Ray Spectral Index Research Articles

X-ray spectroscopic survey of highly accreting AGN

Improving our understanding of the nuclear properties of high-Eddington-ratio (λEdd) active galactic nuclei (AGN) is necessary since at this regime the radiation pressure is expected to affect the structure and efficiency of the accretion disc-corona system. This may cause departures from the typical nuclear properties of low-λEdd AGN, which have been largely studied so far. We present here the X-ray spectral analysis of 14 radio-quiet, λEdd ≳ 1 AGN at 0.4 ≤ z ≤ 0.75, observed with XMM-Newton. Optical/UV data from simultaneous Optical Monitor observations have also been considered. These quasars were selected to have relatively high values of black hole mass (MBH ∼ 108 − 8.5 M⊙) and bolometric luminosity (Lbol ∼ 1046 erg s−1) in order to complement previous studies of high-λEdd AGN at lower MBH and Lbol. We studied the relation between λEdd and other key X-ray spectral parameters, such as the photon index (Γ) of the power-law continuum, the X-ray bolometric correction (kbol, X), and the optical/UV-to-X-ray spectral index (αox). Our analysis reveals that, despite the homogeneous optical and supermassive black hole accretion properties, the X-ray properties of these high-λEdd AGN are quite heterogeneous. We indeed measured values of Γ between 1.3 and 2.5, at odds with the expectations based on previously reported Γ − λEdd relations, for which Γ ≥ 2 would be a ubiquitous hallmark of AGN with λEdd ∼ 1. Interestingly, we found that ∼30% of the sources are X-ray weak, with an X-ray emission about a factor of ∼10 − 80 fainter than that of typical AGN at similar UV luminosities. The X-ray weakness seems to be intrinsic and not due to the presence of absorption along the line of sight to the nucleus. This result may indicate that high-λEdd AGN commonly undergo periods of intrinsic X-ray weakness. Furthermore, results from follow-up monitoring with Swift of one of these X-ray weak sources suggest that these periods can last for several years.

Multi-frequency study of spectral indices of BL Lacertae objects and flat-spectrum radio quasars

We present statistical analyses of large homogenous data sample of Fermi-detected blazars thoroughly studied in order to reassess the relationship between flat-spectrum radio quasars (FSRQs) and subclasses of BL Lacertae objects (BL Lacs) blazar populations. We discovered from the average values of γ-ray (αγ−ray) and X-ray (αX−ray) spectral indices that the sequence of distribution is indicative of blazar unification scheme. Analyses of FSRQs and BL Lacs data show difference in the shape of γ-ray and X-ray spectral indices: significant anti-correlation (r> - 0.79) exists between αγ−ray and αX−ray spectral indices. The spectral energy distributions of the blazar subclasses show that FSRQs and BL Lacs have similar spectral properties which can be unified through evolutionary sequence. Nevertheless, there is a significant difference between the shapes of X-ray and γ-ray spectra of blazars suggestive that different mechanisms are responsible for spectral variations in the two energy bands. All these results suggest that there is a form of a unified scheme for all blazars.

Spectral Analysis of Forbush Decreases Using a New Yield Function

The Forbush decreases of the cosmic ray intensity observed on 24 December 2014 and on 8 September 2017 were chosen for cosmic ray spectral analysis. At first an analytical study of the solar and geomagnetic parameters of these events was carried out due to the fact that both are typical cosmic ray events. Hourly cosmic ray data of the neutron monitor stations obtained from the high-resolution neutron monitor database were used for calculating the cosmic ray density and anisotropy variations. Following the method of the coupling coefficients, the galactic cosmic ray spectral index was calculated using the technique of Wawrzynczak and Alania (Adv. Space Res. 45, 622, 2010). A newly presented yield function by Mishev, Usoskin, and Kovaltsov (J. Geophys. Res. 118, 2783, 2013) including a geometrical correction factor, already used in the spectral analysis of the cosmic ray ground level enhancements, was applied for the first time to the case of Forbush decreases. A comparison of these results during the events is performed by using two other coupling functions: the function presented in the work of Clem and Dorman (Space Sci. Rev. 93, 335, 2000) and the one in the work of Belov and Struminsky (Proc. 25th Int. Cosmic Ray Conf. 1, 201, 1997). The latter includes an extension for neutron monitor stations with rigidity $1~\mbox{GV} < R < 2.78~\mbox{GV}$ . The obtained spectral index and the calculated cosmic ray intensity in the heliosphere during the two Forbush decreases after the coupling by these three functions are presented and discussed.

