Photon Spectral Index Research Articles

Discovery of Very High Energy Gamma-Ray Emissions from the Low-luminosity AGN NGC 4278 by LHAASO

Abstract The first source catalog of the Large High Altitude Air Shower Observatory (LHAASO) reported the detection of a very high energy gamma-ray source, 1LHAASO J1219+2915. This Letter presents a further detailed study of the spectral and temporal behavior of this pointlike source. The best-fit position of the TeV source (R.A. = 185.°05 ± 0.°04, decl. = 29.°25 ± 0.°03) is compatible with NGC 4278 within ∼0.°03. Variation analysis shows an indication of variability on a timescale of a few months in the TeV band, which is consistent with low-frequency observations. Based on these observations, we report the detection of TeV γ-ray emissions from this low-luminosity active galactic nucleus. The observation by LHAASO's Water Cherenkov Detector Array during the active period has a significance level of 8.8σ with a best-fit photon spectral index Γ = 2.56 ± 0.14 and a flux f 1–10 TeV = (7.0 ± 1.1sta ± 0.35syst) × 10−13 photons cm−2 s−1, or approximately 5% of the Crab Nebula. The discovery of VHE gamma-ray emission from NGC 4278 indicates that compact, weak radio jets can efficiently accelerate particles and emit TeV photons.

Study of γ-Ray Emission from a Compact Radio Galaxy with the Fermi Large Area Telescope

The radio galaxy PKS 1007+142 is classified as a compact steep-spectrum source (CSS) and belongs to the class of young active galactic nuclei (AGNs). In this paper, we investigate the γ-ray emission from this CSS by conducting a comprehensive analysis of the 15 yr Fermi Large Area Telescope (Fermi-LAT) observation data. The Fermi-LAT latest Source Catalog, 4FGL-DR4, includes an unassociated γ-ray source, 4FGL J1010.0+1416, located at 0.°24 away from the radio position of PKS 1007+142. Using the 15 yr Fermi-LAT observation data, we reestimate the best-fit position of the γ-ray source and find that PKS 1007+142 is in close proximity to the γ-ray source and falls within its 68% error circle. Therefore, we conclude that PKS 1007+142 is the most plausible counterpart to the unassociated LAT source with detection test statistics ∼ 43.4 (∼6.6σ). PKS 1007+142 exhibits a steep power-law spectrum in the 0.1–300 GeV band, with a photon spectral index (Γ γ ) of 2.86 ± 0.17. The average flux in the considered time interval is (2.14 ± 0.34) × 10−12 erg cm−2 s−1. Comparing PKS 1007+142 with other γ-ray emitting AGNs in both the L γ –Γ γ and L γ –L 1.4 GHz planes, it shows a softer γ-ray spectrum and lower luminosity compared to other γ-ray emitting CSSs. Furthermore, the possible origins of γ-ray in PKS 1007+142 are also discussed.

Discovery of an extended GeV counterpart to the TeV source 1LHAASO J1945+2424 in Fermi-LAT data

ABSTRACT With almost 15 yr of data taken by the Large Area Telescope (LAT) onboard the Fermi satellite we discovered an extended source of GeV emission in the region of the very high energy (TeV) source 1LHAASO J1945+2424. This TeV source is more extended than the LAT source. The spectrum of the GeV emission is hard (with a photon spectral index ∼1.5) and connects smoothly with that of the TeV source, indicating a likely common origin. In order to explain the origin of the γ-rays we explore scenarios that are typically used for supernova remnants (SNRs) and pulsar wind nebulae (PWNe). For an SNR with a single particle population, a leptonic particle distribution in the form of a broken power law with a break energy of ∼3.7 TeV explains the spectra well, while in the hadronic scenario a simple power law with a hard spectral index of ∼1.64 is necessary. In the PWN scenario, reasonable parameters are obtained for a source age of 10 kyr and current pulsar spin-down luminosity of ∼1034 erg s−1.

