We report on the spectral cross-calibration results of the Konus-Wind, the Suzaku/WAM, and the Swift/BAT instruments using simultaneously observed gamma-ray bursts (GRBs). This is the first attempt ...to use simultaneously observed GRBs as a spectral calibration source to understand systematic problems among the instruments. Based on these joint spectral fits, we find that (1) although a constant factor (a normalization factor) agrees within 20% among the instruments, the BAT constant factor shows a systematically smaller value by 10%–20% compared to that of Konus-Wind, (2) there is a systematic trend that the low-energy photon index becomes steeper by 0.1–0.2 and
$E_{\rm peak}$
becomes systematically higher by 10%–20% when including the BAT data in the joint fits, and (3) the high-energy photon index agrees within 0.2 among the instruments. Our results show that cross-calibration based on joint spectral analysis is an important step to understanding the instrumental effects that could be affecting the scientific results from the GRB prompt emission data.
We have assembled the largest sample of ultra hard X-ray selected (14-195 keV) active galactic nucleus (AGN) with host galaxy optical data to date, with 185 nearby (z < 0.05), moderate luminosity ...AGNs from the Swift BAT sample. The BAT AGN host galaxies have intermediate optical colors (u -- r and g -- r) that are bluer than a comparison sample of inactive galaxies and optically selected AGNs from the Sloan Digital Sky Survey (SDSS) which are chosen to have the same stellar mass. Based on morphological classifications from the RC3 and the Galaxy Zoo, the bluer colors of BAT AGNs are mainly due to a higher fraction of mergers and massive spirals than in the comparison samples. BAT AGNs in massive galaxies (log M * >10.5) have a 5-10 times higher rate of spiral morphologies than in SDSS AGNs or inactive galaxies. We also see enhanced far-infrared emission in BAT AGN suggestive of higher levels of star formation compared to the comparison samples. BAT AGNs are preferentially found in the most massive host galaxies with high concentration indexes indicative of large bulge-to-disk ratios and large supermassive black holes. The narrow-line (NL) BAT AGNs have similar intrinsic luminosities as the SDSS NL Seyferts based on measurements of O III Delta *l5007. There is also a correlation between the stellar mass and X-ray emission. The BAT AGNs in mergers have bluer colors and greater ultra hard X-ray emission compared to the BAT sample as a whole. In agreement with the unified model of AGNs, and the relatively unbiased nature of the BAT sources, the host galaxy colors and morphologies are independent of measures of obscuration such as X-ray column density or Seyfert type. The high fraction of massive spiral galaxies and galaxy mergers in BAT AGNs suggest that host galaxy morphology is related to the activation and fueling of local AGN.
The 9-month Swift Burst Alert Telescope (BAT) catalog provides the first unbiased (NH < 1024 cm-2) look at local (z = 0.03) active galactic nuclei (AGNs). In this paper, we present the collected ...X-ray properties (0.3-12 keV) for the 153 AGNs detected. In addition, we examine the X-ray properties for a complete sample of nonbeamed sources, above the Galactic plane (b >= 15°). Of these, 45% are best fit by simple power law models, while 55% require the more complex partial covering model. One of our goals was to determine the fraction of 'hidden' AGNs, which we define as sources with scattering fractions less than or equal to 0.03 and ratios of soft to hard X-ray flux less than or equal to 0.04. We found that 'hidden' AGNs constitute a high percentage of the sample (24%), proving that they are a very significant portion of local AGNs. Further, we find that the fraction of absorbed sources does increase at lower unabsorbed 2-10 keV luminosities, as well as accretion rates. This suggests that the unified model requires modification to include luminosity dependence, as suggested by models such as the 'receding torus' model. Some of the most interesting results for the BAT AGN sample involve the host galaxy properties. We found that 33% are hosted in peculiar/irregular galaxies and only 5/74 are hosted in ellipticals. Further, 54% are hosted in interacting/merger galaxies. Finally, we present both the average X-ray spectrum (0.1-10 keV) and log N-log S in the 2-10 keV band. With our average spectrum, we have the remarkable result of reproducing the measured CXB X-ray power law slope of Gamma 1.4. From the log N-log S relationship, we show that we are complete to log S >= -11 in the 2-10 keV band. Below this value, we are missing as many as 3000 sources at log S = -12. Both the collected X-ray properties of our uniform sample and the log N-log S relationship will now provide valuable input to X-ray background models for z 0.
