Average Source Spectrum Research Articles

Another look at the 2009 seismic activity, Harrat Lunayyir, Saudi Arabia

This study aims to recognize the influence of source- and path-effects on seismic spectra using the waveform data from the 2009 seismic activity, Harrat Lunayyir (HL), eastern flank of the Red Sea coastline, Saudi Arabia. This seismic activity is considered as a series of continuously evolving events reaching at some stage the maximum on 19 May 2009, with the largest shock of Mw 5.4, and then decayed. A total number of 218 events of magnitudes ML ≥ 2.0 were analyzed, which include the largest shock, 101 and 116 seismic events representing the seismic events within the early and decay stages, respectively. The data analysis using Frequency-time analysis revealed diversity patterns of frequency contents along the recorded seismic stations. Moreover, the modified WADATI diagram showed VP/VS variations of 1.7 ± 0.003 and 1.72 ± 0.002 before and after the occurrence of the largest-sized event among the HL seismic activity, respectively. Using the grid-search method to recover the space parameters representing the average source spectrum of each earthquake over the recorded seismic stations, estimates of source sizes and stress drops imply variations in self-similarity scaling relationship among the HL seismic activity. According to the results obtained from the current analyses, we propose a scenario began with an emplacement of seismic activity due to the local stress accumulation imports from an upward dike injection that is blocked by cooled and hardened magmatic materials in the upper crust from the earlier intrusions. The stress accumulation continued and was then released, at some stage, by the event having the maximum magnitude followed by seismic activities released due to stress relaxations.

The NuSTAR Hard X-Ray Survey of the Norma Arm Region

Abstract We present a catalog of hard X-ray sources in a square-degree region surveyed by the Nuclear Spectroscopic Telescope Array (NuSTAR) in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and the typical and maximum exposure depths are 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of 5 × 10−14 and 4 × 10−14 erg s−1 cm−2 in the 3–10 and 10–20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected, and 10 are detected with low significance; 8 of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multiwavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of ≈10–20 keV, consistent with the Galactic ridge X-ray emission spectrum but lower than the temperatures of CVs near the Galactic center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic center. The NuSTAR logN–logS distribution in the 10–20 keV band is consistent with the distribution measured by Chandra at 2–10 keV if the average source spectrum is assumed to be a thermal model with kT ≈ 15 keV, as observed for the CV candidates.

The character of scaling earthquake source spectra for Kamchatka in the 3.5–6.5 magnitude range

The properties of the source spectra of local shallow-focus earthquakes on Kamchatka in the range of magnitudes M w = 3.5–6.5 are studied using 460 records of S-waves obtained at the PET station. The family of average source spectra is constructed; the spectra are used to study the relationship between M w and the key quasi-dimensionless source parameters: stress drop Δσ and apparent stress σa. It is found that the parameter Δσ is almost stable, while σa grows steadily as the magnitude M w increases, indicating that the similarity is violated. It is known that at sufficiently large M w the similarity hypothesis is approximately valid: both parameters Δσ and σa do not show any noticeable magnitude dependence. It has been established that M w ≈ 5.7 is the threshold value of the magnitude when the change in regimes described occurs for the conditions on Kamchatka.

Inverse problem for extragalactic transport of ultra-high energy cosmic rays

The energy spectra and composition of ultra-high energy cosmic rays are changing in a course of propagation in the expanding Universe filled with background radiation. We developed a numerical code for solution of inverse problem for cosmic-ray transport equations that allows the determination of average source spectra of different nuclei from the cosmic ray spectra observed at the Earth. Employing this approach, the injection spectra of protons and Iron nuclei in extragalactic sources are found assuming that only these species are accelerated at the source. The data from the Auger experiment and the combined data from the Telescope Array + HiRes experiments are used to illustrate the method.