Hunting for the candidates of misclassified sources in LSP BL Lacs using machine learning

ABSTRACT An equivalent–width-based classification may cause the erroneous judgement to the flat spectrum radio quasars (FSRQs) and BL Lacerate objects (BL Lac) due to the diluting the line features by dramatic variations in the jet continuum flux. To help address the issue, this work explores the possible intrinsic classification on the basis of a random forest supervised machine learning algorithm. In order to do so, we compile a sample of 1680 Fermi blazars that have both gamma-rays and radio-frequencies data available from the 4LAC-DR2 catalogue, which includes 1352 training and validation samples and 328 forecast samples. By studying the results for all of the different combinations of 23 characteristic parameters, we found that there are 178 optimal parameter combinations (OPCs) with the highest accuracy (≃98.89 per cent). Using the combined classification results from the nine combinations of these OPCs to the 328 forecast samples, we predict that there are 113 true BL Lacs (TBLs) and 157 false BL Lacs (FBLs) that are possible intrinsically FSRQs misclassified as BL Lacs. The FBLs show a clear separation from TBLs and FSRQs in the gamma-ray photon spectral index, Γph, and X-band radio flux, logFR, plot. Phenomenally, existence a BL Lac to FSRQ (B-to-F) transition zone is suggested, where the FBLs are in the stage of transition from BL Lacs to FSRQs. Comparing the LSP changing-look blazars (CLBs) reported in the literatures, the majority of LSP CLBs are located at the B-to-F zone. We argue that the FBLs located at B-to-F transition zone are the most likely candidates of CLBs.

VERITAS Discovery of Very High Energy Gamma-Ray Emission from S3 1227+25 and Multiwavelength Observations

We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on 2015 May 15 with the Fermi-Large Area Telescope (LAT). A combined 5 hr VERITAS exposure on May 16 and 18 resulted in a strong 13σ detection with a differential photon spectral index, Γ = 3.8 ± 0.4, and a flux level at 9% of the Crab Nebula above 120 GeV. This also triggered target-of-opportunity observations with Swift, optical photometry, polarimetry, and radio measurements, also presented in this work, in addition to the VERITAS and Fermi-LAT data. A temporal analysis of the gamma-ray flux during this period finds evidence of a shortest variability timescale of τ obs = 6.2 ± 0.9 hr, indicating emission from compact regions within the jet, and the combined gamma-ray spectrum shows no strong evidence of a spectral cutoff. An investigation into correlations between the multiwavelength observations found evidence of optical and gamma-ray correlations, suggesting a single-zone model of emission. Finally, the multiwavelength spectral energy distribution is well described by a simple one-zone leptonic synchrotron self-Compton radiation model.

The dichotomy between low-frequency-peaked BL Lacs and flat-spectrum radio quasars

ABSTRACT By comparing the Fourth Fermi Large Area Telescope active galactic nuclei (AGNs) Catalog Data Release 2 (4LAC-DR2) and the Radio Fundamental Catalog (RFC-2021b), we obtain an AGN sample containing 2816 AGNs. We calculate the γ-ray loudness (Gγ) of the sample sources, and establish the relevant relationship between several gamma/radio properties. The results show that: 1) the synchrotron peak frequency ($\nu _\mathrm{peak}^S$) strongly correlates with the γ-ray photon spectral index (Γγ), γ-ray luminosity (Lγ), and radio luminosity (LR); 2) the low-synchrotron-peaked BL Lac objects (LBLs) and the flat-spectrum radio quasars (FSRQs) partially overlap in log $\nu _\mathrm{peak}^S\!-\!\Gamma _\gamma$, log Lγ–Γγ, log LR–Γγ, log Gγ–Γγ, log $\nu _\mathrm{peak}^S\!-\!$log Lγ, log $\nu _\mathrm{peak}^S\!-\!$log LR, and log $\nu _\mathrm{peak}^S\!-\!$log Gγ space. We plotted the distribution of redshift (z), $\nu _\mathrm{peak}^S$, Γγ, Lγ, VLBI 8.4 GHz flux density ($S_{\rm {8.4\, GHz}}$), and Gγ to distinguish FSRQs and LBLs. We note that $\nu _\mathrm{peak}^S$, Γγ, and $S_{8.4\, {\rm GHz}}$ show similar bimodal distributions. To further determine whether FSRQs are misjudged as LBLs, we examined the Compton dominance (CD), and found that CD < 1 for only two out of 39 sources. This measurement provides further evidence that FSRQs may be mistaken for LBLs.

