X-ray variability of AGN

X-ray variability of AGN Magnus Axelsson October 20, 2006 Abstract X-ray variability has proven to be an effective diagnostic both for Galactic black-hole binaries and active galactic nuclei (AGN). This report gives a short overview of observations in the 1 10 kev band. The focus is primarily toward Seyfert 1 type galaxies, as they are generally the brightest. Both the RMS-flux relation and power density spectra of AGN are presented, and constraints on physical models are discussed. The results are briefly compared to the characteristics of black-hole binary systems, and some open questions are highlighted. 1 Introduction Active Galactic Nuclei (AGN) are among the most powerful objects observed. The central engine for these sources is thought to be a supermassive (M 10 5 M ) black hole accreting matter. X-ray radiation is generally believed to be emitted from the innermost region surrounding the black hole. As such, the variability properties of the X-ray emission provides a probe into the core of the AGN, and may provide constraints on physical models. 1.1 Schematic model The current model for the AGN engine is based primarily on spectroscopic observations. The components usually invoked are an accretion disc and a hot electron or positron/electron pair corona close to the black hole. The geometry is somewhat more uncertain. A schematic figure of the AGN engine is shown in Fig. 1. Figure 1: Schematic view of an AGN. Matter is accreted through an accretion disc onto a supermassive black hole. The X-ray emission is produced in the region closest to the black hole. The central engine may be surrounded by a toroidal structure, and our viewing angle will then determine whether the central engine is obscured or not [2]. The core of the AGN is believed to be a supermassive black hole accreting matter through an accretion disc. The disc is truncated at some radius close to the black hole (it may extend to the innermost stable orbit) and in the inner region is a hot, optically thin corona, which may extend outwards, sandwiching the disc. Soft seed photons from the disc are Comptonized in the corona, producing the observed X-ray emission. Figure 1 also gives an illustration to the usual explanation for the different classes of AGN observed. When the AGN is viewed at low inclination, the central engine will be seen. As the inclination increases, 1

the core will be obscured by the surrounding torus, and the resulting emission will of course be fainter. Two types of galaxies explained by this picture are Seyfert 1 and Seyfert 2 sources, where Seyfert 2 sources are suggested to be obscured Seyfert 1s. Important to note is that if the difference between the classes is purely due to obscuration, their intrinsic properties, such as X-ray variability, should be the same. 1.2 Different types of variability The term (X-ray) variability can be used to refer to any change of the observed (X-ray) flux with time. In principle, this can be both changes in the spectral energy distribution (SED) of the source, as well as changes in the observed flux. This work will concentrate on the variability in the 1 10 kev range. As this band is rather narrow, the focus will lie on changes in the overall flux rather than in the SED. 1.3 Classes of AGN The term of AGN or Active Galactic Nucleus is a general one used to describe the core of a galaxy, which displays a larger than average luminosity that cannot be attributed directly to stars. There are several types of galaxies which display an AGN, but the most common is the Seyfert type galaxy. Other members of the AGN class are radio loud galaxies, quasars and blazars. Seyfert 1 sources are among the brightest AGN in X-ray emission, and therefore fairly well studied. The SED of these sources roughly fall into two categories, termed broad- and narrow-line Seyfert 1. Both these sources show similar variability properties, and no distinction will be made between them here. Furthermore, the results presented are all from Seyfert 1 type AGN. If the unification picture outlined above is correct, the intrinsic variability properties of Seyfert 2 type AGN are expected to be the same. Whether this is the case or not thus serves as a test for the interpretation of Seyfert 2 sources as obscured Seyfert 1. 2 Measurements of variability Since the X-ray emission from AGN is thought to originate in the region close to the central black hole, studying the X-ray variability is one of the most promising techniques used to derive the properties of the core. Early studies were mainly focused on variability on shorter timescales, but with monitoring instruments such as the All-Sky Monitor (ASM) aboard the RXTE satellite, studies of long timescales are now becoming feasible as well. AGN variability has been detected on all timescales, from hours to years. Figure 2 shows two example lightcurves from XMM-Newton and RXTE, illustrating this variability. Figure 2: Example lightcurves illustrating the X-ray variability of AGN. The left panel shows an observation made with XMM-Newton, covering about 1 day [6]. The right panel shows data from RXTE, covering a period of about 60 days [9]. As can be seen in the figure, the count rate can change by a factor of 2, on both long and short timescales. There have also been studies looking into the spectral variability of AGN, and searches for correlations with spectral parameters such as the spectral index [3]. There is a general trend that higher flux corresponds to softer spectra, i.e., relatively more flux at lower energies. Two mechanisms proposed to explain this are a two-component model and thermal Comptonization. The two-component model assumes that the SED is the result of a soft component of variable flux (but constant shape) and 2

