Spectral Break Frequency Research Articles

Relating the Solar Wind Turbulence Spectral Break at the Dissipation Range with an Upstream Spectral Bump at Planetary Bow Shocks

Abstract At scales much larger than the ion inertial scale and the gyroradius of thermal protons, the magnetohydrodynamic (MHD) theory is well equipped to describe the nature of solar wind turbulence. The turbulent spectrum itself is defined by a power law manifesting the energy cascading process. A break in the turbulence spectrum develops near-ion scales, signaling the onset of energy dissipation. The exact mechanism for the spectral break is still a matter of debate. In this work, we use the 20 Hz Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) magnetic field data during four planetary flybys at different heliocentric distances to examine the nature of the spectral break in the solar wind. We relate the spectral break frequencies of the solar wind MHD turbulence, found in the range of 0.3–0.7 Hz, with the well-known characteristic spectral bump at frequencies ∼1 Hz upstream of planetary bow shocks. Spectral breaks and spectral bumps during three planetary flybys are identified from the MESSENGER observations, with heliocentric distances in the range of 0.3–0.7 au. The MESSENGER observations are complemented by one Magnetospheric Multiscale observation made at 1 au. We find that the ratio of the spectral bump frequency to the spectral break frequency appears to be r- and B-independent. From this, we postulate that the wavenumber of the spectral break and the frequency of the spectral bump have the same dependence on the magnetic field strength ∣B∣. The implication of our work on the nature of the break scale is discussed.

Cosmology with powerful radio-loud AGNs

Immensely bright quasars and radio-loud active galactic nuclei (AGNs) provide an enticing opportunity to construct standard candles detectable up to the very early universe. An analytic theory is proposed to measure the distance to powerful \citeauthor{FR+1974} type-II radio sources based on their integrated flux density across a broad range of radio frequencies, and the angular size and axis ratio of their synchrotron-emitting lobes. This technique can be used at low-redshift to construct absolute standard candles in conjunction with X-ray observations of the host cluster, or at high-redshift to measure the relative distances of objects and constrain the curvature of our universe. Distances calculated with this method are consistent for dissimilar objects at the same redshift; the two lobes of Cygnus A have flux densities, linear sizes and spectral break frequencies varying by between 15-35\% yet their fitted distances are the same to within 7\%. These distance estimates together yield a transverse comoving distance to Cygnus A of $261_{-55}^{+70}\rm\, Mpc$ corresponding to a Hubble constant of $H_0 = 64_{-13}^{+17}\rm\, km\, s^{-1}\, Mpc^{-1}$. Large samples of suitable FR-II sources could provide a measure of the Hubble constant independent of existing techniques such as the cosmic microwave background, baryon acoustic oscillations, and type 1a supernovae.

SUB-mm JET PROPERTIES OF THE X-RAY BINARY SWIFT J1745–26

We present the results of our observations of the early stages of the 2012--2013 outburst of the transient black hole X-ray binary (BHXRB), Swift J1745$-$26, with the VLA, SMA, and JCMT (SCUBA--2). Our data mark the first multiple-band mm & sub-mm observations of a BHXRB. During our observations the system was in the hard accretion state producing a steady, compact jet. The unique combination of radio and mm/sub-mm data allows us to directly measure the spectral indices in and between the radio and mm/sub-mm regimes, including the first mm/sub-mm spectral index measured for a BHXRB. Spectral fitting revealed that both the mm (230 GHz) and sub-mm (350 GHz) measurements are consistent with extrapolations of an inverted power-law from contemporaneous radio data (1--30 GHz). This indicates that, as standard jet models predict, a power-law extending up to mm/sub-mm frequencies can adequately describe the spectrum, and suggests that the mechanism driving spectral inversion could be responsible for the high mm/sub-mm fluxes (compared to radio fluxes) observed in outbursting BHXRBs. While this power-law is also consistent with contemporaneous optical data, the optical data could arise from either jet emission with a jet spectral break frequency of $\nu_{{\rm break}}\gtrsim1\times10^{14}\,{\rm Hz}$ or the combination of jet emission with a lower jet spectral break frequency of $\nu_{{\rm break}}\gtrsim2\times10^{11}\,{\rm Hz}$ and accretion disc emission. Our analysis solidifies the importance of the mm/sub-mm regime in bridging the crucial gap between radio and IR frequencies in the jet spectrum, and justifies the need to explore this regime further.