Long-term variability of AGN at hard X-rays Soldi, S.; Beckmann, V.; Baumgartner, W. H. ...
Astronomy and astrophysics (Berlin),
03/2014, Volume:
563, Issue:
A57
Journal Article
Peer reviewed
Open access
Aims. Variability at all observed wavelengths is a distinctive property of active galactic nuclei (AGN). Hard X-rays provide us with a view of the innermost regions of AGN, mostly unbiased by ...absorption along the line of sight. Characterizing the intrinsic hard X-ray variability of a large AGN sample and comparing it to the results obtained at lower X-ray energies can significantly contribute to our understanding of the mechanisms underlying the high-energy radiation. Methods. Swift/BAT provides us with the unique opportunity to follow, on time scales of days to years and with regular sampling, the 14–195 keV emission of the largest AGN sample available up to date for this kind of investigation. As a continuation of an early work using the first 9 months of BAT data, we study the amplitude of the variations and their dependence on subclass and on energy, for a sample of 110 radio quiet and radio loud AGN selected from the BAT 58-month survey. Results. About 80% of the AGN in the sample are found to exhibit significant variability on month-to-year time scales. In particular, radio loud sources are the most variable, and Seyfert 1.5−2 galaxies are slightly more variable than Seyfert 1, while absorbed and unabsorbed objects show similar timing properties. The amplitude of the variations and their energy dependence are incompatible with variability being driven at hard X-rays by changes in the absorption column density. In general, the variations in the 14–24 and 35–100 keV bands are correlated well, suggesting a common origin to the variability across the BAT energy band. However, radio quiet AGN display on average 10% larger variations at 14–24 keV than at 35–100 keV, and a softer-when-brighter behavior for most of the Seyfert galaxies with detectable spectral variability on a time scale of a month. In addition, sources with harder spectra are found to be more variable than softer ones, unlike what it is observed below 10 keV. These properties are generally consistent with a variable, in flux and shape, power law continuum, pivoting at energies ≳ 50 keV, to which a constant reflection component is superposed. When the same time scales are considered, the timing properties of AGN at hard X-rays are comparable to those at lower energies, with at least some of the differences possibly ascribable to components contributing differently in the two energy domains (e.g., reflection, absorption).
We present the Suzaku broadband observations of two AGNs detected by the Swift BAT hard X-ray (>15 keV) survey that did not have previous X-ray data, SWIFT J0601.9-8636 and SWIFT J0138.6-4001. The ...Suzaku spectra reveal in both objects a heavily absorbed power-law component with a column density of N sub(H) unk 10 super(23.5)-10 super(24) cm super(-2) that dominates above 10 keV and an intense reflection component with a solid angle unk2n from a cold, optically thick medium. We find that these AGNs have an extremely small fraction of scattered light from the nucleus, unk0.5% with respect to the intrinsic power-law component. This indicates that they are burled in a very geometrically thick torus with a small opening angle and/or have an unusually small amount of gas responsible for scattering. In the former case, the geometry of SWIFT J0601.9-8636 should be nearly face-on as inferred from the small absorption for the reflection component. The discovery of two such objects in this small sample implies that there must be a significant number of yet unrecognized, very Compton thick AGNs viewed at larger inclination angles in the local universe, which are difficult to detect even in the currently most sensitive optical or hard X-ray surveys.
We present the first analysis of the all-sky Swift Burst Alert Telescope (BAT) ultra-hard X-ray (14-195 keV) data for a targeted list of objects. We find that the BAT data can be studied at ...three-times-fainter limits than in previous blind detection catalogs based on prior knowledge of source positions and using smaller energy ranges for source detection. We determine the active galactic nucleus (AGN) fraction in 134 nearby (z < 0.05) luminous infrared galaxies (LIRGs) from the GOALS sample. We find that LIRGs have a higher detection frequency than galaxies matched in stellar mass and redshift at 14-195 keV and 24-35 keV. In agreement with work at other wavelengths, the AGN detection fraction increases strongly at high IR luminosity with half of the high-luminosity LIRGs (50%, 6/12, log L sub(IR)/L sub(middot in circle) > 11.8) detected. The BAT AGN classification shows 97% (37/38) agreement with Chandra and XMM-Newton AGN classification using hardness ratios or detection of an iron K alpha line. This confirms our statistical analysis and supports the use of the Swift/BAT all-sky survey to study fainter populations of any category of sources in the ultra-hard X-ray band. BAT AGNs in LIRGs tend to show higher column densities with 40% + or - 9% showing 14-195 keV/2-10 keV hardness flux ratios suggestive of high or Compton-thick column densities (log N sub(H) > 24 cm super(-2)), compared to only 12% + or - 5% of non-LIRG BAT AGNs. We also find that using specific energy ranges of the BAT detector can yield additional sources over total band detections with 24% (5/21) of detections in LIRGs at 24-35 keV not detected at 14-195 keV.