Time variations in Galactic cosmic rays as measured from Southeast Asia

We summarizetime variations in Galactic cosmic ray (GCR) measurements from the Princess Sirindhorn Neutron Monitor(PSNM) at the summit of Doi Inthanon, Thailand’s highest mountain, since 2007. PSNM is the first neutron monitor (NM) station making long-term measurements in Southeast Asia, with the world’s highest cutoff (threshold) rigidity (momentum per charge) of ≈17 GV. GCR variations with the ∼11-year sunspot cycle or ∼22-year solar magnetic cycle are known as solar modulation. Compared with measurements at lower cutoff, PSNM reveals a distinct pattern of solar modulation related to the interplanetary magnetic field strength. NM viewing directions rotate with Earth, so daily (“diurnal”) variations indicate the GCR anisotropy, and we identified time intervals with unusually strong anisotropy due to a unidirectional GCR gradient. From worldwide NM data, we determine an hourly anisotropy during GCR decreases associated with solar storms, indicating GCR flows consistent with our previous theory that cosmic rays drift into one leg of an interplanetary flux rope and out the other. PSNM was also the first fixed NM to monitor time delays between successive neutrons, providing a proxy for the cosmic ray spectral index using data from a single station, avoiding the systematic uncertainties of cross-station comparisons.

An artificial intelligence based approach for constraining the redshift of blazars using γ–ray observations

In this paper, we discuss an artificial intelligence based approach to constrain the redshift of blazars using combined $\gamma$--ray observations from the \emph{Fermi} Large Area Telescope (LAT) and ground based atmospheric Cherenkov telescopes (ACTs) in GeV and \emph{sub} TeV energy regimes respectively. The spectral measurements in GeV and TeV energy bands show a redshift dependent spectral break in the $\gamma$--ray spectra of blazars. We use this observational feature of blazars to constrain their redshift. The observed spectral information of blazars with known redshifts reported in the \emph{Fermi} catalogs (3FGL and 1FHL) and TeV catalog are used to train an Artificial Neural Network (ANN) based algorithm. The training of the ANN methodology is optimized using \emph{Levenberg - Marquardt} algorithm with $\gamma$--ray spectral indices and redshifts of 35 well observed blazars as input and output parameters respectively. After training, we use only observed spectral indices in GeV and sub TeV regimes for 10 blazars as inputs to predict their redshifts. The comparison of predicted redshifts by the ANN with the known redshift suggests that both the values are consistent within $\sim$ 18$\%$ uncertainty. The method proposed in the present work would be helpful in future for constraining or predicting the redshifts of the blazars using only observational $\gamma$--ray spectral informations obtained from the \emph{Fermi}-LAT and current generation IACTs as well as from the next generation Cherenkov Telescope Array (CTA) with improved source statistics.

Probing galactic cosmic ray distribution with TeV gamma-ray sky

The distribution of cosmic rays in the Galaxy is still uncertain and this affects the expectations for the diffuse gamma-ray emission produced by hadronic interactions of cosmic rays with the interstellar gas. We evaluate the diffuse gamma-ray flux at TeV energies by considering different assumptions for the cosmic ray distribution, including the recently emerged possibility of a decreasing cosmic ray spectral index in the inner Galaxy. The diffuse emission from the galactic central region (i.e. in the longitude range | l | ≤ 60̂) changes in the different scenarios by a relatively large factor and can be probed by TeV scale gamma-ray observations. By comparing the total flux produced by diffuse emission and point-like and extended sources resolved by HESS with the gamma-ray flux observed by Argo-YBJ, HESS, HAWC and Milagro, we show that experimental data can already discriminate among different hyphoteses for cosmic ray distribution. The constraints can be strengthened if the contribution of sources not resolved by HESS is taken into account.

Galactic cosmic ray spectral index: the case of Forbush decreases of March 2012

During the burst of solar activity in March 2012, close to the maximum of solar cycle 24, a number of X-class and M-class flares and halo CMEs with velocity up to $2684~\mbox{km}/\mbox{s}$ were recorded. During a relatively short period (7–21 March 2012) two Forbush decreases were registered in the ground-level neutron monitor data. In this work, after a short description of the solar and geomagnetic background of these Forbush decreases, we deduce the cosmic ray density and anisotropy variations based on the daily cosmic ray data of the neutron monitor network ( http://www.nmdb.eu ; http://cosray.phys.uoa.gr ). Applying to our data two different coupling functions methods, the spectral index of these Forbush decreases was calculated following the technique of Wawrzynczak and Alania (Adv. Space Res. 45:622–631, 2010). We pointed out that the estimated values of the spectral index $\gamma$ of these events are almost similar for both cases following the fluctuation of the Forbush decrease. The study and the calculation of the cosmic ray spectrum during such cosmic ray events are very important for Space Weather applications.