MID-INFRARED SPECTRAL DIAGNOSTICS OF LUMINOUS INFRARED GALAXIES

We present a statistical analysis of the mid-infrared (MIR) spectra of 248 luminous infrared (IR) galaxies (LIRGs) which comprise the Great Observatories All-sky LIRG Survey (GOALS) observed with the Infrared Spectrograph (IRS) on-board the Spitzer Space Telescope. The GOALS sample enables a direct measurement of the relative contributions of star-formation and active galactic nuclei (AGN) to the total IR emission from a large sample of local LIRGs. The AGN contribution to the MIR emission (f-AGN) is estimated by employing several diagnostics based on the properties of the [NeV], [OIV] and [NeII] fine structure gas emission lines, the 6.2 microns PAH and the shape of the MIR continuum. We find that 18% of all LIRGs contain an AGN and that in 10% of all sources the AGN contributes more than 50% of the total IR luminosity. Summing up the total IR luminosity contributed by AGN in all our sources suggests that AGN supply ~12% of the total energy emitted by LIRGs. The average spectrum of sources with an AGN looks similar to the average spectrum of sources without an AGN, but it has lower PAH emission and a flatter MIR continuum. AGN dominated LIRGs have higher IR luminosities, warmer MIR colors and are found in interacting systems more often than pure starbursts LIRGs. However we find no linear correlations between these properties and f-AGN. We used the IRAC colors of LIRGs to confirm that finding AGN on the basis of their MIR colors may miss ~40% of AGN dominated (U)LIRGs

Observational upper limits on the gravitational wave production of core collapse supernovae

Abstract The upper limit on the energy density of a stochastic gravitational wave (GW) background obtained from the 2-yr science run (S5) of the Laser Interferometer Gravitational-Wave Observatory (LIGO) is used to constrain the average GW production of core collapse supernovae (ccSNe). We assume that the ccSNe rate tracks the star formation history of the Universe and show that the stochastic background energy density depends only weakly on the assumed average source spectrum. Using the ccSNe rate for z≤ 10, we scale the generic source spectrum to obtain an observation-based upper limit on the average GW emission. We show that the mean energy emitted in GWs can be constrained within < (0.49–1.98) M⊙c2 depending on the average source spectrum. While these results are higher than the total available gravitational energy in a core collapse event, second- and third-generation GW detectors will enable tighter constraints to be set on the GW emission from such systems.

BROADBANDSUZAKUOBSERVATIONS OF IGR J16207-5129

An analysis of IGR J16207-5129 is presented based on observations taken with Suzaku. The data set represents ~80 ks of effective exposure time in a broad energy range between 0.5 and 60 keV, including unprecedented spectral sensitivity above 15 keV. The average source spectrum is well described by an absorbed power law in which we measured a large intrinsic absorption of nH = 16.2(-1.1/+0.9)x10^22 /cm2. This confirms that IGR J16207-5129 belongs to the class of absorbed HMXBs. We were able to constrain the cutoff energy at 19(-4/+8) keV which argues in favor of a neutron star as the primary. Our observation includes an epoch in which the source count rate is compatible with no flux suggesting a possible eclipse. We discuss the nature of this source in light of these and of other recent results.

SpitzerQuasar and ULIRG Evolution Study (QUEST). I. The Origin of the Far‐Infrared Continuum of QSOs

This paper addresses the origin of the far-infrared (FIR) continuum of QSOs, based on the Quasar and ULIRG Evolution Study (QUEST) of nearby QSOs and ULIRGs using observations with the Spitzer Space Telescope. For 27 Palomar-Green QSOs at z ≲ 0.3, we derive luminosities of diagnostic lines ([Ne II] 12.8 μm, [Ne V] 14.3 μm, [O IV] 25.9 μm) and emission features (PAH 7.7 μm emission, which is related to star formation), as well as continuum luminosities over a range of mid- to far-infrared wavelengths between 6 and 60 μm. We detect star-formation-related PAH emission in 11/26 QSOs and fine-structure line emission in all of them, often in multiple lines. The detection of PAHs in the average spectrum of sources that lack individual PAH detections provides further evidence for the widespread presence of PAHs in QSOs. Similar PAH/FIR and [Ne II]/FIR ratios are found in QSOs and in starburst-dominated ULIRGs and lower luminosity starbursts. We conclude that the typical QSO in our sample has at least 30% but likely most of the far-infrared luminosity (~10^(10)-10^(12) L_☉) arising from star formation, with a tendency for larger star formation contribution at the largest FIR luminosities. In the QSO sample, we find correlations between most of the quantities studied including combinations of AGN tracers and starburst tracers. The common scaling of AGN and starburst luminosities (and fluxes) is evidence for a starburst-AGN connection in luminous AGN. Strong correlations of far-infrared continuum and starburst related quantities (PAH, low-excitation [Ne II]) offer additional support for the starburst origin of far-infrared emission.

