Probing the Geometry and Physics of the Emission Region in Active Galactic Nuclei using hard X-ray and Gamma-ray observations
Natasha L. Woods
Office of Science, Science Undergraduate Laboratory Internship (SULI)
The University of Texas at Dallas
Stanford Linear Accelerator Center
Menlo Park, CA
August 14, 2009
Prepared in partial fulfillment of the requirements of the Office of Science, Department of Energy's Science Undergraduate Laboratory Internship under the direction of Marco Ajello and Masaaki Hayashida at the Kavli Institute for Particle Astrophysics and Cosmology, Stanford Linear accelerator Center.
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1. INTRODUCTION
The study of Active Galactic Nuclei (AGN) began as early as 1908 when Arthur Fath noticed six emission lines in the spectra of NGC 1068 that could not be attributed to any stellar processes. Less than two decades thereafter Carl Seyfert recognized similar emission lines in the spectra of spiral galaxies (Fig. 1). Over time galaxies with these emission lines were named AGN. These objects are characterized by the fact that the majority of their energy output are unrelated to stellar processes. Formally, AGN are compact regions of space at the center of galaxies that are exceedingly luminous over all or part of the electromagnetic spectrum [1].
1.1 AGN Classification
AGN are classified by their radio luminosity and spectral properties. Radio-loud AGN emit radio waves, radio-quiet have minimal or no radio flux. Additionally, if the source has a bright optical continuum with broad emission lines it is classified as Type 1. Sources that have a weak optical continuum and narrow emission lines are described as Type 2 (Fig. 1) [2].
1.2 Unified Model of AGN
The Unified Model of AGN posits that all AGN are the same. and the perceived differences can be attributed to the viewing orientation of AGN. In this model all AGN are axis symmetric and consist of a 106-10 M⊙super-massive black hole (SMBH) that actively accretes matter from a disk around it. Outside this disk is a molecular torus composed of gas and dust (Fig. 2). The viewing orientation of the torus is believed to obscure or reveal the luminous continuum emission from the SMBH and its accretion disk.
The orientation of the torus could create the observed differences between Seyfert 1 and 2 galaxies (Sy1 and Sy2 respectively), two classes of radio-quiet AGN. These objects and AGN in general are believed to have a broad line region (BLR) of 3,000-30,000K gas clouds orbiting the SMBH at 3000–10000 km/s, 0.05pc from the SMBH. Doppler shifting of the photons emitted from the BLR broadens emission lines from this region. Another region called the narrow line region (NLR) consists of gas clouds also orbiting the SMBH but 100-300pc from the black hole at 300-100 km/s which produce much narrower emission lines (Fig. 2) [1]. In Sy1 the torus is viewed face-on making both the NLR and BLR visible. Conversely, when the torus is viewed edge-on only the NLR are usually observed and the object is called a Sy2 (Fig. 3) [1].
Much evidence has been found to support this model. Antonucci and Miller found hidden polarized broad emission lines in Sy2 spectra in 1985, proving that Sy2 had BLR hidden by the torus [10]. The luminosity, rapid energy variability, and velocity dispersion of stars near SMBHs best match the observations of the central engine of AGN in the X-ray energy band[1].
However, opposing and unaccounted for evidence for this model exists. For instance, AGN have been observed to change from Sy1 to Sy2 and vice versa, which is unaccounted for in the Unified Model[12]. According to the Unified Model the classification of AGN depends only on orientation, which is constant. Therefore, AGN cannot change classes under this model. This suggests that other factors also determine AGN classification, such as the density and motions of material in the torus.
Another piece of contradictory evidence is the possible difference in electron temperatures in the coronas of Sy1 and Sy2 [1]. If the electron temperatures are truly different, the magnetic fields of Sy1 and Sy2 might be different, which the Unified Model does not mention. Essentially, this would suggest that these objects are not the same, namely their differences could not be attributed to their orientation alone as the Unified Model states.
Additionally, the fraction of obscured sources might depend on luminosity as well as orientation [3]. If this is true more luminous objects would tend to be unobscured due to their increased luminosity not just their orientation.
1.3 Investigating the Unified Model of AGN using X-rays
To further investigate this model the X-ray spectrum of AGN will be examined, which probes the innermost regions of AGN as X-rays are transparent to the torus (excluding Compton-thick sources, the population studied contains very few of these sources). X-ray emission from AGN are believed to be a product of inverse Compton radiation. In this process, photons from the accretion disk are upscattered by hot (possibly relativistic) electrons in a corona above the disk and torus in the X-ray energy band [1]. This corona is most likely heated via magnetic reconnection [5]. The origin and exact geometry of the hot corona remains highly debatable and will be further examined.