GAMMA-RAY BURST AFTERGLOW LIGHT CURVES FROM A LORENTZ-BOOSTED SIMULATION FRAME AND THE SHAPE OF THE JET BREAK

GRB afterglow jets have been notoriously difficult to resolve numerically using 2D hydrodynamical simulations due to high outflow Lorentz factors. By performing simulations in a boosted frame, it is possible to calculate light curves from numerically computed flows in sufficient detail to accurately quantify the shape of the jet break and the post-break steepening of the light curve. We study jet breaks for jets with opening angles of theta_0 = 0.05, 0.1 and 0.2 radians decelerating in a constant density medium, observed at angles (theta_obs) ranging from on-axis to the jet edge. We present a single set of scale-invariant functions describing the time evolution of synchrotron spectral break frequencies and peak flux, depending only on theta_0 and theta_obs, sufficient to reconstruct light curves for arbitrary explosion energy, circumburst density and synchrotron slope p. We compare our light curves to fit functions using sharp breaks and smooth transitions and confirm that the measured jet break time can be delayed due to theta_obs. We find that the difference in temporal indices across the jet break is larger than theoretically anticipated leading to post-break slopes of roughly 0.25 - 1.3 p, sufficiently steep to not easily reconcile with post-break slopes measured for the Swift sample, suggesting that most Swift GRBs do not explode in homogeneous media or that jet breaks are hidden by additional physics such as prolonged energy injection or viewing angle effects. Light curves and spectral break and peak flux evolution functions will be made publicly available on-line at http://cosmo.nyu.edu/afterglowlibrary .

ON SPECTRAL BREAKS IN THE POWER SPECTRA OF MAGNETIC FLUCTUATIONS IN FAST SOLAR WIND BETWEEN 0.3 AND 0.9 AU

We analyze the radial variation of the power spectra of the magnetic field from 0.3 to about 0.9?AU, using Helios 2 spacecraft measurements in the fast solar wind. The time resolution of the magnetic field data allows us to study the power spectra up to 2?Hz. Generally, the corresponding spectral break frequency fb and the Doppler-shifted frequencies, which are related to the proton gyroradius and inertial scales, are close to a frequency f of about 0.5?Hz at a distance of 1?AU from the Sun. However, studying the radial evolution of the power spectra offers us the possibility to distinguish between those scales. Recent Ulysses observations show that, while the proton scales vary, fb stays nearly constant with the heliocentric distance R. In our study we confirm that fb varies within a small interval of [0.2, 0.4]?Hz only, as R varies from 0.3 to 0.9?AU. Moreover, if we assume parallel propagating fluctuations (with respect to the solar wind flow or background magnetic field), we can show that none of the proton scales are coincident with the break scale. If, however, we take into account the two-dimensional nature of the turbulent fluctuations, then we can show that the spatial scale corresponding to fb (R) does follow the proton inertial scale, ? p (R), but not the proton gyroradius scale, ? p (R), as a function of heliocentric distance. These observations indicate that the spectral break at the proton inertial scale might be related to the Hall effect, or be controlled by the ion-cyclotron damping of obliquely propagating fluctuations or the formation of current sheets scaling like ? p , which could be responsible for ion heating through magnetic reconnection.

USING RADIO HALOS AND MINIHALOS TO MEASURE THE DISTRIBUTIONS OF MAGNETIC FIELDS AND COSMIC RAYS IN GALAXY CLUSTERS

Some galaxy clusters show diffuse radio emission in the form of giant halos (GHs) on Mpc scales or minihalos (MHs) on smaller scales. Comparing VLA and XMM radial profiles of several such clusters, we find a universal linear correlation between radio and X-ray surface brightness, valid in both types of halos. It implies a halo central emissivity nu*j_nu=10^{-31.4+-0.2}*(n/10^-2cm^-3)^2*(T/T_0)^{0.2+-0.5} erg/s/cm^3, where T and T_0 are the local and central temperatures, and n is the electron number density. We argue that the tight correlation and the scaling of j_nu, combined with morphological and spectral evidence, indicate that both GHs and MHs arise from secondary electrons and positrons, produced in cosmic-ray ion (CRI) collisions with a strongly magnetized, B >3 muG intracluster gas. When the magnetic energy density drops below that of the microwave background, the radio emission weakens considerably, producing halos with a clumpy morphology (e.g. RXCJ2003.5-2323 and A2255) or a distinct radial break. We thus measure a magnetic field B=3 muG at a radius r~110 kpc in A2029 and r~50 kpc in Perseus. The spectrum of secondaries, produced from hadronic collisions of ~20 GeV CRIs, reflects the energy dependence of the collision cross section. We use the observed spectra of halos, in particular where they steepen with increasing radius or frequency, to (i) measure B~10(nu/700 MHz) muG, with nu the spectral break frequency; (ii) identify a correlation between the average spectrum and the central magnetic field; and (iii) infer a CRI spectral index s <-2.7 and energy fraction xi_p~10^{-3.6+-0.2} at particle energies above 10 GeV. Our results favor a model where CRIs diffuse away from their sources (which are probably supernovae, according to a preliminary correlation with star formation), whereas the magnetic fields are generated by mergers in GHs and by core sloshing in MHs.