Correlation between Brightness Variability and Spectral Index Variability for Fermi Blazars

In this work, the γ-ray photon flux, photon spectral index (α ph), variability index (), and the synchrotron peak frequency () are compiled for 851 common blazars from the 3FGL and 4FGL catalogs and Fan et al. to investigate variability properties for Fermi blazars. Our calculations and analyses reach following results: (1) the averaged luminosity, spectral index, and variability index of FSRQs are higher than those of BL Lacs for the whole sample. (2) It is found that the spectral index variation is closely anti-correlated with the luminosity variation implying that the spectrum becomes harder when the source becomes brighter in the γ-ray band. (3) Positive correlations are found between the photon spectral index and both γ-ray luminosity and variability index () for the whole sample, but anti-correlations are found in the two correlations for FSRQs. For BL Lac subclass, there is a marginal anti-correlation between the photon spectral index and both γ-ray luminosity, and a positive correlation between the photon spectral index and the variability index (). We think those two positive correlations found for the whole sample are apparent. (4) We adopted the SVM machine learning method to separate BL Lacs and FSRQs in the and plots and proposed that a BCU is an FSRQ candidate if it satisfies , or , otherwise, it is a BL Lac candidate. Our classification results are quite consistent with those by Kang et al (2019).

GeV γ-Ray Emission of Compact Steep-spectrum Source 4C +39.23B

Abstract Thirteen yr observation data of 4FGL J0824.9+3915 with the Large Area Telescope on board the Fermi Gamma Ray Space Telescope (Fermi/LAT) are analyzed for revisiting whether 4C +39.23B, a compact steep-spectrum (CSS) source close to a flat-spectrum radio quasar (FSRQ) 4C +39.23A in the γ-ray emitting region of 4FGL J0824.9+3915, is a γ-ray emitter. We find that the time-integrated γ-ray emission of 4FGL J0824.9+3915 is overwhelmingly dominated by 4C +39.23A. It shows significant variability at a 6.7σ confidence level and the average γ-ray flux in the 0.1–300 GeV energy band is (1.60 ± 0.15) × 10−8 ph cm−2 s−1 with a power-law photon spectral index of 2.48 ± 0.05. During MJD 57500–58500, 4FGL J0824.9+3915 is in a low state with a steady γ-ray flux. Through the analysis of the Fermi/LAT observation data in this time interval, it is found that the test statistic values of the γ-ray emission from 4C +39.23A and 4C +39.23B are ∼5 and ∼31, respectively, indicating that the γ-ray emission in this time interval is dominated by CSS 4C +39.23B. The derived average flux in this time interval for 4C +39.23B is (9.40 ± 4.10) × 10−9 ph cm −2 s−1 with Γ γ = 2.45 ± 0.17. Attributing the spectral energy distribution (SED) of 4C +39.23B to the radiations from its core and extended region, we show that the SED can be represented with a two-zone leptonic model. Its γ-ray emission is contributed to by the core region. The derived magnetic field strength and Doppler boosting factor of the core are 0.13 G and 6.5. Comparing 4C +39.23B with other γ-emitting CSSs and compact symmetric objects (CSOs) in the Γ γ − L γ plane, it resembles CSSs.