A Medium Survey of the Hard X‐Ray Sky withASCA. II. The Source’s Broadband X‐Ray Spectral Properties

A complete sample of 60 serendipitous hard X-ray sources with flux in the range ~1 × 10-13 ergs cm-2 s-1 to ~4 × 10-12 ergs cm-2 s-1 (2-10 keV), detected in 87 ASCA GIS2 images, was recently presented in the literature. Using this sample it was possible to extend the description of the 2-10 keV log N(> S)- log S down to a flux limit of ~6 × 10-14 ergs cm-2 s-1 (the faintest detectable flux), resolving about a quarter of the cosmic X-ray background (CXB). In this paper we have combined the ASCA GIS2 and GIS3 data of these sources to investigate their X-ray spectral properties using the hardness ratios and the stacked-spectra method. Because of the sample statistical representativeness, the results presented here, which refer to the faintest hard X-ray sources that can be studied with the current instrumentation, are relevant to the understanding of the CXB and of the active galactic nucleus (AGN) unification scheme. The stacked spectra show that the average source's spectrum hardens toward fainter fluxes; it changes from an energy spectral index = 0.87 ± 0.08 for the 20 brightest sources (2-10 keV count rate ≥3.9 × 10-3 counts s-1, the sample) to = 0.36 ± 0.14 for the remaining 40 fainter sources (the sample). The dividing line of 3.9 × 10-3 counts s-1 corresponds to unabsorbed 2-10 keV fluxes in the range ~5.4 × 10-13 to ~3.1 × 10-13 ergs cm-2 s-1 for a source described by a power-law model with energy spectral index between 0.0 and 2.0. It thus seems that we are now beginning to detect those sources that have the correct spectral shape to be responsible for the 2-10 keV CXB. The hardness-ratio analyses indicate that this flattening is due to a population of sources with very hard spectra showing up in the faint sample; about half of the sources in this sample require αE 0.5, while only ~10% of the sources in the bright sample are consistent with an energy spectral index so flat. A number of sources (~30%) in the faint sample seem to be characterized by an apparently inverted X-ray spectrum (i.e., αE 0.0). These objects are probably extremely absorbed sources, as expected from the CXB synthesis models based on the AGN unification scheme, if not a new population of very hard serendipitous sources. The broadband (0.7-10 keV) spectral properties of the selected sources, as inferred from the hardness-ratios diagram, seem to be more complex than is expected from a simple absorbed power-law model. We have thus investigated more complex models, in line with the AGN unification scheme, and we find that these models seem to be able to explain the overall spectral properties of the present sample; this result also seems to be suggested by a comparison of the hardness-ratio diagram of the serendipitous ASCA sources with that obtained using a sample of nearby and well-known Seyfert 1 and Seyfert 2 galaxies observed with ASCA.

The [ITAL]ASCA[/ITAL] Medium-Sensitivity Survey (The GIS Catalog Project): Source Counts and Evidence for Emerging Population of Hard Sources

We present first results from the ASCA Medium-Sensitivity Survey (or the GIS catalog project). From the serendipitous fields amounting to 106 deg2, we determined the log N-log S relations in the 0.7-7 and 2-10 keV bands with the best statistical accuracy obtained so far, over the flux range from 1 × 10-11 to 5 × 10-14 and 1 × 10-11 to 7 × 10-14 ergs s-1 cm-2, respectively. When the sources detected in the 0.7-7 keV band are divided into two subsamples with higher and lower spectral hardness, the former exhibits a significantly steeper slope than the latter at fluxes below ~10-12 ergs s-1 cm-2 (0.7-7 keV). The average spectrum of sources becomes continuously harder toward fainter fluxes, from a photon index of 2.1 in the 0.7-10 keV range at the flux of ~10-11 to 1.6 at ~10-13 ergs s-1 cm-2 (0.7-7 keV). This is consistent with the comparison of source counts between the 2-10 and the 0.7-2 keV bands and solves the puzzle of their discrepancy reported previously. Our results demonstrate rapid emergence of hard X-ray sources with a decreasing flux from ~10-12 to ~10-13 ergs s-1 cm-2 (2-10 keV).

