Photoionization calculations of the radiation force due to spectral lines in AGNs. (arXiv:1812.01773v1 [astro-ph.GA])

<a href="http://arxiv.org/find/astro-ph/1/au:+Dannen_R/0/1/0/all/0/1">Randall C. Dannen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Proga_D/0/1/0/all/0/1">Daniel Proga</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kallman_T/0/1/0/all/0/1">Timothy R. Kallman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Waters_T/0/1/0/all/0/1">Tim Waters</a>

One of the main physical mechanisms that could drive mass outflows in AGNs is

radiation pressure on spectral lines. Although this mechanism is conceptually

straightforward to understand, the actual magnitude of the radiation force is

challenging to compute because the force depends on the physical conditions in

the gas, as well as the strength, spectral energy distribution (SED), and

geometry of the radiation field. We present results from our photoionization

and radiation transfer calculations of the so-called force multiplier, $M$,

using the same radiation field to compute the gas photoionization and thermal

balance. Our method is general and can be used for an arbitrary SED. Here we

focus on describing results for two SEDs that correspond to a Type 1 and Type 2

AGN. We use the photoionization code XSTAR and take into account the most

up-to-date and complete atomic data and line list. Our main results are the

following: 1) for a fixed value of the optical depth parameter, $M$ is not a

monotonic function of the photoionization parameter $xi$. Although $M$ starts

to decrease with $xi$ for $xi> 1$ as shown by others, this decrease in our

calculations is relatively gradual and in fact $M$ can increase by a factor of

few at $xi approx 10-1000$ (details depend on the assumed SED). The main

dynamically relevant effect of this behavior is that the multiplier can stay

larger than 1 for $xi$ as high as $1000$; 2) at these same $xi$ for which the

multiplier is higher than in previous calculations, the gas is thermally

unstable to isobaric perturbations. We discuss implications of our results in

the context of AGN winds that are observed in the UV and X-ray bands, and we

confirm previous findings that the force multiplier depends on very many lines,

not a few very optically thick lines.

One of the main physical mechanisms that could drive mass outflows in AGNs is

radiation pressure on spectral lines. Although this mechanism is conceptually

straightforward to understand, the actual magnitude of the radiation force is

challenging to compute because the force depends on the physical conditions in

the gas, as well as the strength, spectral energy distribution (SED), and

geometry of the radiation field. We present results from our photoionization

and radiation transfer calculations of the so-called force multiplier, $M$,

using the same radiation field to compute the gas photoionization and thermal

balance. Our method is general and can be used for an arbitrary SED. Here we

focus on describing results for two SEDs that correspond to a Type 1 and Type 2

AGN. We use the photoionization code XSTAR and take into account the most

up-to-date and complete atomic data and line list. Our main results are the

following: 1) for a fixed value of the optical depth parameter, $M$ is not a

monotonic function of the photoionization parameter $xi$. Although $M$ starts

to decrease with $xi$ for $xi> 1$ as shown by others, this decrease in our

calculations is relatively gradual and in fact $M$ can increase by a factor of

few at $xi approx 10-1000$ (details depend on the assumed SED). The main

dynamically relevant effect of this behavior is that the multiplier can stay

larger than 1 for $xi$ as high as $1000$; 2) at these same $xi$ for which the

multiplier is higher than in previous calculations, the gas is thermally

unstable to isobaric perturbations. We discuss implications of our results in

the context of AGN winds that are observed in the UV and X-ray bands, and we

confirm previous findings that the force multiplier depends on very many lines,

not a few very optically thick lines.

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