The light observed from AGN falls into three categories: direct, transmitted, and reflected. Direct light comes from photons scattered off the corona that are detected by an observer with no other noteworthy interactions. Transmitted light also originates from photons scattered off the corona, but these photons travel through the torus. Reflected light is scattered off the corona, reflected off the accretion disk or torus via inverse Compton reflection and later detected. This type of reflection peaks at 30 keV and then sharply declines. Consequently, the larger the reflection component the softer the spectrum in the 15-200 keV band. Due to the mere geometry of the model, Sy1 should have a larger reflection component than Sy2 in the Unified Model [10].
The reflection component of the total light emitted by AGN and the photon index (see Methods) are directly related. By examining the distribution of the photon indices the Unified model will be tested. If the photon indices of Sy1 and Sy2 are found to be different this could indicate that the two objects are intrinsically different and further disprove the Unified Model. Previous research using Swift-BAT, OSSE, and INTEGRAL have reached this conclusion [10]. The average covering factor and high energy cutoff of the reflective media in Sy1 and Sy2 will be determined (if the indices are different).
1.4 Relevance
Ultimately, the Unified Model of AGN will be tested. The geometry and temperature of electrons in the corona of Sy1 and Sy2 will be estimated and compared to determine if Sy1 and Sy2 are intrinsically different. This research matters as AGN are believed to account for 100% of the Cosmic X-ray Background (CXB) (Fig. 4). However, for this to be true a third of the AGN population should be Compton thick, and very few of these objects have been detected thus far. Other explanations of the CXB posit that AGN have a larger reflection component than previously estimated, and the sum of all AGN reflection peaks at ~30 keV will resolve the spectra feature produced by Compton-thick AGN. Better understanding AGN and their relationship to the CXB will help place constraints on cosmological models and black hole physics.
- METHODS
2.1 Swift-BAT Mission
Swift is a revolutionary multi-wavelength observatory focused on the gamma-ray burst science (GRB). Launched in 2004, Swift uses three instruments: the Burst Alert Telescope (BAT), the X-ray Telescope (XRT), and the Ultraviolet/Optical Telescope (UVOT), to monitor GRBs and afterglows in gamma-ray, X-ray, optical, and ultraviolet wavebands. BAT the largest instrument on board detects ~100 GRBs per year. Once a GRB is detected XRT and UVOT are aimed at the burst to help determine the location and spectra of the GRB. The data collected is circulated publicly worldwide for follow-up observations and studies. BAT is a coded mask telescope that utilizes a photon-counting CdZnTe detector with a 5200 cm2 detecting area in the 15-150 keV range [18]. BAT surveys the X-ray sky with unprecedented sensitivity due to its wide FOV and pointing strategy enabling it to continuously monitor ~80% of the sky daily. It has reached a sensitivity of ~1mCrab in 1Ms of exposure, making it a perfect instrument for studying objects whose emission is faint in hard X-rays such as AGN[20].
2.2 Data Sources
This study is different from previous work in that an unbiased sample (blind search) of ~200 sources from a three year exposure will be used from [20] (See Fig. 5&6 and Table 1). As mentioned before as the sources are being studied in the 15-200 keV range, absorption by the torus can be ignored as the torus in transparent to photons at this energy level, further strengthening the integrity of this sample. As seen in Fig. 5 Sy1 tend to have a larger luminosity and redshift than Sy2, suggesting that these two classes are not the same and that their central engines might be different.
Type / Sy1 / Sy2 / Other78 / 58 / 63
Table 1: Table with number of sources for each class in this paper. The “Other” class represents AGN between Sy1 and Sy2 (i.e. Sy1.5).
2.3 Data Manipulation in Xspec and Root
Once the sources were chosen, spectra were generated (see [20]). Using these spectra photon indices for each source were determined using a simple power law model from [20] in Xspec 11.3.2p (see (1)).
The spectra examined were fit to a power law such that:
(1) dN/DE=AE-Γ
where gamma is the photon index, A is the normalization, N is the number of photons and E is energy in keV.
This model takes the spectra entered by the user and determines the best fit using the chi-squared statistic for the data by varying model parameters (see Fig. 7). To better fit the model two calibration files were used. The first accounted for the detectors' effective area at given energy levels (Fig. 8) [20]. The second calibration file accounted for systematic errors as a function of detectors' energy level (Fig. 9) [20]. Using these two calibration files and the data from the Swift-BAT mission, Xspec11 outputs the photon index and its associated positive and negative error, the reduced chi-squared statistic among other quantities. The photon index is of particular interest to this research as it will help determine the angle subtended by the reflective media in AGN and whether or not Sy1 and Sy2 are the same, as the Unified Model states.
Moreover, to achieve this goal statistical analysis of the results will be done using Root and other applications in the HEAsoft package. For instance, the distribution of photon indices for each class will be determined and compared to examine the Unified Model.
REFERENCES
[1] Marco Ajello Swift/BAT studies of AGN and CXB
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[3] The Second INTEGRAL AGN Catalogue Beckmann 2009
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