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Resolving the Extragalactic γ-ray Background above 50 GeV with Fermi-LAT

Journal Paper
Ajello and Di Mauro for the Fermi-LAT collaboration

The Fermi Large Area Telescope (LAT) Collaboration has recently released a catalog of 360

sources detected above 50 GeV (2FHL). This catalog was obtained using 80 months of data re-

processed with Pass 8, the newest event-level analysis, which significantly improves the acceptance

and angular resolution of the instrument. Most of the 2FHL sources at high Galactic latitude are

blazars. Using detailed Monte Carlo simulations, we measure, for the first time, the source count

distribution, dN/dS, of extragalactic γ-ray sources at E > 50 GeV and find that it is compatible

with a Euclidean distribution down to the lowest measured source flux in the 2FHL (∼ 8 × 10−12 ph

cm−2 s−1). We employ a one-point photon fluctuation analysis to constrain the behavior of dN/dS

below the source detection threshold. Overall the source count distribution is constrained over

three decades in flux and found compatible with a broken power law with a break flux, Sb, in the

range [8 × 10−12, 1.5 × 10−11] ph cm−2 s−1 and power-law indices below and above the break of

α2 ∈ [1.60, 1.75] and α1 = 2.49 ± 0.12 respectively. Integration of dN/dS shows that point sources

account for at least 86+16% of the total extragalactic γ-ray background. The simple form of the −14

derived source count distribution is consistent with a single population (i.e. blazars) dominating the source counts to the minimum flux explored by this analysis. We estimate the density of sources detectable in blind surveys that will be performed in the coming years by the Cherenkov Telescope Array.

2FHL: The Second Catalog of Hard Fermi-LAT Sources

Journal Paper
Ackermann, M.; Ajello, M.; Atwood, W. B.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; et al
ApJS, 2016, 222, 5

We present a catalog of sources detected above 50 GeV by the Fermi-Large Area Telescope (LAT) in 80 months of data. The newly delivered Pass 8 event-level analysis allows the detection and characterization of sources in the 50 GeV-2 TeV energy range. In this energy band, Fermi-LAT has detected 360 sources, which constitute the second catalog of hard Fermi-LAT sources (2FHL). The improved angular resolution enables the precise localization of point sources (˜1.‧7 radius at 68% C. L.) and the detection and characterization of spatially extended sources. We find that 86% of the sources can be associated with counterparts at other wavelengths, of which the majority (75%) are active galactic nuclei and the rest (11%) are Galactic sources. Only 25% of the 2FHL sources have been previously detected by Cherenkov telescopes, implying that the 2FHL provides a reservoir of candidates to be followed up at very high energies. This work closes the energy gap between the observations performed at GeV energies by Fermi-LAT on orbit and the observations performed at higher energies by Cherenkov telescopes from the ground.

The Origin of the Extragalactic Gamma-Ray Background and Implications for Dark Matter Annihilation

Journal Paper
Ajello, M.; Gasparrini, D.; Sánchez-Conde, M.; Zaharijas, G.; Gustafsson, M.; Cohen-Tanugi, J.; Dermer, C. D.; Inoue, Y.; Hartmann, D.; Ackermann, M.; Bechtol, K.; Franckowiak, A.; Reimer, A.; Romani, R. W.; Strong, A. W.
ApJ Letters, 800, 27

The origin of the extragalactic γ-ray background (EGB) has been debated for some time. The EGB comprises the γ-ray emission from resolved and unresolved extragalactic sources, such as blazars, star-forming galaxies, and radio galaxies, as well as radiation from truly diffuse processes. This Letter focuses on the blazar source class, the most numerous detected population, and presents an updated luminosity function and spectral energy distribution model consistent with the blazar observations performed by the Fermi-Large Area Telescope (LAT). We show that blazars account for 50-11+12% of the EGB photons (>0.1 GeV), and that Fermi-LAT has already resolved ∼70% of this contribution. Blazars, and in particular hard-spectrum sources such as BL Lacs, are responsible for most of the EGB emission above 100 GeV. We find that the extragalactic background light, which attenuates blazars’ high-energy emission, is responsible for the high-energy cutoff observed in the EGB spectrum. Finally, we show that blazars, star-forming galaxies, and radio galaxies can naturally account for the amplitude and spectral shape of the background in the 0.1–820 GeV range, leaving only modest room for other contributions. This allows us to set competitive constraints on the dark matter annihilation cross section.