a hard component with constant flux. An increase in flux would then mean an increase in the soft component, and thus a softer overall spectrum. In the thermal Comptonization model, the spectral index is determined by soft seed photons scattering in a hotter corona. If the soft seed photons increase, cooling of the corona becomes more efficient and the spectrum becomes softer [7]. 3 RMS-flux relation Figure 3: RMS of the Seyfert 1 galaxy SAXJ1808.4-3654 plotted against flux. The data is best fit by a linear relation [8]. When a lightcurve has been obtained from a source, a rough estimate of the variability can be obtained by simply calculating the root-mean-square (RMS) value. This value gives an indication of how much the flux varies about the mean, but does not say anything on what timescales the variability occurs (one large deviation can give the same RMS value as many small ones). An interesting result is however achieved when plotting the RMS against the flux. This is done in Fig. 3. The points in the figure show that there is a linear relationship between flux and RMS. The variability thus responds to flux changes on all timescales. 4 Power density spectra To better determine the variability on different timescales, a power density spectrum (PDS) may be constructed. This directly gives a measure of variability as a function of frequency. An example is shown in Fig. 4. At low frequencies, the PDS is rather flat, with an index of 1. However, there is a break and at higher frequencies the index steepens to 2. Thus the power of the faster variability is suppressed compared with that on longer timescales. This trend appears to be general in the PDS of AGN, and the break frequency is particularly interesting as it may be related with the size of the region emitting the X-rays. Another interesting feature of the PDS is its similarity to the PDS of X-ray binary systems in the Galaxy, thus raising the question whether similar physical processes are at work in the two systems, but at different scales. 5 Comparison with Black Hole binaries When comparing variability properties of AGN and black-hole binaries (BHBs), it is clear that there are strong similarities. Figure 5 shows such a comparison of the RMS-flux relationship and the PDS. Evident from the comparison is that both types of sources show a linear RMS-flux relationship. Perhaps Figure 4: Power density spectrum of the Seyfert 1.5 galaxy NGC 3227. The dashed line is a bending power-law model, with an index of -1 at lower frequencies. Above the bend frequency of 2.6 10 5 Hz, the index changes to -2. The solid line shows the predicted results of the model when considering the sampling of data points [9]. even more intriguing is that AGN in general show similar PDS as BHBs, but shifted to lower frequencies. However, this similarity only seems to apply to BHBs in the soft state. In the hard state, BHBs display a low-frequency break that (as yet) has not been seen in AGN. Further similarities between AGN and BHBs are suggested when plotting the black hole mass against the break frequency, shown in Fig. 6. In the figure, linear relationships have been extended from the measurements of Cygnus X-1, a well-studied BHB source, in its two different spectral states. Strikingly, the majority of AGN data points fall close to these two lines, despite the fact that their masses are orders of magnitude larger. 3

Figure 5: Top: Comparison of the RMS-flux relation in SAX J1808.4-3658 and the black-hole binary Cygnus X-1. Both display a linear relationship [10]. Bottom: Comparison of the PDS of two AGN and Cygnus X-1 in both hard and soft states. The PDS of the AGN both resemble that of Cygnus X-1 in the soft state, with a single break. The shift in frequency is expected as the AGN systems are on a much larger scale than that of Galactic sources [6]. 6 Physical models The information gained from the linear RMS-flux relationship and the PDS can be used to constrain physical models proposed to explain the X-ray emission in AGN. A linear RMS-flux relationship requires the emission processes to be multiplicative, thus ruling out models where the flux arises from several independent components. An example of this would be a scenario of multiple flares, where the individual flares do not correlate. A scenario, perhaps inspired by models of BHBs, which has been proposed for the X-ray emission is illustrated in Fig. 7. In this model, variability arises due to changes in the local mass accretion rate in the disc. These variations then propagate inwards, and are realized in the X-ray producing region the Comptonizing region close to the black hole [4]. The information from the PDS can also be used to gain information on physical parameters of the AGN. In particular, the break frequency is often associated with the inner edge of the accretion disc. Connecting the frequency to some timescale (e.g., thermal or viscous) of the inner disc, the break frequency gives a relation between the location of the region and the mass of the black hole. Given an independent estimate of the black hole mass, for instance from measurements of the velocity dispersion of stars orbiting the core, will then give the inward extent of the accretion disc. In addition, there is a trend toward more power at higher frequencies (a flatter PDS above the break) when studying higher energies. This could indicate variability in the corona, where the higher energies are produced, strengthening the connection between the break frequency and inner edge of the accretion disc [6]. 4