A deep ROSAT survey — XII. The X-ray spectra of faint ROSAT sources

Optical spectroscopy has enabled us to identify the optical counterparts to over 200 faint X-ray sources to a flux limit of S_(0.5-2keV)=4x10^-15 erg s^-1 cm^-2 on 5 deep ROSAT fields. Here we present a spectral analysis of all the X-ray sources to investigate claims that the average source spectra harden at faint X-ray flux. From a hardness ratio analysis we confirm that the average spectra from 0.5-2 keV harden from an equivalent photon index of $\Gamma=2.2$ at S_(0.5-2keV)=1x10^-13 erg s^-1 cm^-2 to $\Gamma=1.7$ below 1x10^-14 erg s^-1 cm^-2. These spectral changes are due to the emergence of an unidentified source population rather than the class of X-ray QSOs already identified. The 128 QSOs detected so far show no evidence for spectral hardening over this energy range and retain a mean photon index of $\Gamma=2.2$. Recent work suggests that many of the remaining unidentified sources are X-ray luminous galaxies. Taking a subset identified as the most likely galaxy candidates we find that these show significantly harder spectra than QSOs. The emission line galaxies in particular show spectra more consistent with the residual X-ray background, with $\Gamma=1.51 \pm 0.1$ from 0.1-2 keV. Individually the galaxies appear to be a mixture of absorbed and unabsorbed X-ray sources. Combined with recent cross-correlation results and work on the source number count distribution, these results suggest that we may be uncovering the missing hard component of the cosmic X-ray background.

Broadband source spectrum, seismic energy, and stress drop of the 1989 MacQuarie Ridge Earthquake

I compute the broadband source spectrum at periods from 1 to 50 seconds using teleseismic P body waves of the May 23, 1989 Macquarie Ridge earthquake (Mw = 8.1) recorded by the GDSN, GEOSCOPE, and IDA networks. The average source spectrum is obtained by windowing, tapering, and Fourier‐transforming P waves, removing from the spectra the effects of attenuation, geometrical spreading, and radiation pattern, and averaging logarithmically over the stations. The source spectrum for the strike‐slip Macquarie Ridge earthquake is higher than an average source spectrum of seven recent large earthquakes (scaled to be comparable to a Mw = 8.1 earthquake) by a factor of 2 to 3 at periods of 1 to 20 seconds. These other earthquakes were underthrusting events in subduction zones. Using Haskell's formulation assuming a point source with no directivity, I estimate the seismically radiated energy from the source spectrum by integrating the square of the source spectrum in velocity and scaling the result. The seismic energy thus estimated for the Macquarie Ridge earthquake is 3 to 8 ×1023 ergs. An Orowan stress drop can be obtained from the seismic energy and moment. The Orowan stress drop for the Macquarie Ridge earthquake is about 20 to 50 bars, much higher than similarly determined stress drops of other recent large earthquakes. There is a correlation between the Orowan stress drops and time since the last earthquake of comparable or larger magnitude for seven recent large earthquakes. This correlation suggests that a healing process operates that may control the mechanical strength of the fault and is important on time scales of tens to hundreds of years.