Spectral Analysis of Fermi-LAT Blazars above 50 GeV

Journal Paper
Domínguez, Alberto & Ajello, Marco
ApJL, 813, 34

We present an analysis of the intrinsic (unattenuated by the extragalactic background light, EBL) power-law spectral indices of 128 extragalactic sources detected up to z∼ 2 with the Fermi-Large Area Telescope (LAT) at very high energies (VHEs, E≥slant 50 GeV). The median of the intrinsic index distribution is 2.20 (versus 2.54 for the observed distribution). We also analyze the observed spectral breaks (i.e., the difference between the VHE and high energy, HE, 100 {MeV}≤slant E≤slant 300 {{GeV}}, spectral indices). The Fermi-LAT has now provided a large sample of sources detected both at VHE and HE with comparable exposure that allows us to test models of extragalactic γ-ray photon propagation. We find that our data are compatible with simulations that include intrinsic blazar curvature and EBL attenuation. There is also no evidence of evolution with redshift of the physics that drives the photon emission in high-frequency synchrotron peak (HSP) blazars. This makes HSP blazars excellent probes of the EBL.

The Cosmic Evolution of Fermi BL Lac Objects

Journal Paper
Ajello, M.; Romani, R. W.; Gasparrini, D.; et al.
2014, APJ, 780,73

Fermi has provided the largest sample of γ-ray-selected blazars to date. In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by Fermi during its first year of operation. We obtained redshift constraints for 206 out of the 211 BL Lac objects in our sample, making it the largest and most complete sample of BL Lac objects available in the literature. We use this sample to determine the luminosity function of BL Lac objects and its evolution with cosmic time. We find that for most BL Lac classes the evolution is positive, with a space density peaking at modest redshift (z ≈ 1.2). Low-luminosity, high-synchrotron-peaked (HSP) BL Lac objects are an exception, showing strong negative evolution, with number density increasing for z <~ 0.5. Since this rise corresponds to a drop-off in the density of flat-spectrum radio quasars (FSRQs), a possible interpretation is that these HSPs represent an accretion-starved end state of an earlier merger-driven gas-rich phase. We additionally find that the known BL Lac correlation between luminosity and photon spectral index persists after correction for the substantial observational selection effects with implications for the so-called “blazar sequence.” Finally, by estimating the beaming corrections to the luminosity function, we find that BL Lac objects have an average Lorentz factor of \gamma =6.1^{+1.1}_{-0.8}, and that most are seen within 10° of the jet axis.

The Fermi All-sky Variability Analysis: A List of Flaring Gamma-Ray Sources and the Search for Transients in Our Galaxy

Journal Paper
Buheler R., Ajello, M., Allafort A. for the Fermi-LAT Collaboration
2013, ApJ, 771, 57

In this paper, we present the Fermi All-sky Variability Analysis (FAVA), a tool to systematically study the variability of the gamma-ray sky measured by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. For each direction on the sky, FAVA compares the number of gamma-rays observed in a given time window to the number of gamma-rays expected for the average emission detected from that direction. This method is used in weekly time intervals to derive a list of 215 flaring gamma-ray sources. We proceed to discuss the 27 sources found at Galactic latitudes smaller than 10° and show that, despite their low latitudes, most of them are likely of extragalactic origin.

