The source of the X-ray photons may also provide additional problems for a simple picture of the transfer function. If a component of the X-ray photons are inverse Compton scattered UV seed photons then it follows that variability in the X-rays may be produced by variability in the UV continuum emission from the inner disk. Hence it is possible to imagine a scenario where UV continuum could be observed both leading and trailing the X-ray variability in time. A fraction of the UV photons are up-scattered to X-rays, which then impact the disk and produce a second contribution to the UV continuum, while the rest of the initial UV photons escape. Hence we can observe a UV emission leading the X-rays. This can be described by negative delays in the transfer function which cannot be produced by the emission mechanism adopted in the model
5.12 Future work
It would be desirable to have such high quality in both the X-ray and optical, for a larger sample of AGN. There are at least four AGN that are being monitored byKepler(Mushotzky et al., 2011) and so if additional simultaneous X-ray observations can be obtained then high quality observational transfer functions could be estimated. This would give an indication of whether the features of the transfer function seen in Zw-229 are seen in other systems and if so whether any correlations with the properties of the BH can be found (i.e. mass, luminosity). Additional data would also allow us to investigate whether the features in the transfer function are time variable. Unfortunately theKeplerdata processing means that only preselected targets in the field are monitored, meaning there is little scope to find additional AGN in archival data.
Multi wavelength observations at a variety of optical wavelengths would provide the abil- ity to substantiate the findings here that the bump is caused by an increase in the flaring. The transfer function is wavelength dependent and we have seen that it should have a different signature at different wavelengths. This data may allow a better determination of the true underlying structure.
In this work the continuum transfer function has been used but with the appropriate obser- vations an emission line transfer function can be obtained. Emission lines encode information not only about the geometry but also about the velocity structure. The doppler shift of the emission lines gives the line of sight velocity of the emitting material and so provides another window to investigate the structure of the accretion disk and its surrounds. Within the Monte Carlo approach adopted here it would be a simple matter to encode a velocity structure into
Chapter 5. Echo Mapping of Accretion Disks
the model and provide additional observables to be compared to data, however this would come at the cost of additional computation time.
In addition a very simple approach to the emission/absorption process is used here to obtain a first order approximation to the transfer function. Again, however, the utility of the Monte Carlo Radiative transfer approach means that it would be straight-forward to increase the complexity of the model to encompass more of the physical properties. For instance a disk structure from MHD simulations (Proga et al., 2000) could be used to self-consistently calculate the disk ionization structure, as well as including the effect of self illumination of the disk by inverse Compton scattered UV photons. Although this would vastly increase the time required for each simulation, and provide a larger number of free parameters, it may allow more insight into the physics of the accretion disk.
5.12.1 Summary
In this chapter I have investigated the continuum transfer function of a simple model for the accretion disks that are thought to surround and power AGN. Motivated by observations that differ from a simple flat accretion disk I look at the effect of flaring of the outer disk on the transfer function at a number of wavelengths. Results indicate that a change in the disk flaring parameter between the inner and outer disk can produce features in the transfer function that are qualitatively similar to those seen in the observations. I propose that the increase in the flaring could be a signature of the emergence of a disk wind or outflow as suggested by other authors.
6
Conclusions and Outlook
The work in this thesis has spanned around nine orders of magnitude in size, from 1 au to 10’s kpc. Its overarching theme has been the use of radiation transfer models to uncover structure from observations which may otherwise have gone unnoticed and to probe the impact of radiation on its surroundings. I have investigated the use of time resolved MCRT models to help uncover the structure of the inner regions of the accretion disks around super-massive black holes. By adopting a simple model for the structure of the X-ray emitting regions and the accretion disk it illuminates, I have been able to discern clues to the shape of the outer accretion disk. The observations along with these models suggest that a wind or outflow may emerge from the surface of the disk and alter the shape of the transfer function in an obvious way.
I have also attempted to assess the impact of massive stars on the large scale star formation process. By coupling a Monte Carlo photoionization code to a series of snapshots of a realistic numerical simulation of a star forming cloud within a spiral galaxy, I have determined the impact of photoionization feedback on the star formation process. The method has been tested against an SPH implementation of photoionization feedback and although the method
Chapter 6. Conclusions and Outlook
is not able to account for the dynamical effects of the ionized gas in inhibiting or triggering further star formation it does provide a first order estimate of the reduction in star formation due to massive stars. PI feedback is able to reduce the mass accreted by stellar sinks by 38% under the assumptions made here with many stellar clusters unable to effectively accrete gas. The maximum mass attained by a stellar cluster is reduced to 53% of its original value when PI feedback is included. I have also highlighted the possible issues with SFRs estimated from indicators which are only sensitive to clusters containing massive stars, such as Hα emission, when applied to the entire star forming region. These can lead to the SFR being underestimated unless under sampling of the high mass end of the IMF is taken into account. By utilizing observations covering a wide range in wavelength to evaluate the structure of the dusty ISM in three LSB disk galaxies I have studied the structure of the dusty ISM. The results indicate that the dusty ISM has a different global structure to that seen in normal higher mass disk galaxies, with scale heights equal to or exceeding the stellar scale heights. This suggests that the cold ISM of low mass, LSB galaxies may be stable against axisymmetic perturbations leading to conditions less favorable for high SFRs.
6.1 Outlook
The Monte Carlo methods used in this work are likely to become more and more powerful in the future. At their heart they require computations of very large numbers of simple calcula- tions. The increasing power of even desktop computers is opening up new possibilities of the kind of problems which can be investigated. Increasing use of parallel and distributed com- puting can be naturally applied to many Monte Carlo problems. In addition to astronomy the field of medical science is also making increased use of Monte Carlo methods to produce new non-invasive diagnosis and measurement techniques. It is encouraging to think that methods developed to study distant astronomical phenomena can find applications able to provide a real benefit to people lives.
The integration of radiative transfer methods with numerical hydrodynamics codes pro- vides an exciting opportunity to produce fully self consistent models with a wide range of observables. Star formation regions may be modeled with a full treatment of feedback from massive stars to produce multi-wavelength views from the UV to the sub-mm, allowing a greater understanding of the physical processes behind observations. Some progress in imple- menting radiative transfer in hydrodynamical codes has already been made (Dale & Bonnell, 2011; Vazquez Semadeni et al., 2010; Nayakshin et al., 2009). Many such problems require
6.1. Outlook
computations over many orders of magnitude in size and the use of techniques like adaptive mesh refinement will be required to make such computations manageable.
The introduction of new observatories and instruments will provide a wealth of new data to motivate and constrain new models. The Herschel archive along with new observations by the Atacama Large Millimeter Array (ALMA) and the Scuba-2 camera at the James Clark Maxwell Telescope will provide a wealth of data in the FIR and sub-mm. These data will allow further investigation of the dusty ISM in galaxies throughout the Universe providing clues to star formation processes and the impact of massive stars.
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