Thermal Instability in Radiation Hydrodynamics: Instability Mechanisms, Position-dependent S-curves, and Attenuation Curves

Proga, Daniel; Waters, Tim; Dyda, Sergei; Zhu, Zhaohuan

doi:10.3847/2041-8213/ac87b0

“…In this paper, we measure variability in the covering fraction and N H of the X-ray obscurer, presumably a disk wind, in Mrk 817 using individual NICER observations. These open a new doorway for testing time-dependent photoionization models of warm absorbers, such as those proposed by García et al (2013), Proga et al (2022), andSadaula et al (2022), by measuring the effect of X-ray source variability on the ionization of the obscuring gas.…”

Section: Introductionmentioning

confidence: 99%

AGN STORM 2. III. A NICER View of the Variable X-Ray Obscurer in Mrk 817

Partington

¹

,

Cackett

²

,

Kara

³

et al. 2023

ApJ

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The AGN STORM 2 Collaboration targeted the Seyfert 1 galaxy Mrk 817 for a year-long multiwavelength, coordinated reverberation mapping campaign including Hubble Space Telescope, Swift, XMM-Newton, NICER, and ground-based observatories. Early observations with NICER and XMM revealed an X-ray state 10 times fainter than historical observations, consistent with the presence of a new dust-free, ionized obscurer. The following analysis of NICER spectra attributes variability in the observed X-ray flux to changes in both the column density of the obscurer by at least one order of magnitude (N H ranges from 2.85 − 0.33 + 0.48 × 10 22 cm − 2 to 25.6 − 3.5 + 3.0 × 10 22 cm − 2 ) and the intrinsic continuum brightness (the unobscured flux ranges from 10−11.8 to 10−10.5 erg s−1 cm−2). While the X-ray flux generally remains in a faint state, there is one large flare during which Mrk 817 returns to its historical mean flux. The obscuring gas is still present at lower column density during the flare, but it also becomes highly ionized, increasing its transparency. Correlation between the column density of the X-ray obscurer and the strength of UV broad absorption lines suggests that the X-ray and UV continua are both affected by the same obscuration, consistent with a clumpy disk wind launched from the inner broad-line region.

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“…At the same time, many recent works [e.g. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] support the original claim by Parker [17] that thermal instability is a fundamental process that can trigger in situ condensations, like prominences and coronal rain in the solar atmosphere, but also denser clumps in galactic winds, or in other astrophysical contexts [18][19][20]. For this reason, the linear mode which grows exponentially due to the energy loss is usually called 'thermal' or 'condensation' mode (see [21]).…”

Section: Introductionmentioning

confidence: 66%

Radiative loss and ion-neutral collisional effects in astrophysical plasmas

Popescu Braileanu,

Keppens

2024

Phil. Trans. R. Soc. A.

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In this paper, we study the role of radiative cooling (RC) in a two-fluid model consisting of coupled neutrals and charged particles. We first analyse the linearized two-fluid equations where we include radiative losses in the energy equation for the charged particles. In a one-dimensional geometry for parallel propagation and in the limiting cases of weak and strong coupling, it can be shown analytically that the instability conditions for the thermal mode and the sound waves, the isobaric and isentropic criteria, respectively, remain unchanged with respect to one-fluid radiative plasmas. For the parameters considered in this paper, representative for the solar corona, the RC produces growth of the thermal mode and damping of the sound waves. In the weak coupling limit, the growth of the thermal instability and the damping of the sound waves is as derived in Field (Field 1965 Astrophys. J. 142 , 531 ( doi:10.1086/148317 )) using the charged fluid properties. When neutrals are included and are sufficiently coupled to the charges, the thermal mode growth rate and the wave damping both reduce by the same factor, which depends on the ionization fraction only. For a heating function that is constant in time, we find that the growth of the thermal mode and the damping of the sound waves are slightly larger. The numerical calculation of the eigenvalues of the general system of equations in a three-dimensional geometry confirm the analytic results. We then run two-dimensional fully nonlinear simulations that give consistent results: a higher ionization fraction or lower coupling will increase the growth rate. The magnetic field contribution is negligible in the linear phase. Ionization-recombination effects might play an important role because the RC produces a large range of temperatures in the system. In the numerical simulation, after the first condensation phase, when the minimum temperature is reached, the fraction of neutrals increases four orders of magnitude because of the recombination. This article is part of the theme issue ‘Partially ionized plasma of the solar atmosphere: recent advances and future pathways’.

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“…The shift in the spectral features due to these effects will be small compared with our spectral resolution. On the other hand, for a supersonic flow, such as in a warm absorber, the bulk forces, whatever they are, dominate over internal dynamics in the cloud (Proga et al 2022). We acknowledge this effect can be important.…”

Section: Assumptionsmentioning

confidence: 99%

Time-dependent Photoionization Modeling of Warm Absorbers in Active Galactic Nuclei

R

¹

,

Bautista

²

,

Garcia

³

et al. 2023

ApJ

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Warm absorber spectra contain bound-bound and bound-free absorption features seen in the X-ray and UV spectra from many active galactic nuclei. The widths and centroid energies of these features indicate they occur in outflowing gas, and the outflow can affect the gas within the host galaxy. Thus, the warm absorber mass and energy budgets are of great interest. Estimates for these properties depend on models that connect the observed strengths of the absorption features with the density, composition, and ionization state of the absorbing gas. Such models assume that the ionization and heating of the gas come primarily from the strong continuum near the central black hole. They also assume that the various heating, cooling, ionization, and recombination processes are in a time-steady balance. This assumption may not be valid, owing to the intrinsic time variability of the illuminating continuum or other factors that change the cloud environment. This paper presents models for warm absorbers that follow the time dependence of the ionization, temperature, and radiation field in warm absorber gas clouds in response to a changing continuum illumination. We show that the effects of time variability are important over a range of parameter values, that time-dependent models differ from equilibrium models in meaningful ways, and that these effects should be included in models that derive properties of warm absorber outflows.

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Thermal Instability in Radiation Hydrodynamics: Instability Mechanisms, Position-dependent S-curves, and Attenuation Curves

Cited by 9 publications

References 42 publications

AGN STORM 2. III. A NICER View of the Variable X-Ray Obscurer in Mrk 817

AGN STORM 2. III. A NICER View of the Variable X-Ray Obscurer in Mrk 817

Radiative loss and ion-neutral collisional effects in astrophysical plasmas

Time-dependent Photoionization Modeling of Warm Absorbers in Active Galactic Nuclei

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