The Luminosity Function of Fermi-detected Flat-spectrum Radio Quasars

Journal Paper
Ajello, M.; Shaw, M. S.; Romani, R. W.; Dermer, C. D.; Costamante, L.; King, O. G.; Max-Moerbeck, W.; Readhead, A.; Reimer, A.; Richards, J. L.; Stevenson, M.
2012, ApJ, 751, 108

Fermi has provided the largest sample of γ-ray-selected blazars to date. In this work we use a complete sample of flat spectrum radio quasars (FSRQs) detected during the first year of operation to determine the luminosity function (LF) and its evolution with cosmic time. The number density of FSRQs grows dramatically up to redshift ~0.5-2.0 and declines thereafter. The redshift of the peak in the density is luminosity dependent, with more luminous sources peaking at earlier times; thus the LF of γ-ray FSRQs follows a luminosity-dependent density evolution similar to that of radio-quiet active galactic nuclei. Also, using data from the Swift Burst Alert Telescope we derive the average spectral energy distribution (SED) of FSRQs in the 10 keV-300 GeV band and show that there is no correlation between the luminosity at the peak of the γ-ray emission component and its peak frequency. Using this luminosity-independent SED with the derived LF allows us to predict that the contribution of FSRQs to the Fermi isotropic γ-ray background is 9.3+1.6 – 1.0% (±3% systematic uncertainty) in the 0.1-100 GeV band. Finally we determine the LF of unbeamed FSRQs, finding that FSRQs have an average Lorentz factor of γ = 11.7+3.3 – 2.2, that most are seen within 5° of the jet axis, and that they represent only ~0.1% of the parent population.

The Deep Look at the Hard X-Ray Sky: The Swift-INTEGRAL X-Ray (SIX) Survey

Journal Paper
Bottacini, Eugenio; Ajello, Marco; Greiner, Jochen
2012, ApJS, 201, 34

The supermassive black holes at the center of active galactic nuclei (AGNs) are surrounded by obscuring matter that can block nuclear radiation. Depending on the amount of blocked radiation, the flux from the AGN can be too faint to be detected by currently flying hard X-ray (above 15 keV) missions. At these energies only ~1% of the intensity of the cosmic X-ray background (CXB) can be resolved into point-like sources that are AGNs. In this work, we address the question of undetected sources contributing to the CXB with a very sensitive and new hard X-ray survey: the Swift-INTEGRAL X-ray survey, which is obtained with the new approach of combining the Swift/BAT and INTEGRAL/IBIS X-ray observations. We merge the observations of both missions, which enhances the exposure time and reduces systematic uncertainties. As a result, we obtain a new survey over a wide sky area of 6200 deg2 that is more sensitive than the surveys of Swift/BAT or INTEGRAL/IBIS alone. Our sample comprises 113 sources: 86 AGNs (Seyfert-like and blazars), 5 galaxies, 2 clusters of galaxies, 3 Galactic sources, 3 previously detected unidentified X-ray sources, and 14 unidentified sources. The scientific outcome from the study of the sample has been properly addressed to study the evolution of AGNs at redshift below 0.4. We do not find any evolution using the 1/V max method. Our sample of faint sources is a suitable target for the new generation hard X-ray telescopes with focusing techniques.

The 60 Month All-sky Burst Alert Telescope Survey of Active Galactic Nucleus and the Anisotropy of nearby AGNs

Journal Paper
Ajello, M.; Alexander, D. M.; Greiner, J.; Madejski, G. M.; Gehrels, N.; Burlon, D.
2012, ApJ, 749, 21

Surveys above 10 keV represent one of the best resources to provide an unbiased census of the population of active galactic nuclei (AGNs). We present the results of 60 months of observation of the hard X-ray sky with Swift/Burst Alert Telescope (BAT). In this time frame, BAT-detected (in the 15-55 keV band) 720 sources in an all-sky survey of which 428 are associated with AGNs, most of which are nearby. Our sample has negligible incompleteness and statistics a factor of ~2 larger over similarly complete sets of AGNs. Our sample contains (at least) 15 bona fide Compton-thick AGNs and 3 likely candidates. Compton-thick AGNs represent ~5% of AGN samples detected above 15 keV. We use the BAT data set to refine the determination of the log N-log S of AGNs which is extremely important, now that NuSTAR prepares for launch, toward assessing the AGN contribution to the cosmic X-ray background. We show that the log N-log S of AGNs selected above 10 keV is now established to ~10% precision. We derive the luminosity function of Compton-thick AGNs and measure a space density of 7.9+4.1 – 2.9 × 10-5 Mpc-3 for objects with a de-absorbed luminosity larger than 2 × 1042 erg s-1. As the BAT AGNs are all mostly local, they allow us to investigate the spatial distribution of AGNs in the nearby universe regardless of absorption. We find concentrations of AGNs that coincide spatially with the largest congregations of matter in the local (<=85 Mpc) universe. There is some evidence that the fraction of Seyfert 2 objects is larger than average in the direction of these dense regions.