Akn564 Mkn766 NGC4051 MCG-6-30-15 Figure 6: Black hole mass plotted against PSD break timescale for several AGN. The two states of the Galactic source Cygnus X-1 are also shown, and lines with index 1.0 drawn through those points. The arrow labelled with ṁ indicates how the relation may shift in response to increases in the mass accretion rate [6]. Figure 7: Possible model for the X-ray variability of AGN. Local changes in the mass accretion rate propagate inwards in the disc, and are realized in the X-ray producing region near the black hole. This model predicts a linear relationship between the RMS and flux, as observed. Figure courtesy of P. Uttley/S. Vaughan. 7 Remaining issues Although many observations have been made, and physical models exist for the X-ray emission in AGN, work still remains to be done. One question is whether the models outlined here, proposed for Seyfert 1 type galaxies, are also valid for Seyfert 2 types. There have not been many studies done on the variability of Seyfert 2 sources, and the results so far do not all support the same intrinsic variability of the two classes. Another issue is the connection between spectral state (as determined from the SED) and the temporal characteristics. This question is particularly relevant when comparing AGN to their Galactic analogues, BHBs. For BHB systems, there is a strong correlation between spectral state and appearance of the PDS. For AGN however, no source has thus far been shown to deviate significantly from the PDS observed in the soft state of BHBs. Despite this, several AGN have SEDs resembling those of BHBs in the hard state. Given the many similarities and indications of similar processes, why is the PDS is a reliable indicator of state in BHBs, but not in AGN? In the end, the study of X-ray variability in AGN (and in some respects also in BHBs) is still largely phenomenological. Until models can be constructed which explain both the observed energy distribution and variability, progress will continue to be somewhat slow. 5

8 Summary X-ray variability of AGN is an important tool for studying the region close to the central black hole, where the X-ray emission is produced. Observations show that AGN display significant variability on both long and short timescales. By studying the X-ray variability, a number of constraints can be inferred. The linear RMS-flux relation rules out models where the flux arises as a result of multiple independent flares. From the break frequency of the PDS it is possible to gain information on the extent of the inner part of the accretion disc. There are strong similarities between the temporal properties of AGN and black hole binaries. These suggest that similar physical processes are at work, but at different scales. However, in BHBs there is a strong correlation between PDS and spectral state, which does not appear to be true for AGN. As it probes the physical processes close to the black hole, X-ray variability is a valuable test for the unification model of AGN. Seyfert 2 galaxies are then obscured Seyfert 1 types, and thereby should have the same intrinsic variability. Current observations do not uniformly support this model. The study of X-ray variability in AGN is still somewhat phenomenological. However, progress is being made toward constructing models that account for both the spectral and temporal properties in these sources. As better instruments are built, X-ray variability will continue to play an important part in helping to understand the physics of AGN. References [1] Awaki, H., Murakami, H., Leighly, K., et al, 2005, ApJ, 632, 793 [2] Gandhi, P. Super Massive Black Holes, http://www-xray.ast.cam.ac.uk/ pg/nakshatra/nakshatra.html (2003). [3] Lamer, G., McHardy, I. M., Uttley, P., & Jahoda, K. 2003, MNRAS, 338, 323 [4] Lyubarskii, Y. E. 1997, MNRAS, 292, 679 [5] McHardy, I. M., Gunn, K. F., Uttley, P., & Goad, M. R. 2005, MNRAS, 359, 1469 [6] McHardy, I. M., Papadakis, I. E., Uttley, P., Page, M. J., & Mason, K. O. 2004, MNRAS, 348, 783 [7] Taylor, R. D., Uttley, P., & McHardy, I. M. 2003, MNRAS, 342, L31 [8] Uttley, P. 2004, MNRAS, 347, L61 [9] Uttley, P., & McHardy, I. M. 2005, MNRAS, 363, 586 [10] Uttley, P., & McHardy, I. M. 2001, MNRAS, 323, L26 6