Gamma-ray and Neutrino Diffuse Emissions of the Galaxy at very High Energy

Several independent analyzes of Fermi-LAT results found evidences of a spatial dependent cosmic ray (CR) spectral index which is not accounted for by conventional models of CR transport in the Galaxy. Moreover, several experiments have established the presence of a CR spectral hardening above few hundred GeV. We show that these results may have a relevant impact on the γ-ray and neutrino diffuse emissions of the Galaxy above the TeV. Indeed a phenomenological model which adopts a spatial dependent diffusion coefficient, so to account for those features, also reproduces the γ-ray excess found by Milagro at 15 TeV and the spectrum measured by H.E.S.S. and Fermi-LAT in the Galactic ridge. The same model predicts a neutrino emission along the Galactic plane which is significantly larger than expected on the basis of conventional models. This emission is compatible with ANTARES upper limits and is a natural target for KM3NeT.

Radio core dominance of Fermi blazars

During the first 4 years of mission, Fermi/LAT detected 1444 blazars (3FGL) (Ackermann et al. in Astrophys. J. 810:14, 2015). Fermi/LAT observations of blazars indicate that Fermi blazars are luminous and strongly variable with variability time scales, for some cases, as short as hours. Those observations suggest a strong beaming effect in Fermi/LAT blazars. In the present work, we will investigate the beaming effect in Fermi/LAT blazars using a core-dominance parameter, \(R = S_{\mathrm{core}}/ S_{\mathrm{ext.}}\), where \(S_{\mathrm{core}}\) is the core emission, while \(S_{\mathrm{ext.}}\) is the extended emission. We compiled 1335 blazars with available core-dominance parameter, out of which 169 blazars have \(\gamma\)-ray emission (from 3FGL). We compared the core-dominance parameters, \(\log R\), between the 169 Fermi-detected blazars (FDBs) and the rest non-Fermi-detected blazars (non-FDBs), and we found that the averaged values are \(\langle \log R\rangle = 0.99\pm0.87\) for FDBs and \(\langle \log R\rangle = -0.62\pm1.15\) for the non-FDBs. A K-S test shows that the probability for the two distributions of FDBs and non-FDBs to come from the same parent distribution is near zero (\(P =9.12\times10^{-52}\)). Secondly, we also investigated the variability index (\(V.I.\)) in the \(\gamma\)-ray band for FDBs, and we found \(V.I.=(0.12 \pm0.07) \log R+(2.25\pm0.10)\), suggesting that a source with larger \(\log R\) has larger \(V.I.\) value. Thirdly, we compared the mean values of radio spectral index for FDBs and non-FDBs, and we obtained \(\langle \alpha_{\mathrm{radio}}\rangle =0.06\pm0.35\) for FDBs and \(\langle \alpha_{\mathrm{radio}}\rangle =0.57\pm0.46\) for non-FDBs. If \(\gamma\)-rays are composed of two components like radio emission (core and extended components), then we can expect a correlation between \(\log R\) and the \(\gamma\)-ray spectral index. When we used the radio core-dominance parameter, \(\log R\), to investigate the relationship, we found that the spectral index for the core component is \(\alpha_{\gamma}|_{\mathrm{core}} = 1.11\) (a photon spectral index of \(\alpha_{\gamma}^{\mathrm{ph}}|_{\mathrm{core}} = 2.11\)) and that for the extended component is \(\alpha_{\gamma}|_{\mathrm{ext.}} = 0.70\) (a photon spectral index of \(\alpha_{\gamma}^{\mathrm{ph}}|_{\mathrm{ext.}} = 1.70\)). Some discussions are also presented.

ESTIMATION OF THE EXTRAGALACTIC BACKGROUND LIGHT USING TeV OBSERVATIONS OF BL Lac OBJECTS

The very high energy (VHE) gamma ray spectral index of high energy peaked blazars correlates strongly with its corresponding redshift whereas no such correlation is observed in the X-ray or the GeV bands. We attribute this correlation to a result of photon-photon absorption of TeV photons with the extragalactic background light (EBL) and utilizing this, we compute the allowed flux range for the EBL, which is independent of previous estimates. The observed VHE spectrum of the sources in our sample can be well approximated by a power-law, and if the de-absorbed spectrum is also assumed to be a power law, then we show that the spectral shape of EBL will be $\epsilon n(\epsilon) \sim k log(\frac{\epsilon}{\epsilon_p}) $. We estimate the range of values for the parameters defining the EBL spectrum, $k$ and $\epsilon_p$, such that the correlation of the intrinsic VHE spectrum with redshift is nullified. The estimated EBL depends only on the observed correlation and the assumption of a power law source spectrum. Specifically, it does not depend on the spectral modeling or radiative mechanism of the sources, nor does it depend on any theoretical shape of the EBL spectrum obtained through cosmological calculations. The estimated EBL spectrum is consistent with the upper and lower limits imposed by different observations. Moreover, it also agrees closely with the theoretical estimates obtained through cosmological evolution models.

