectral index evolution for each pulsation. The timing of the associated filament eruption coincides with the early impulsive phase. Conclusions. Our results indicate that periodic acceleration and/or injection of electrons from multiple sites along the flare arcade is responsible for the pulsations observed in HXR and microwave emission. The evolution of pulsation timescales is likely a result of changes in the 3D magnetic field configuration over time related to the associated filament eruption.

A Small-scale Structure Model of a Jet Based on Observations of Microvariability

We developed a multiregion radiation model for the evolution of flux and spectral index with time. In this model, each perturbation component in the jet produces an independent flare. The model can be used to study the decomposition of microvariability, the structural scale of the perturbed components, and the physical parameters of the acceleration processes. Based on the shock acceleration model for a relativistic jet, the influence of the acceleration parameters on multiband flare parameters is calculated. We present the results of multiband optical microvariability of the blazar BL Lacertae observed during 89 nights in the period from 2009 to 2021, and use them as a sample for model fitting. The results show that both the amplitude and duration of flares decomposed from the microvariability light curves conform to a log-normal distribution. The time delays between the optical bands follow a normal distribution and amount to several minutes, which corroborate with both predictions from the theoretical model and the calculation of the discrete correlation function. Using the spectral index evolution and the simultaneous fitting of the multiband variability curves, we obtain the acceleration and radiation parameters to constrain and distinguish the origins of different flares. Based on the flare decomposition, we can effectively reproduce the time-domain evolution trends of the optical variations and energy spectrum, and explain the various redder-when-brighter and bluer-when-brighter behaviors.

Radio QPO in the γ-ray-loud X-ray binary LS I +61°303

Abstract LS I +61°303 is a γ-ray emitting X-ray binary with periodic radio outbursts with time-scales of one month. Previous observations have revealed microflares superimposed on these large outbursts with periods ranging from a few minutes to hours. This makes LS I +61°303, along with Cyg X-1, the only TeV emitting X-ray binary exhibiting radio microflares. To further investigate this microflaring activity in LS I +61°303 we observed the source with the 100-m Effelsberg radio telescope at 4.85, 8.35 and 10.45 GHz, and performed a timing analysis on the obtained data. Radio oscillations of 15 h time-scales are detected at all three frequencies. We also compare the spectral index evolution of radio data to that of the photon index of GeV data observed by Fermi-Large Area Telescope. We conclude that the observed Quasi Periodic Oscillations could result from multiple shocks in a jet.

Steep Decay Phase Shaped by the Curvature Effect. II. Spectral Evolution

Abstract We derive a simple analytical formula to describe the evolution of spectral index β in the steep decay phase shaped by the curvature effect with the assumption that the spectral parameters and Lorentz factor of the jet shell are the same for different latitudes. Here, the value of β is estimated in the 0.3−10 keV energy band. For a spherical thin shell with a cutoff power-law (CPL) intrinsic radiation spectrum, the spectral evolution can be read as a linear function of observer time. For the situation with the Band function intrinsic radiation spectrum, the spectral evolution may be complex. If the observed break energy of the radiation spectrum is larger than 10 keV, the spectral evolution is the same as that shaped by jet shells with a CPL spectrum. If the observed break energy is less than 0.3 keV, the value of β would be a constant. For others, the spectral evolution can be approximated as a logarithmal function of the observer time in general.