The Imprint of the Extragalactic Background Light in the Gamma-Ray Spectra of Blazars

Journal Paper
Ajello, M., Buehler R., Reimer A., for the Fermi-LAT Collaboration
2012, Science, 338, 1190A

The light emitted by stars and accreting compact objects through the history of the universe is encoded in the intensity of the extragalactic background light (EBL). Knowledge of the EBL is important to understand the nature of star formation and galaxy evolution, but direct measurements of the EBL are limited by galactic and other foreground emissions. Here, we report an absorption feature seen in the combined spectra of a sample of gamma-ray blazars out to a redshift of z~1.6. This feature is caused by attenuation of gamma rays by the EBL at optical to ultraviolet frequencies and allowed us to measure the EBL flux density in this frequency band.

Three-year Swift-BAT Survey of Active Galactic Nuclei: Reconciling Theory and Observations?

Journal Paper
Burlon, D.; Ajello, M.; Greiner, J.; Comastri, A.; Merloni, A.; Gehrels, N.
2011, ApJ, 728, 58

It is well accepted that unabsorbed as well as absorbed active galactic nuclei (AGNs) are needed to explain the nature and shape of the Cosmic X-ray background (CXB), even if the fraction of highly absorbed objects (dubbed Compton-thick sources) still substantially escapes detection. We derive and analyze the absorption distribution using a complete sample of AGNs detected by Swift-BAT in the first three years of the survey. The fraction of Compton-thick AGNs represents only 4.6% of the total AGN population detected by Swift-BAT. However, we show that once corrected for the bias against the detection of very absorbed sources the real intrinsic fraction of Compton-thick AGNs is 20+9 -6%. We proved for the first time (also in the Burst Alert Telescope (BAT) band) that the anti-correlation of the fraction of absorbed AGNs and luminosity is tightly connected to the different behavior of the X-ray luminosity functions (XLFs) of absorbed and unabsorbed AGNs. This points toward a difference between the two subsamples of objects with absorbed AGNs being, on average, intrinsically less luminous than unobscured ones. Moreover, the XLFs show that the fraction of obscured AGNs might also decrease at very low luminosity. This can be successfully interpreted in the framework of a disk cloud outflow scenario as the disappearance of the obscuring region below a critical luminosity. Our results are discussed in the framework of population synthesis models and the origin of the CXB.

Fueling Lobes of Radio Galaxies: Statistical Particle Acceleration and the Extragalactic γ-ray Background

Journal Paper
Massaro, F.; Ajello, M.
2011, ApJL, 729, 12

The recent discovery of the γ-ray emission from the lobes of the closest radio galaxy Centaurus A by Fermi implies the presence of high-energy electrons at least up to γ ~ 105-106. These high-energy electrons are required to interpret the observed γ-ray radiation in terms of inverse Compton emission off the cosmic microwave background (IC/CMB), the widely accepted scenario to describe the X-ray emission of radio galaxy lobes. In this Letter, we consider the giant radio lobes of FR II radio galaxies showing that it is possible to maintain electrons at energies γ ~ 105-106, assuming an acceleration scenario (driven by turbulent magnetic fields) that compensates radiative losses. In addition, we consider the contribution to the diffuse extragalactic γ-ray background due to the IC/CMB emission of FR IIs’ lobes, showing its relevance in the keV to MeV energy range.