he burst alert telescope (BAT) is one of three instruments on the Swift MIDEX spacecraft to study gamma-ray bursts (GRBs). The BAT first detects the GRB and localizes the burst direction to an ...accuracy of 1-4 arcmin within 20 s after the start of the event. The GRB trigger initiates an autonomous spacecraft slew to point the two narrow field-of-view (FOV) instruments at the burst location within 20-70 s so to make follow-up X-ray and optical observations. The BAT is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane. The detector plane is composed of 32,768 pieces of CdZnTe (4×4×2 mm), and the coded-aperture mask is composed of 52,000 pieces of lead (5×5×1 mm) with a 1-m separation between mask and detector plane. The BAT operates over the 15-150 keV energy range with 7 keV resolution, a sensitivity of 10^sup -8^ erg s^sup -1^ cm^sup -2^, and a 1.4 sr (half-coded) FOV. We expect to detect > 100 GRBs/year for a 2-year mission. The BAT also performs an all-sky hard X-ray survey with a sensitivity of 2 m Crab (systematic limit) and it serves as a hard X-ray transient monitor.PUBLICATION ABSTRACT
We report on a large stellar flare from the nearby dMe flare star EV Lac observed by the Swift and Konus-Wind satellites and the Liverpool Telescope. It is the first large stellar flare from a dMe ...flare star to result in a Swift trigger based on its hard X-ray intensity. Its peak f{sub X} from 0.3 to 100 keV of 5.3 x 10{sup -8} erg cm{sup -2} s{sup -1} is nearly 7000 times larger than the star's quiescent coronal flux, and the change in magnitude in the white filter is {>=}4.7. This flare also caused a transient increase in EV Lac's bolometric luminosity (L{sub bol}) during the early stages of the flare, with a peak estimated L{sub X}/L{sub bol} {approx} 3.1. We apply flare loop hydrodynamic modeling to the plasma parameter temporal changes to derive a loop semi-length of l/R{sub *} = 0.37 {+-} 0.07. The soft X-ray spectrum of the flare reveals evidence of iron K{alpha} emission at 6.4 keV. We model the K{alpha} emission as fluorescence from the hot flare source irradiating the photospheric iron, and derive loop heights of h/R{sub *} = 0.1, consistent within factors of a few with the heights inferred from hydrodynamic modeling. The K{alpha} emission feature shows variability on timescales of {approx}200 s which is difficult to interpret using the pure fluorescence hypothesis. We examine K{alpha} emission produced by collisional ionization from accelerated particles, and find parameter values for the spectrum of accelerated particles which can accommodate the increased amount of K{alpha} flux and the lack of observed nonthermal emission in the 20-50 keV spectral region.
We present a systematic spectral analysis with Suzaku of six active galactic nuclei (AGNs) detected in the Swift/BAT hard X-ray (15-200 keV) survey: Swift J0138.6-4001, J0255.2-0011, J0350.1-5019, ...J0505.7-2348, J0601.9-8636, and J1628.1-5145. This is considered to be a representative sample of new AGNs without X-ray spectral information before the BAT survey. We find that the 0.5-200 keV spectra of these sources can be uniformly fitted with a base model consisting of heavily absorbed (log N H > 23.5 cm-2) transmitted components, scattered lights, a reflection component, and an iron-K emission line. There are two distinct groups: three 'new-type' AGNs (including the two sources reported by Ueda et al.) with an extremely small scattered fraction (f scat < 0.5%) and strong reflection component (R = /2p 0.8, where is the solid angle of the reflector), and three 'classical-type' ones with f scat > 0.5% and R 0.8. The spectral parameters suggest that the new type has an optically thick torus for Thomson scattering (N H ~ 1025 cm-2) with a small opening angle c ~ 20° viewed in a rather face-on geometry, while the classical type has a thin torus (N H ~ 1023-24 cm-2) with c 30°. We infer that a significant number of new-type AGNs with an edge-on view are missing in the current all-sky hard X-ray surveys.
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