THE HISTORY OF GRB OUTFLOWS: EJECTION LORENTZ FACTOR AND RADIATION LOCATION OF X-RAY FLARES

ABSTRACT We present time-resolved spectral analysis of the steep decay segments of 29 bright X-ray flares of gamma-ray bursts (GRBs) observed with the Swift/X-ray telescope, and model their light curves and spectral index evolution behaviors with the curvature effect model. Our results show that the observed rapid flux decay and strong spectral index evolution with time can be well fitted with this model, and the derived characteristic timescales (t c ) are in the range of 23 ∼ 264 s. Using an empirical relation between the peak luminosity and the Lorentz factor derived from the prompt gamma-rays, we estimate the Lorentz factors of the flares (ΓX). We obtain ΓX = 17 ∼ 87 with a median value of 52, which is smaller than the initial Lorentz factors of prompt gamma-ray fireballs. With the derived t c and ΓX, we constrain the radiating regions of 13 X-ray flares, yielding R X = (0.2 ∼ 1.1) × 1016 cm, which are smaller than the radii of the afterglow fireballs at the peak times of the flares. A long evolution feature from prompt gamma-ray phase to the X-ray epoch is found by incorporating our results with a sample of GRBs whose initial Lorentz factors are available in the literature, i.e., . These results may shed light on the long-term evolution of GRB central engines.

Evolution of spectral index of energetic protons in the magnetopause crossing at the subsolar point

The evolution of the spectral index of the omnidirectional differential flux of protons (within a range of 40 to 600 keV) in the magnetopause crossing near the subsolar point was analyzed. The work is based on the measurement data of the THEMIS international project as of July 18, 2007. A specific feature of the event under study is the possibility of determining the spectral index at energies >40 keV with a time resolution of 3 s. It is shown that the ion distribution functions, both outside and inside the magnetopause, can be approximated by kappa-like distributions with power low high-energy tail in the analyzed crossing. A high level of fluctuations of the spectral index of energetic ions was shown to have been observed near the magnetopause. The fluctuation level substantially reduces inside the magnetosphere. In this case, the spectral index has a value of ~6 on average and remains constant up to inner regions of the magnetosphere.

Evolution of the spectral index after inflation

In this article we investigate the time evolution of the adiabatic (curvature) and isocurvature (entropy) spectral indices after inflation era for all cosmological scales with two different initial conditions. For this purpose, we first extract an explicit equation for the time evolution of the comoving curvature perturbation (which may be known as the generalized Mukhanov-Sasaki equation). It would be cleared that the evolution of adiabatic spectral index severely depends on the initial conditions moreover, as expected it is constant only for the super-Hubble scales and adiabatic initial conditions. Additionally, the adiabatic spectral index after recombination approaches a constant value for the isocurvature perturbations. Finally, we re-investigate the Sachs-Wolfe effect and show that the fudge factor 1/3 in the adiabatic ordinary Sachs-Wolfe formula must be replaced by 0.4.

Evolution of the far-infrared-radio correlation and infrared spectral energy distributions of massive galaxies over z= 0-2

We investigate the far-infrared-radio correlation (FRC) of stellar-mass-selected galaxies in the Extended Chandra Deep Field South using far-infrared imaging from Spitzer and radio imaging from the Very Large Array and Giant Metre-Wave Radio Telescope. We stack in redshift bins to probe galaxies below the noise and confusion limits. Radio fluxes are K-corrected using observed flux ratios, leading to tentative evidence for an evolution in spectral index. We compare spectral energy distribution (SED) templates of local galaxies for K-correcting FIR fluxes, and show that the data are best fit by a quiescent spiral template (M51) rather than a warm starburst (M82) or ULIRG (Arp220), implying a predominance of cold dust in massive galaxies at high redshift. In contrast we measure total infrared luminosities that are consistent with high star-formation rates. We observe that the FRC index (q) does not evolve significantly over z=0-2 when computed from K-corrected 24 or 160-mum photometry, but that using 70-mum fluxes leads to an apparent decline in q beyond z~1. This suggests some change in the SED at high redshift, either a steepening of the spectrum at rest-frame ~25-35mum or a deficiency at ~70mum leading to a drop in the total infrared/radio ratios. We compare our results to other work in the literature and find synergies with recent findings on the high-redshift FRC, high specific star-formation rates of massive galaxies and the cold dust temperatures in these galaxies.