Maximally hard radio spectra from Fermi acceleration in pulsar-wind nebulae

ABSTRACT The processes leading to the exceptionally hard radio spectra of pulsar-wind nebulae (PWNe) are not yet understood. Radio photon spectral indices among 29 PWNe from the literature show an approximately normal, α = 0.2 ± 0.2 distribution. We present ∼3σ evidence for a distinct sub-population of PWNe, with a hard spectrum α = 0.01 ± 0.06 near the termination shock and significantly softer elsewhere, possibly due to a recent evacuation of the shock surroundings. Such spectra, especially in the hard sub-population, suggest a Fermi process, such as diffusive shock acceleration (DSA), at its extreme, α = 0 limit. In particular, we show that this limit is approached in DSA for sufficiently anisotropic small-angle scattering, enhanced on either side of the shock for particles approaching the shock front. In the upstream, the spectral hardening is mostly associated with an enhanced energy gain, possibly driven by the same beamed particles crossing the shock. Downstream, the main effect is a diminished escape probability, but this lowers the acceleration efficiency to $\lesssim 25{{\ \rm per\ cent}}$ for α = 0.3 and $\lesssim 1{{\ \rm per\ cent}}$ for α = 0.03.

Multiwavelength flare observations of the blazar S5 1803+784

ABSTRACTThe radio, optical, and γ-ray light curves of the blazar S5 1803+784, from the beginning of the Fermi-Large Area Telescope (LAT) mission in 2008 August–2018 December, are presented. The aim of this work is to look for correlations among different wavelengths useful for further theoretical studies. We analysed all the data collected by Fermi-LAT for this source, taking into account the presence of nearby sources, and we collected optical data from our own observations and public archive data to build the most complete optical and γ-ray light curve possible. Several γ-ray flares (F > 2.3 10−7ph(E > 0.1 GeV) cm−2 s−1) with optical coverage were detected, all but one with corresponding optical enhancement; we also found two optical flares without a γ-ray counterpart. We obtained two Swift Target of Opportunity observations during the strong flare of 2015. Radio observations performed with VLBA and EVN through our proposals in the years 2016–2020 were analysed to search for morphological changes after the major flares. The optical/γ-ray flux ratio at the flare peak varied for each flare. Very minor optical V − I colour changes were detected during the flares. The X-ray spectrum was well fitted by a power law with photon spectral index α = 1.5, nearly independent of the flux level: No clear correlation with the optical or the γ-ray emission was found. The γ-ray spectral shape was well fitted by a power law with average photon index α = 2.2. These findings support an inverse-Compton origin for the high-energy emission of the source, nearly co-spatial with the optically emitting region. The radio maps showed two new components originating from the core and moving outwards, with ejection epochs compatible with the dates of the two largest γ-ray flares.

Testing Comptonization as the Origin of the Continuum in Nonmagnetic Cataclysmic Variables: The Photon Index of X-Ray Emission

Abstract The X-ray spectra of nonmagnetic cataclysmic variables (nmCVs) in the ∼0.3–15 keV energy band have been described by either one or several optically thin thermal plasma components or by cooling flow models. We tested whether the spectral continuum in nmCVs could be successfully described by Comptonization of soft photons off hot electrons presented in a cloud surrounding the source (the transition layer (TL)). We used public XMM-Newton EPIC-pn, Chandra HETG/ACIS and LETG/HRC, and RXTE PCA and HEXTE observations of four dwarf novae (U Gem, SS Cyg, VW Hyi, and SS Aur) observed in the quiescent and outburst states. In total, we analyzed 18 observations, including a simultaneous 0.4–150 keV Chandra/RXTE spectrum of SS Cyg in quiescence. We fitted the spectral continuum with up to two thermal Comptonization components (the compTT or compTB models in XSPEC) using only one thermal plasma temperature and one optical depth. In this framework, the two seed photon components are presumably coming from the innermost and outer parts of the TL (or innermost part of the disk). We obtained that the thermal Comptonization can successfully describe the spectral continuum of these nmCVs in the ∼0.4–150 keV energy band. Moreover, we present the first principal radiative transfer model that explains the quasi-constancy of the spectral photon index observed around 1.8, which strongly supports the Comptonization framework in nmCVs.