The Fermi-LAT High-Latitude Survey: Source Count Distributions and the Origin of the Extragalactic Diffuse Background

Journal Paper
Ajello M., Tramacere, A., for the Fermi-LAT Collaboration
2010, ApJ, 720, 435

This is the first of a series of papers aimed at characterizing the populations detected in the high-latitude sky of the Fermi-LAT survey. In this work, we focus on the intrinsic spectral and flux properties of the source sample. We show that when selection effects are properly taken into account, Fermi sources are on average steeper than previously found (e.g., in the bright source list) with an average photon index of 2.40 ± 0.02 over the entire 0.1-100 GeV energy band. We confirm that flat spectrum radio quasars have steeper spectra than BL Lacertae objects with an average index of 2.48 ± 0.02 versus 2.18 ± 0.02. Using several methods, we build the deepest source count distribution at GeV energies, deriving that the intrinsic source (i.e., blazar) surface density at F 100 >= 10-9 ph cm-2 s-1 is 0.12+0.03 -0.02 deg-2. The integration of the source count distribution yields that point sources contribute 16(±1.8)% (±7% systematic uncertainty) of the GeV isotropic diffuse background. At the fluxes currently reached by LAT, we can rule out the hypothesis that pointlike sources (i.e., blazars) produce a larger fraction of the diffuse emission.

Galaxy Clusters in the Swift/BAT Era. II. 10 More Clusters Detected Above 15 keV

Journal Paper
Ajello, M.; Rebusco, P.; Cappelluti, N.; Reimer, O.; Böhringer, H.; La Parola, V.; Cusumano, G.
2010, ApJ, 725, 1688

We report on the discovery of 10 additional galaxy clusters detected in the ongoing Swift/Burst Alert Telescope (BAT) all-sky survey. Among the newly BAT-discovered clusters there are Bullet, A85, Norma, and PKS 0745-19. Norma is the only cluster, among those presented here, which is resolved by BAT. For all the clusters, we perform a detailed spectral analysis using XMM-Newton and Swift/BAT data to investigate the presence of a hard (non-thermal) X-ray excess. We find that in most cases the clusters’ emission in the 0.3-200 keV band can be explained by a multi-temperature thermal model confirming our previous results. For two clusters (Bullet and A3667), we find evidence for the presence of a hard X-ray excess. In the case of the Bullet cluster, our analysis confirms the presence of a non-thermal, power-law-like, component with a 20-100 keV flux of 3.4 × 10-12 erg cm-2 s-1 as detected in previous studies. For A3667, the excess emission can be successfully modeled as a hot component (kT ~ 13 keV). We thus conclude that the hard X-ray emission from galaxy clusters (except the Bullet) has most likely a thermal origin.

PKS 0537-286, carrying the information of the environment of SMBHs in the early Universe

Journal Paper
Bottacini, E.; Ajello, M.; Greiner, J.; Pian, E.; Rau, A.; Palazzi, E.; Covino, S.; Ghisellini, G.; Krühler, T.; Küpcü Yoldaş, A.; Cappelluti, N.; Afonso, P.
2010, A&A, 509, 69

Context. The high-redshift (z = 3.1) blazar PKS 0537-286, belonging to the flat spectrum radio quasar blazar subclass, is one of the most luminous active galactic nuclei (AGN) in the Universe. Blazars are very suitable candidates for multiwavelength observations. Indeed, the relativistic beaming effect at work within the jet enhances their luminosity. This in turn allows the properties of the extragalactic jets, the powering central engine, and the surrounding environment to be derived.
Aims: Our aim is to present the results of a multifrequency campaign from the near-IR to hard X-ray energies on PKS 0537-286 and give insight into the physical environment where the radiation processes take place.
Methods: We observed the source at different epochs from 2006 to 2008 with INTEGRAL and Swift, and nearly simultaneously with ground-based optical telescopes. We also analyzed two archival spectra taken with XMM-Newton in 1999 and 2005. A comparative analysis of the results is performed.
Results: The X-ray continuum of the blazar, as sampled by XMM, is described by a power law of index Γ = 1.2, modified by variable absorption at the soft X-rays, as found in other high-redshift QSOs. Modest X-ray continuum variability is found in the Swift observations. The combined Swift/BAT and Swift/XRT spectrum is very hard (Γ = 1.3). This, together with the non simultaneous EGRET detection and the more recent non detection by Fermi-LAT, constrains the peak of the high-energy component robustly. The optical/UV data, heavily affected by intervening Ly α absorption, indicate the presence of a bright thermal accretion disk that decreased in luminosity between 2006 and 2008. We infer from this a reduction of the BLR radius. When taking this into account, the 2006 and 2008 SEDs are compatible with a model based on synchrotron radiation and external inverse Compton scattering where the accretion-disk luminosity decreases between the 2 epochs by a factor 2, while the bulk Lorentz factor remains unchanged and the magnetic field changed only marginally.