Thunderstorm ground enhancements: Gamma ray differential energy spectra

The shape and evolution of the energy spectra of the thunderstorm ground enhancement (TGE) electrons and gamma rays shed light on the origin of TGEs, on the relationship of modification of the energy spectra (MOS) and relativistic runaway electron avalanche processes, on the nature of the seed particles, and on the strength and elongation of an atmospheric electric field. However, till now the measurements of energy spectra of TGE electrons and gamma rays have been rather scarce. For the first time, we present differential energy spectra of gamma rays in the wide energy range 4--100 MeV for five TGE events detected in 2012--2013 at Aragats. We use the special technique of electron/gamma ray fraction determination to select TGE events with very small contamination of electrons. The network of large NaI spectrometers located 3200 m above sea level measured energy spectra of gamma rays. The power law indices of ``small'' TGEs are rather close to the background cosmic gamma ray spectrum ($\ensuremath{\gamma}\ensuremath{\sim}\ensuremath{-}2$); thus, we may deduce that these small events are due to MOS of cosmic ray electrons in the electric field of a thundercloud. Larger TGEs measured by the NaI network and the two largest TGE events earlier recovered from energy releases in a 60-cm-thick scintillator have much steeper energy spectra typical for the avalanche process in atmosphere. The classification of TGEs according to intensity and gamma ray spectral index pointed toward two main mechanisms of the TGE gamma ray origin: the runaway process and modification of electron energy spectra in the thunderstorm atmospheres.

THE SPECTRAL INDEX PROPERTIES OFFERMIBLAZARS

In this paper, a sample of 451 blazars (193 flat spectrum radio quasars (FSRQs), 258 BL Lacertae objects) with corresponding X-ray and Fermi ?-ray data is compiled to investigate the correlation both between the X-ray spectral index and the ?-ray spectral index and between the spectral index and the luminosity, and to compare the spectral indexes ?X, ??, ?X?, and ??X? for different subclasses. We also investigated the correlation between the X-ray and the ?-ray luminosity. The following results have been obtained. (1) Our analysis indicates that an anti-correlation exists between the X-ray and the ?-ray spectral indexes for the whole sample. However, when we considered the subclasses of blazars (FSRQs, the low-peaked BL Lacertae objects (LBLs) and the high-peaked BL Lacertae objects (HBLs)) separately, there is not a clear relationship for each subclass. For the average values, , we have ; and (2) For the correlations between the X-ray and the ?-ray luminosities, we have log ?L ? = (1.033 ? 0.002)log ?L X + (0.213 ? 0.094) for the whole sample, log ?L ? = (0.741 ? 0.004)log ?L X + (12.378 ? 0.162) for HBLs, and log ?L ? = (1.032 ? 0.003)log ?L X + (0.643 ? 0.138) for the LBLs and FSRQs; the correlation slope for HBLs is different from that for FSRQs and LBLs. For the average values, , we have , , and for the X-ray luminosity; and , , and for the ?-ray luminosity. (3) There is an anti-correlation between the effective spectral index, ?X?, and log ?L ? for the whole sample, and the subclasses (FSRQs, LBLs, and HBLs). The effective spectral index shows that (4) For the spectral index difference, ??X?(=?X? ? ??) of the effective spectral index, ?X?, and the ?-ray spectral index, ??, we obtain (5) Based on the Fermi-detected sources, we can say that the HBLs are different from FSRQs, while the LBLs are similar to FSRQs.

The Hot Disk Corona and Magnetic Turbulence in Radio-quiet Active Galactic Nuclei: Observational Constraints

We compile a sample consisting of 56 radio-quiet active galactic nuclei so as to investigate the statistical properties of the hot corona in accretion disks from ASCA observations. The black hole masses in the sample are estimated via several popular methods, and the bolometric luminosities from the multiwavelength continuum. This allows us to estimate the Eddington ratio (E = L-bol/L-Edd) so that the undergoing physical processes can be tested via hard X-ray data. We use the least- squares method of multivariate regression and find a strong correlation between F-X = L2-10 keV/L-bol and E as F-X proportional to E-064. This indicates that the release of gravitational energy in the hot corona is controlled by the Eddington ratio. On the other hand, the correlation between the hard X- ray spectral index (Gamma) and E depends critically on the types of objects: Gamma is nearly constant (Gamma proportional to E-0) in broad- line Seyfert 1 galaxies, whereas Gamma proportional to log E-0.18 in narrow- line Seyfert 1 galaxies, although this is not very significant. We can set constraints on the forms of the magnetic stress tensor on the condition that F-X is proportional to the fraction f of gravitational energy dissipated in the hot corona and that f is proportional to the magnetic energy density in the disk. We find that the shear stress tensor is favored by the correlation in the present sample, t. P where P-gas is the gas pressure.