Long‐Term Radio Monitoring of SN 1993J

We present our observations of the radio emission from supernova (SN) 1993J, in M 81 (NGC 3031), made with the VLA, from 90 to 0.7 cm, as well as numerous measurements from other telescopes. The combined data set constitutes probably the most detailed set of measurements ever established for any SN outside of the Local Group in any wavelength range. Only SN 1987A in the LMC has been the subject of such an intensive observational program. The radio emission evolves regularly in both time and frequency, and the usual interpretation in terms of shock interaction with a circumstellar medium (CSM) formed by a pre-SN stellar wind describes the observations rather well considering the complexity of the phenomenon. However: 1) The 85 - 110 GHz measurements at early times are not well fitted by the parameterization, unlike the cm wavelength measurements. 2) At mid-cm wavelengths there is some deviation from the fitted radio light curves. 3) At a time ~3100 days after shock breakout, the decline rate of the radio emission steepens without change in the spectral index. This decline is best described as an exponential decay starting at day 3100 with an e-folding time of ~1100 days. 4) The best overall fit to all of the data is a model including both non-thermal synchrotron self-absorption (SSA) and a thermal free-free absorbing (FFA) components at early times, evolving to a constant spectral index, optically thin decline rate, until the break in that decline rate. Moreover, neither a purely SSA nor a purely FFA absorbing models can provide a fit that simultaneously reproduces the light curves, the spectral index evolution, and the brightness temperature evolution. 5) The radio and X-ray light curves exhibit similar behavior and suggest a sudden drop in the SN progenitor mass-loss rate at ~8000 years prior to shock breakout.

The multi-wavelength correlations and the evolution of spectral index on the quasar 3C 273

The observed data from 2 keV to 5 GHz were complied to investigate possible correlations between different frequency variations by means of discrete correlation function (DCF) analysis. The results show that there are significant flux correlations between different radio bands and the weak correlations between radio and optical variability. The variations of higher frequency components would lead those of lower frequency components. Due to the sparse and uneven data in the X-ray band, the correlations between X-ray and the lower energy bands (radio and optical) cannot be confirmed. In addition, we give the spectral index evolution in each energy band and find out that there are strong anti-correlation between spectral index and flux, in the sense that the spectral index decreases with the flux increases.

The Evolution of Outbursts in Cygnus X-3 at 2.25 and 8.3 GHz

view Abstract Citations (60) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution of Outbursts in Cygnus X-3 at 2.25 and 8.3 GHz Waltman, E. B. ; Ghigo, F. D. ; Johnston, K. J. ; Foster, R. S. ; Fiedler, R. L. ; Spencer, J. H. Abstract Detailed observations of a long outburst in the X-ray binary Cygnus X-3 at 2.25 and 8.3 GHz in early 1994 are presented. The source has been monitored at dual radio frequencies on a daily basis with the Green Bank Interferometer since 1982. This outburst was predicted based on the behavior of the quiescent flux densities prior to the major flare. The data show that the 1994 February 20 flare occurred just one day after 19 days of very low quiescent flux densities and exhibited higher opacity at 2 GHz than usual for Cygnus X-3 flares. Later flares in the 1994 February-March outburst sequence displayed progressively less opacity as determined from the observed ratios of peak flux densities at the two frequencies, the lags between the peaks, and the spectral index. Examination of the evolution of spectral index during the 1994 February outburst, compared to previous outbursts in the history of Cygnus X-3, shows that the trend toward optically thin flares begins with the onset of outburst activity immediately following the very low quiescent flux densities and may relate to the very low flux densities. This behavior can be interpreted either as evidence for a declining opacity from thermal gas in the binary system or for evolution of the exponential lateral expansion rate of the jet during the series of flares in the outburst. Publication: The Astronomical Journal Pub Date: July 1995 DOI: 10.1086/117518 Bibcode: 1995AJ....110..290W Keywords: BINARIES: CLOSE; X-RAYS: STARS full text sources ADS |