An Empirical “High-confidence” Candidate Zone for Fermi BL Lacertae Objects

Abstract In the third catalog of active galactic nuclei detected by the Fermi Large Area Telescope Clean (3LAC) sample, there are 402 blazar candidates of uncertain type (BCU). The proposed analysis will help to evaluate the potential optical classification flat spectrum radio quasars (FSRQs) versus BL Lacertae (BL Lac) objects of BCUs, which can help to understand which is the most elusive class of blazar hidden in the Fermi sample. By studying the 3LAC sample, we found some critical values of γ-ray photon spectral index (Γph), variability index (VI), and radio flux ( ) of the sources separate known FSRQs and BL Lac objects. We further utilize those values to defined an empirical “high-confidence” candidate zone that can be classified as BCUs. Within such a zone (Γph < 2.187, log F R < 2.258, and log VI < 1.702), we found that 120 BCUs can be classified as BL Lac object candidates with a higher degree of confidence (with a misjudged rate <1%). Our results suggest that an empirical “high-confidence” diagnosis is possible to distinguish the BL Lac objects from the Fermi observations based on only the direct observational data of Γph, VI, and F R.

Spectral index−mass accretion rate correlation and evaluation of black hole masses in AGNs 3C 454.3 and M 87

We present the discovery of correlations between the X-ray spectral (photon) index and mass accretion rate observed in active galactic nuclei (AGNs) 3C 454.3 and M 87. We analyzed spectral transition episodes observed in these AGNs using Chandra, Swift, Suzaku, BeppoSAX, ASCA and RXTE data. We applied a scaling technique for a black hole (BH) mass evaluation which uses a correlation between the photon index and normalization of the seed (disk) component which is proportional to a mass accretion rate. We developed an analytical model that shows that the photon index of the BH emergent spectrum undergoes an evolution from lower to higher values depending on disk mass accretion rate. To estimate a BH mass in 3C 454.3 we consider extra-galactic SMBHs NGC 4051 and NGC 7469 as well as Galactic BHs Cygnus X–1 and GRO J1550–564 as reference sources for which distances, inclination angles are known and the BH masses are already evaluated. For M 87 on the other hand, we provide the BH mass scaling using extra-galactic sources (IMBHs: ESO 243–49 HLX–1 and M 101 ULX–1) and Galactic sources (stellar mass BHs: XTE J1550–564, 4U 1630–472, GRS 1915+105 and H 1743–322) as reference sources. Application of the scaling technique for the photon index−mass accretion rate correlation provides estimates of the BH masses in 3C 454.3 and M 87 to be about 3.4 × 109 and 5.6 × 107 solar masses, respectively. We also compared our scaling BH mass estimates with a recent BH mass estimate of M87 = 6.5 × 109 M⊙ made using the Event Horizon Telescope which gives an image at 1.3 mm and is based on the angular size of the “BH event horizon”. Our BH mass estimate in M 87 is at least two orders of magnitude lower than that made by the EHT team.

Evaluating the Optical Classification of Fermi BCUs Using Machine Learning

Abstract In the third catalog of active galactic nuclei detected by the Fermi-LAT (3LAC) Clean Sample, there are 402 blazar candidates of uncertain type (BCUs). Due to the limitations of astronomical observation or intrinsic properties, it is difficult to classify blazars using optical spectroscopy. The potential classification of BCUs using machine-learning algorithms is essential. Based on the 3LAC Clean Sample, we collect 1420 Fermi blazars with eight parameters of γ-ray photon spectral index; radio flux; flux density; curve significance; the integral photon flux in 100–300 MeV, 0.3–1 GeV, and 10–100 GeV; and variability index. Here we apply four different supervised machine-learning (SML) algorithms (decision trees, random forests, support vector machines, and Mclust Gaussian finite mixture models) to evaluate the classification of BCUs based on the direct observational properties. All four methods can perform exceedingly well with more accuracy and can effectively forecast the classification of Fermi BCUs. The evaluating results show that the results of these methods (SML) are valid and robust, where about one-fourth of sources are flat-spectrum radio quasars (FSRQs) and three-fourths are BL Lacertae (BL Lacs) in 400 BCUs, which are consistent with some other recent results. Although a number of factors influence the accuracy of SML, the results are stable at a fixed ratio 1:3 between FSRQs and BL Lacs, which suggests that the SML can provide an effective method to evaluate the potential classification of BCUs. Among the four methods, Mclust Gaussian Mixture Modeling has the highest accuracy for our training sample (4/5, seed = 123).