The Evolution of Swift/BAT Blazars and the Origin of the MeV Background

Journal Paper
Ajello, M.; Costamante, L.; Sambruna, R. M.; Gehrels, N.; Chiang, J.; Rau, A.; Escala, A.; Greiner, J.; Tueller, J.; Wall, J. V.; Mushotzky, R. F.
2009, ApJ, 699, 603

We use three years of data from the Swift/Burst Alert Telescope (BAT) survey to select a complete sample of X-ray blazars above 15 keV. This sample comprises 26 flat-spectrum radio quasars (FSRQs) and 12 BL Lacertae (BL Lac) objects detected over a redshift range of 0.03 < z < 4.0. We use this sample to determine, for the first time in the 15-55 keV band, the evolution of blazars. We find that, contrary to the Seyfert-like active galactic nuclei (AGNs) detected by BAT, the population of blazars shows strong positive evolution. This evolution is comparable to the evolution of luminous optical quasi-stellar objects (QSOs) and luminous X-ray-selected AGNs. We also find evidence for an epoch dependence of the evolution as determined previously for radio-quiet AGNs. We interpret both these findings as a strong link between accretion and jet activity. In our sample, the FSRQs evolve strongly, while our best fit shows that BL Lac objects might not evolve at all. The blazar population accounts for 10%-20% (depending on the evolution of the BL Lac objects) of the cosmic X-ray background (CXB) in the 15-55 keV band. We find that FSRQs can explain the entire CXB emission for energies above 500 keV solving the mystery of the generation of the MeV background. The evolution of luminous FSRQs shows a peak in redshift (zc = 4.3 ± 0.5) which is larger than the one observed in QSOs and X-ray-selected AGNs. We argue that FSRQs can be used as tracers of massive elliptical galaxies in the early universe.

Galaxy Clusters in the Swift/Burst Alert Telescope Era: Hard X-rays in the Intracluster Medium

Journal Paper
Ajello, M.; Rebusco, P.; Cappelluti, N.; Reimer, O.; Böhringer, H.; Greiner, J.; Gehrels, N.; Tueller, J.; Moretti, A.
2009, ApJ, 690, 367

We report the detection of 10 clusters of galaxies in the ongoing Swift/Burst Alert Telescope (BAT) all-sky survey. This sample, which mostly comprises merging clusters, was serendipitously detected in the 15-55 keV band. We use the BAT sample to investigate the presence of excess hard X-rays above the thermal emission. The BAT clusters do not show significant (e.g., >=2σ) nonthermal hard X-ray emission. The only exception is represented by Perseus whose high-energy emission is likely due to NGC 1275. Using XMM-Newton, Swift/XRT, Chandra and BAT data, we are able to produce upper limits of the inverse Compton (IC) emission mechanism which are in disagreement with most of the previously-claimed hard X-ray excesses. The coupling of the X-ray upper limits of the IC mechanism to radio data shows that, in some clusters, the magnetic field might be larger than 0.5 μG. We also derive the first log N-log S and luminosity function distributions of galaxy clusters above 15 keV.