Source characteristics of the 1985 Michoacan, Mexico Earthquake at periods of 1 to 30 seconds

Source characteristics of the Sept. 19, 1985 Michoacan, Mexico earthquake and its aftershock on Sept. 21 were inferred from broadband and short‐period teleseismic GDSN records. We Fourier‐transformed the P waves, corrected for instrument response, attenuation, geometrical spreading, and radiation pattern (including the depth phases), and then averaged to obtain the teleseismic source spectrum from 1 to 30 s. The Michoacan source spectrum is enriched at 30 s and depleted at 1 to 10 s relative to an average source spectrum of large interplate subduction events. Source spectra for the Sept. 21 aftershock, 1981 Playa Azul, 1979 Petatlan, and 1978 Oaxaca events follow a trend similar to that of the 1985 Michoacan event. This spectral trend may characterize the Mexican subduction zone.A station‐by‐station least‐squares inversion of the Michoacan earthquake records for the source time function yielded three source pulses, which we interpreted as events on the fault plane. The first two are similar in moment, and the third contains only 20% of the moment of the first. Directivity is evident in the timing. At each station, we measured the time differences between the pulses, and performed a least‐squares nonlinear estimation of the strike, distance, and time separation between the events to locate them relative to one another. The second event occurred 26 s after the first, and 82 km southeast of it, indicating southeastward rupture along the trench. The third event occurred 21 s after the second, and about 40 km seaward of it. The two large events are also seen in the near‐field strong motions.The mainshock records, spectrum, and time functions contain less high frequency radiation than those of the 1985 Valparaiso, Chile earthquake. Apparently, the Michoacan earthquake ruptured two relatively smooth, strong patches which generated large 30 s waves, but small 1 to 10 s waves. Such behavior contrasts with the Valparaiso event which had a more complex rupture process and generated more 1 to 5 s energy. This difference is consistent with the higher near‐field accelerations recorded for the Valparaiso event.

Source spectra of great earthquakes: Teleseismic constraints on rupture process and strong motion

Short-period body waves recorded at teleseismic distances from great earthquakes provide information about source rupture processes and strong motions. First, we examine mostly WWSSN records of 19 earthquakes of moment magnitude M_w of 6.5 to 9.5. Four parameters are measured from the short-period P-wave train: the maximum amplitude; the period at maximum amplitude; the time between the first arrival and when the maximum amplitude is attained; and coda length. An extension, m_b, of the teleseismic magnitude, m_b, is defined using the maximum amplitude of the entire short-period P-wave rather than the amplitude achieved in the first few P-wave cycles. A least-squares fit to the data yields the following relationship between m_b and M_w: m_b = 0.53 M_w + 2.70 for M_w 6.5 to 9.5. The time from the first arrival until the maximum amplitude is achieved and the coda length are roughly proportional to M_w, but are further interpreted by a simple asperity model of the rupture process. These data support that short-period waves are, on average, generated preferentially in the same regions of the fault plane as long-period waves (with periods of 10 to 50 sec). We analyze the spectra of short- and intermediate-period teleseismic GDSN records for seven earthquakes with M_w's of 6.4 to 7.8 and hand-digitized short-period WWSSN records of the 1971 San Fernando earthquake. Significant differences exist between the spectra of different events, due partly to variations in tectonic setting or seismic coupling. Using the digital data, we also investigate the relationship between time-domain amplitude and spectral amplitude for short-period P waves. From our empirical relation between spectral amplitude and time-domain amplitude, we estimate the spectral amplitudes implied by the m_b data. We compare our results to the ω^(−2) and Gusev spectral models. Neither model can completely represent the data. Nevertheless, we consider the ω^(−2) model a useful reference model for comparing different events. The average source spectrum of six large events with M_w 7.4 to 7.8 does not have the spectral structure suggested by Gusev. An application to strong motion modeling is presented in which a 1971 San Fernando teleseismic short-period record is summed up to simulate teleseismic records produced by five great earthquakes. The summation procedure matches the moment of the event to be simulated, and includes rupture propagation, fault plane roughness, and randomness. The m_b data provide an important constraint on the summation procedures. Thus constrained, this summation procedure can be more confidently used with near-field strong motion records as Green's functions to predict strong motions from great earthquakes.

Review of source spectrum level density of wind-generated noise

The source spectrum level density of wind-generated noise has been deduced from ocean measurements, laboratory experiments, and theoretical studies in the past 25 years. Selected contributions are reviewed and the results are compared. Since the source level results compare favorably, at least in deep water, an average source spectrum level density as a function of frequency and surface wind speed is proposed for use in noise models. The differences between the dipole and “rocking” dipole source directivity patterns is discussed and wind-generated noise modeling issues are addressed. There seems to be little agreement on the underlying physical mechanisms of wind-generated noise. The question of what information would be added to the source spectrum level densities if a theory for the underlying physical mechanisms were known is also addressed.