Characteristics of SEP Events and Their Solar Origin During the Evolution of Active Region NOAA 10069

We present the results of a comparative analysis of the properties of a series of successive solar flares, which occurred in active region (AR) 10069 in August 2002, and the associated solar energetic particle (SEP) events. The active region was extremely flare productive during its evolution. The solar flare characteristics are based on X-ray and radio emission data: maximum detected photon energies and spectral index, delays between microwave, metric-radio and, hard X-ray emissions. The coronal mass ejections (CMEs) are characterized by their projected speed. The SEP properties are described by the relative electron to proton abundance as well as by the abundance of lower relative to higher energy particles. The analysis carried out supports some previous results obtained by large statistical studies, but at the same time refutes others. For example, the set of analyzed events that occurred in the AR did not show clear evidence of the big flare syndrome though the large proton events observed near Earth were always accompanied by CMEs. Some of the peculiar observations could be the result of the magnetic topology of the AR.

Systematic Investigation of X-Ray Spectral Variability of TeV Blazars during Flares in the RXTE Era

Abstract Utilizing all the 16 yr RXTE observations, we analyze the X-ray spectra of 32 TeV blazars and perform a systematic investigation of X-ray spectral variability for the five brightest sources during their major flares that lasted several days. We obtain photon spectral index (α), flux and synchrotron radiation peak energy (E p) from empirical spectral fitting, and electron spectral index (p) from theoretical synchrotron radiation modeling. We find that both α and p generally display a harder-when-brighter trend, confirming the results of many previous works. Furthermore, we confirm and strengthen the result that p must vary in order to explain the observed X-ray spectral variability during flares, which would have useful implications for interpreting the associated higher-energy spectral variability. We see apparent electron spectral hysteresis in many but not all p-flux plots that takes a form of “loop” or oblique “8.” We obtain a tight p–hardness ratio (HR) relation and a tighter p–α relation using spectra of flaring periods, both of which are also applicable to stacked data of quiescent periods. We demonstrate that these two empirical relations can be used efficiently to estimate p from HR or α that is readily achieved. Finally, we find that, when considering TeV blazars as a whole, α and X-ray luminosity are positively correlated, E p is negatively correlated with p and α, and E p is positively correlated with HR; all these correlations are in line with the blazar sequence. However, after correcting for the Doppler boosting effect, α and intrinsic X-ray luminosity follow an anticorrelation.

Multi-wavelength study of the short term TeV flaring activity from the blazar Mrk 501 observed in June 2014

In this work, we study the short term flaring activity from the high synchrotron peaked blazar Mrk 501 detected by the FACT and H.E.S.S. telescopes in the energy range 2–20 TeV during June 23–24, 2014 (MJD 56831.86–56831.94). We revisit this major TeV flare of the source in the context of near simultaneous multi-wavelength observations of γ–rays in MeV-GeV regime with Fermi-LAT, soft X-rays in 0.3–10 keV range with Swift-XRT, hard X-rays in 10–20 keV and 15–50 keV bands with MAXI and Swift-BAT respectively, UV-Optical with Swift-UVOT and 15 GHz radio with OVRO telescope. We have performed a detailed temporal and spectral analysis of the data from Fermi-LAT, Swift-XRT and Swift-UVOT during the period June 15–30, 2014 (MJD 56823–56838). Near simultaneous archival data available from Swift-BAT, MAXI and OVRO telescope along with the V-band optical polarization measurements from SPOL observatory are also used in the study of giant TeV flare of Mrk 501 detected by the FACT and H.E.S.S. telescopes. No significant change in the multi-wavelength emission from radio to high energy γ–rays during the TeV flaring activity of Mrk 501 is observed except variation in soft X-rays. The varying soft X-ray emission is found to be correlated with the γ–ray emission at TeV energies during the flaring activity of the source. The soft X-ray photon spectral index is observed to be anti-correlated with the integral flux showing harder-when-brighter behavior. An average value of 4.5% for V-band optical polarization is obtained during the above period whereas the corresponding electric vector position angle changes significantly. We have used the minimum variability timescale from the H.E.S.S. observations to estimate the Doppler factor of the emission region which is found to be consistent with the previous studies of the source.