The Swift BAT X-Ray Survey. III. X-Ray Spectra and Statistical Properties

Journal Paper
Ajello, M.; Rau, A.; Greiner, J.; Kanbach, G.; Salvato, M.; Strong, A. W.; Barthelmy, S. D.; Gehrels, N.; Markwardt, C. B.; Tueller, J.
2008, ApJ, 673, 96

In this concluding part of the series of three papers dedicated to the Swift BAT hard X-ray survey (BXS), we focus on the X-ray spectral analysis and statistical properties of the source sample. Using a dedicated method to extract time-averaged spectra of BAT sources, we show that Galactic sources have, generally, softer spectra than extragalactic objects and that Seyfert 2 galaxies are harder than Seyfert 1s. The averaged spectrum of all Seyfert galaxies is consistent with a power-law with a photon index of 2.00+/-0.07. The cumulative flux-number relation for the extragalactic sources in the 14-170 keV band is best described by a power-law with a slope α=1.55+/-0.20 and a normalization of 9.6+/-1.9×10-3 AGNs deg-2 (or 396+/-80 AGNs all-sky) above a flux level of 2×10-11 ergs cm-2 s-1 (~0.85 mcrab). The integration of the cumulative flux per unit area indicates that BAT resolves 1%-2% of the X-ray background emission in the 14-170 keV band. A subsample of 24 extragalactic sources above the 4.5 σ detection limit is used to study the statistical properties of AGNs. This sample is composed of local Seyfert galaxies (z=0.026, median value) and ~10% blazars. We find that 55% of the Seyfert galaxies are absorbed by column densities of NH>1022 H atoms cm-2 but that none is genuinely bona fide Compton thick. This study shows the capabilities of BAT to probe the hard X-ray sky to the millicrab level.

Cosmic X-Ray Background and Earth Albedo Spectra with Swift BAT

Journal Paper
Ajello, M.; Greiner, J.; Sato, G.; Willis, D. R.; Kanbach, G.; Strong, A. W.; Diehl, R.; Hasinger, G.; Gehrels, N.; Markwardt, C. B.; Tueller, J.
2008, ApJ, 689, 666

We use Swift BAT Earth occultation data at different geomagnetic latitudes to derive a sensitive measurement of the cosmic X-ray background (CXB) and of the Earth albedo emission in the 15-200 keV band. We compare our CXB spectrum with recent (INTEGRAL, BeppoSAX) and past results (HEAO-1) and find good agreement. Using an independent measurement of the CXB spectrum we are able to confirm our results. This study shows that the BAT CXB spectrum has a normalization ~8%+/-3% larger than the HEAO-1 measurement. The BAT accurate Earth albedo spectrum can be used to predict the level of photon background for satellites in low Earth and mid inclination orbits.

BAT X-Ray Survey. I. Methodology and X-Ray Identification

Journal Paper
Ajello, M.; Greiner, J.; Kanbach, G.; Rau, A.; Strong, A. W.; Kennea, J. A.
2008, ApJ, 678, 102

We applied the maximum likelihood (ML) method, as an image reconstruction algorithm, to the BAT X-Ray Survey (BXS). This method was specifically designed to preserve the full statistical information in the data and to avoid mosaicking of many exposures with different pointing directions, thus reducing systematic errors when co-adding images. We reconstructed, in the 14-170 keV energy band, the image of a 90×90 deg2 sky region, centered on (R.A., decl.)=(105deg, -25deg), which BAT surveyed with an exposure time of ~1 Ms (in 2005 November). The best sensitivity in our image is ~0.85 mcrab or 2.0×10-11 ergs cm-2. We detect 49 hard X-ray sources above the 4.5 σ level; of these, only 12 were previously known as hard X-ray sources (>15 keV). Swift XRT observations allowed us to firmly identify the counterparts for 15 objects, while 2 objects have Einstein IPC counterparts (Harris et al. 1990); in addition to those, we found a likely counterpart for 13 objects by correlating our sample with the ROSAT All-Sky Survey Bright Source Catalog (Voges et al. 1999). Seven objects remain unidentified. Analysis of the noise properties of our image shows that ~75% of the area is surveyed to a flux limit of ~1 mcrab. This study shows that the coupling of the ML method to the most sensitive, all-sky surveying, hard X-ray instrument, BAT, is able to probe for the first time the hard X-ray sky to the millicrab flux level. The successful application of this method to BAT demonstrates that it could also be applied with advantage to similar instruments such as INTEGRAL IBIS.