The dual imaging cerenkov experiment (DICE) and the elemental composition in the “Knee” region

An outstanding problem in cosmic ray physics is the cosmic ray elemental abundance in the “knee” region of the charged particle spectrum. The Dual Imaging Cerenkov Experiment (DICE) is designed to measure the mean elemental mass and energy spectrum around the “knee” energy by using composition sensitive properties of Extensive Air Showers (EAS). We present preliminary results on one EAS parameter, the average depth of shower maximum, 〈 X max 〉. We also show evidence for a change in the cosmic ray spectral index, a result which is in qualitative agreement with other experiments. A small increase in 〈 X max 〉 is present at the onset of the “knee”, implying an increase in the relative number of lighter elements. Further consistency tests of these results which are in progress are described.

Ultraviolet observations of a bright southern quasar

view Abstract Citations (1) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ultraviolet Observations of a Bright Southern Quasar Kirhakos, Sofia ; Strauss, Michael A. ; Yahil, Amos ; Griffith, Mark ; Fisher, Karl B. Abstract We present low-resolution ultraviolet spectra of the QSO 09149 - 6206 (z = 0.057) obtained with the International Ultraviolet Explorer. The spectrum shows strong emission lines of Ly α, C IV, C III], Mg II, and Fe II. The Fe II blend centered at 2080 A is unusually strong; this may be a consequence of overcorrection for the 2175 A silicate feature in the reddening curve. Absorption features from the Milky Way are clearly present. There is no evidence for further absorption features due to objects beyond our Galaxy and in the foreground of the QSO. After correcting for Galactic reddening, the spectral energy distribution is very flat from ultraviolet to far-infrared wavelengths; there is no big blue bump in this object. The QSO is detected weakly at 6 cm; we have only upper limits at x-ray wavelengths. The upper limit to the optical-x- ray spectral index is - 1.35. We have measured the Hα line strength from a spectrum taken under photometric conditions; the Ly α/Hα line ratio is 0.8 after correction for reddening. A spectrum of the fuzz surrounding the QSO shows narrow emission lines of Hα, [N II], [S II], and [O I]. Publication: The Astronomical Journal Pub Date: December 1991 DOI: 10.1086/116014 Bibcode: 1991AJ....102.1933K Keywords: Quasars; Sky Surveys (Astronomy); Southern Sky; Ultraviolet Spectra; Iue; Ultraviolet Absorption; Ultraviolet Astronomy; Astrophysics; ULTRAVIOLET: SPECTRA; QUASARS full text sources ADS | data products SIMBAD (1) NED (1) MAST (1) INES (1)

The Influence of Extended Source Distributions on Cosmic ray Spectral Index Variations in the Galactic Wind Model

The solution of the steady-state transport equation describing the propagation of relativistic electrons perpendicular to the galactic plane in a galactic wind is discussed for extended source distributions. The wind velocity is assumed to be zero in the plane and to increase with galactic height. The electrons undergo simultaneously diffusion, convection, adiabatic deceleration, radiative losses and injection. We contrast the resulting spectra with those derived for an infinitely thin source distribution and discuss the dependence of the variation of the spectral index inside the source distribution on the form of this distribution in any transport model. Apart from a new region at very small energies the asymptotic spectra agree with those of the line-source model (LS). Inside the source distribution the spectral index break is doubled from δγlin =0.5 to δγext=1.0.

Cosmic gamma rays from extragalactic objects

The authors have analysed the database of cosmic gamma rays above 100 MeV from the SAS-2 satellite to search for gamma -ray fluxes from extragalactic active galactic nuclei (AGN). The correlation function method devised by the COS-B group to search for galactic gamma -ray sources has been used. They have thus identified 15 gamma -ray AGN, including 11 quasars, 3 BL Lac objects and 1 radiogalaxy. They have further investigated the relationship between their gamma -ray fluxes in different energy regions (>100 MeV and 35-100 MeV) and the relationship between their gamma -ray fluxes and their red shifts, and they have also derived their gamma -ray spectral indices. The results give further support to the claim that the sources detected are genuine extragalactic objects.