Establishing the spectral turnover of blazar PKS 2155−304 as an outcome of radiative losses

Abstract The broad-band optical/UV and X-ray spectra of blazars have been often modelled as synchrotron component arising from a broken power-law distribution of electrons. A broken power-law distribution is expected, since the high-energy electrons undergo radiative losses effectively. The change in the energy index should then be ≈1 and corresponds to a spectral index difference of 0.5. However, one of the long outstanding problems has been that the observed index change is significantly different. On the other hand, recent high-quality observations of blazars suggest that their local spectra may not be a power law, instead have a slight curvature and often represented by a log-parabola model. Using XMM–Newtonobservations spanning over 12 yr for the BL Lac PKS 2155−304, we show that the optical/UV and X-ray spectra can be well represented by a broken log-parabola model. Further, we show that such a spectrum can indicate the energy dependence of the electron escape time-scale from the main acceleration zone. This novel approach, besides addressing the observed difference in the photon spectral indices, also tries to explain the spectral turn over in far-UV/soft X-rays as a result of the radiative losses.

NGC 4051: Black hole mass and photon index-mass accretion rate correlation

We present a discovery of the correlation between the X-ray spectral (photon) index and mass accretion rate observed in an active galactic nucleus, NGC 4051. We analyzed spectral transition episodes observed in NGC 4051 usingXMM-Newton,Suzakuand RXTE. We applied a scaling technique for a black hole (BH) mass evaluation which uses a correlation between the photon index and normalization of the seed (disk) component, which is proportional to a mass accretion rate. We developed an analytical model that shows the spectral (photon) index of the BH emergent spectrum undergoes an evolution from lower to higher values depending on a mass accretion rate in the accretion disk. We considered Cygnus X-1 and GRO J1550–564 as reference sources for which distances, inclination angles and the BH masses are evaluated by dynamical measurements. Application of the scaling technique for the photon index−mass accretion rate correlation provides an estimate of the black hole mass in NGC 4051 to be more than 6 × 105solar masses.

The Chandra COSMOS Legacy Survey: Energy Spectrum of the Cosmic X-Ray Background and Constraints on Undetected Populations

Abstract Using Chandra observations in the 2.15 deg2 COSMOS-legacy field, we present one of the most accurate measurements of the Cosmic X-ray Background (CXB) spectrum to date in the [0.3–7] keV energy band. The CXB has three distinct components: contributions from two Galactic collisional thermal plasmas at kT ∼ 0.27 and 0.07 keV and an extragalactic power law with a photon spectral index Γ = 1.45 ± 0.02. The 1 keV normalization of the extragalactic component is 10.91 ± 0.16 keV cm−2 s−1 sr−1 keV−1. Removing all X-ray-detected sources, the remaining unresolved CXB is best fit by a power law with normalization 4.18 ± 0.26 keV cm−2 s−1 sr−1 keV−1 and photon spectral index Γ = 1.57 ± 0.10. Removing faint galaxies down to leaves a hard spectrum with and a 1 keV normalization of ∼1.37 keV cm−2 s−1 sr−1 keV−1. This means that ∼91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute ∼8%–9% of the total CXB show marginal evidence of being harder and possibly more obscured than resolved sources. Another ∼1% of the CXB can be attributed to still undetected star-forming galaxies and absorbed active galactic nuclei. According to these limits, we investigate a scenario where early black holes totally account for non-source CXB fraction and constrain some of their properties. In order to not exceed the remaining CXB and the accreted mass density, such a population of black holes must grow in Compton-thick envelopes with 1.6 × 1025 cm−2 and form in extremely low-metallicity environments .