Scaling relations for earthquake source parameters and magnitudes

A data set of 41 moderate and large earthquakes has been used to derive scaling rules for kinematic fault parameters. If effective stress and static stress drop are equal, then fault rise time, τ, and fault area, S, are related by τ = 16S^(1/2)/(7π^(3/2)β), where β is shear velocity. Fault length (parallel to strike) and width (parallel to dip) are empirically related by L=2W. Scatter for both scaling rules is about a factor of two. These scaling laws combine to give width and rise time in terms of fault length. Length is then used as the sole free parameter in a Haskell type fault model to derive scaling laws relating seismic moment to M_S (20-sec surface-wave magnitude), M_S to S and m_b (1-sec body-wave magnitude) to M_S. Observed data agree well with the predicted scaling relation. The “source spectrum” depends on both azimuth and apparent velocity of the phase or mode, so there is a different “source spectrum” for each mode, rather than a single spectrum for all modes. Furthermore, fault width (i.e., the two dimensionality of faults) must not be neglected. Inclusion of width leads to different average source spectra for surface waves and body waves. These spectra in turn imply that m_b and M_S reach maximum values regardless of further increases in L and seismic moment. The m_b versus M_S relation from this study differs significantly from the Gutenberg-Richter (G-R) relation, because the G-R equation was derived for body waves with a predominant period of about 5 sec and thus does not apply to modern 1-sec m_b determinations. Previous investigators who assumed that the G-R relation was derived from 1-sec data were in error. Finally, averaging reported rupture velocities yields the relation v_R = 0.72β.

A magnitude scale for local earthquakes in South Australia

Abstract To minimize dispersion in local magnitude estimates due to different instrumental bandwidths, a scale has been established to allow for the average source spectrum, geometrical attenuation, and frequency-dependent absorption. The data used to derive the scale parameters were from S waves recorded in South Australia from 1967 through 1970, in the frequency range 1 to 10 Hz, and for epicentral distances up to 5°. The magnitudes were mainly in the range 1.5 to 3.5. The local scale MN is given by M N = 4.85 + log A g + 0.84 log Δ + 0.0003 f Δ / 2.3 − 2.89 log f + 2.45 ( log f ) 2 + c where Ag mm is the ground amplitude at f Hz at Δ km epicentral distance, and c is a station correction. The dispersion in observations of magnitude has probably been reduced by use of the scale to near the theoretical limits, allowing for possible source radiation patterns. The relationship of the scale to other measures of magnitude is uncertain, but MN may be approximately equivalent to the local Richter magnitude ML for the magnitude range (1.5 < ML < 3.5) commonly observed in South Australia. The scle is limited in use to data in the ranges given above, for local earthquakes in South Australia. Modification is necessary before such a magnitude scale can be applied elsewhere or to different data ranges.

Some experimental results of the study of VLF-propagation by means of sferics

An analysis of 48 atmospherics waveforms has been made to yield the distances of origin and the frequency-amplitude spectra. By using the formula for the resultant field strength of the dominant mode in the Earth-ionosphere wave-guide, attenuation coefficients for the frequency range of 3–15 kHz have been determined and compared with the results of earlier workers and with theory. Having determined the attenuation coefficients, the average source-spectrum is calculated back and is discussed in relation to the excitation factor.

Analysis of smooth type atmospheric waveforms

To resolve conflicting reports concerning smooth type waveforms, over 250 traces were extensively examined, as exemplifying wave-guide pulse dispersion. With very precise measurement, regular traces and those having various minor irregularities were distinguished. Over half the waveforms had regular traces and were directly explicable but analysis, accurate enough to give independent values of ionosphere height and propagation distance, was unattainable. Independent estimates of storm distances from a Sferics network suggested unexpected constancy of ionosphere height at 83 ± 2 km by day and night. Amplitude analysis gave identical average source spectra for day- and night-time waveforms and indicated unsuspectedly trivial variations of attenuation with frequency in the range 4–10 kc/s, and for the day to night transition.