Characteristics of magnetic fluctuations within coronal mass ejections: The January 1997 event

We determine the geometry of the fluctuations of the magnetic field at frequencies just above the proton gyrofrequency for the January 10, 1997 CME and the magnetic cloud within. The transverse magnetic fluctuations represent a greater fraction of the magnetic energy than is the case in the typical undisturbed solar wind. The break in the power spectrum that is associated with the the onset of magnetic dissipation falls within the frequency range of interest. The fluctuation geometry is markedly different above and below the spectral break frequency. The inertial range geometry remains unchanged in the cloud with only ∼30% of the energy associated with field‐aligned wave vectors. The dissipation range wave vectors are highly oblique to the mean magnetic field B with up to 96% of the energy associated with oblique wave vectors.

TheEXOSATData on GX 339−4: Further Evidence for an “Intermediate” State

We have studied the fast timing and spectral behavior of the black hole candidate (BHC) GX 339-4 using all 1 - 20 keV EXOSAT ME data: the July 1983, March 1984, May 1984 and April 1985 observations. In April 1985 GX 339-4 was in a weak low state (0.03E-9 erg cm^{-2} s^{-1}, 2 - 10 keV). The X-ray spectrum was a hard power law with a photon index \alpha of 1.82 and the power spectrum, though ill-constrained by the data, was consistent with a typical BHC low state spectrum. During the other three pointings the system was brighter (1.5E-9 erg cm^{-2} s^{-1}, 2 - 10 keV), an ultrasoft component was present in the spectrum, and the power law was steeper (\alpha ~3.5 in 1984; in 1983 it was not well-measured); the power spectrum showed a flat-topped band-limited noise component with a break frequency of ~4.5 Hz and a fractional rms amplitude of ~7%. Comparing flux levels, X-ray spectra and power spectra we conclude that in these observations GX 339-4 was in a state intermediate between the usual BHC low and high states. The system was ~4 times brighter than in its usual low state, 4 times fainter than in its usual high state, and an order of magnitude fainter than in its very high state. We compare these results to those recently obtained on other BHCs, and conclude that this ``intermediate state'' behavior is a common characteristic of BHCs, that occurs at $\dot M$ levels intermediate between the high and the low state. We argue that this result can be used to resolve the long-standing issue of the dependence of the power spectral break frequency in the low state on mass accretion rate, and strengthens the idea that low-state noise and very-high state noise may have a common origin. We briefly discuss a possible interpretation for the changes in break frequency in the low state and between low state and intermediate state.

Multifrequency radio observations of Cygnus A - Spectral aging in powerful radio galaxies

A detailed analysis of the radio spectrum across the lobes of Cygnus A is presented in order to critically test the synchroton spectral aging theory. The results are in good agreement with the jet model for powerful radio galaxies, involving particle acceleration at the hot spots and outflow into the radio lobes, with subsequent energy loss due to synchrotron radiation. The hot spot spectra are well represented by a spectral aging model involving continuous injection of relativistic particles. Both hot spots have spectral break frequencies around 10 GHz. An injection index of 0.5 is found for both hot spots, consistent with diffusive shock acceleration at a strong nonrelativistic shock in a Newtonian fluid. The LF hot spot emission spectrum falls below the injected power law. This effect is isolated to the hot spots, and is best explained by a low-energy cutoff in the particle distribution.

The jet of PKS 0521 - 36 - an aging counterpart of M87?

view Abstract Citations (60) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Jet of PKS 0521-36: an Aging Counterpart of M87? Keel, W. C. Abstract Imaging photometry and VLA mapping at 2 and 6 cm have been used to study the structure and spectral energy distribution of the jet in PKS 0521 - 36. The jet exhibits a strong knot (1.6 kpc from the nucleus) which is resolved at 2 cm and elongated perpendicular to the jet, in a manner suggesting an internal shock. The optical continuum has a spectral index of -2.0 + or - 0.3, with a mean radio-to-optical index of 0.76; this steepens by 0.07 between the inner and outer parts of the jet. A spectral break frequency about 3 times lower than what is seen in M87 is inferred. The radio data suggest that this difference is mostly due to a lower average magnetic field in PKS 0521 - 36 and that the electron energy distributions in both objects extend to gamma about 10 to the 6th. Galactic environments and jet morphologies are compared for the two galaxies. Publication: The Astrophysical Journal Pub Date: March 1986 DOI: 10.1086/163991 Bibcode: 1986ApJ...302..296K Keywords: Astronomical Spectroscopy; Energy Spectra; Radio Galaxies; Radio Jets (Astronomy); Charge Coupled Devices; Elliptical Galaxies; Galactic Radio Waves; H Alpha Line; Red Shift; Spectral Line Width; Astrophysics; GALAXIES: INDIVIDUAL ALPHANUMERIC: PKS 0521-36; GALAXIES: JETS; INTERFEROMETRY; RADIO SOURCES: GALAXIES full text sources ADS | data products NED (3) SIMBAD (2)