A possible model for the long-term flares of Sgr A*

Okuda, Tôru; Singh, Chandra B.; Das, Santabrata; Aktar, Ramiz; Nandi, Anuj; Pino, E. M. de Gouveia Dal

doi:10.1093/pasj/psz021

Cited by 17 publications

(21 citation statements)

References 79 publications

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“…The present results for cases with low resistivity of η = 10 −6 and 0.01 are very similar to those for the previous magnetized flow without resistivity (Okuda et al (2019)). Adopting the same parameters of the flow and magnetic field as the present study, they found that the centrifugally supported shock moves back and forth between 60 R g ≤ R ≤ 170R g and that another inner weak shock appears irregularly with rapid variations due to the interaction of the expanding high magnetic blob with the accreting matter below the outer shock.…”

Section: Astrophysical Significancesupporting

confidence: 90%

“…However, in many cases of the low angular momentum flow with the standing shock, h/r ∼ 0.1 -0.5 because such flows are intrinsically advective and geometrically thick. Therefore if the outer radial boundary is chosen to be very far from the predicted theoretical shock location, the difference between the numerical and theoretical shock locations becomes significant (Okuda et al (2019)).…”

Section: Basic Equationsmentioning

confidence: 99%

“…Recently general relativistic high-resolution shock-capturing simulation code was used to study the scenario in the Schwarzschild (Kim et al (2017)) and Kerr (Kim et al (2019)) space-time which further established the formation of standing shock in hydrodynamic flow around non-rotating as well as rotating black hole in full general relativistic treatment. However, till now there has been only one work taking into account different magnitudes of magnetic field strength in such flows in the presence of standing shocks (Okuda et al (2019)). The long term evolution properties were investigated and long term flares in connection with Sgr A* could be explained.…”

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confidence: 99%

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Effects of resistivity on standing shocks in low angular momentum flows around black holes

Singh,

Okuda,

Aktar

2021

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We study two dimensional low angular momentum flow around the black hole using the resistive magnetohydrodynamic module of PLUTO code. Simulations have been performed for the flows with parameters of specific angular momentum, specific energy, and magnetic field which may be expected for the flow around Sgr A*. For flows with lower resistivity η = 10 −6 and 0.01, the luminosity and the shock location on the equator vary quasi-periodically. The power density spectra of luminosity variation show the peak frequencies which correspond to the periods of 5 ×10 5 , 1.4 ×10 5 , and 5×10 4 seconds, respectively. These quasi-periodic oscillations (QPOs) occur due to the interaction between the outer oscillating standing shock and the inner weak shocks occurring at the innermost hot blob. While for cases with higher resistivity η = 0.1 and 1.0, the high resistivity considerably suppresses the magnetic activity such as the MHD turbulence and the flows tend to be steady and symmetric to the equator. The steady standing shock is formed more outward compared with the hydrodynamical flow. The low angular momentum flow model with the above flow parameters and with low resistivity has a possibility for the explanation of the long-term flares with ∼ one per day and ∼ 5 -10 days of Sgr A* in the latest observations by Chandra, Swift, and XMM-Newton monitoring of Sgr A*. 1 INTRODUCTION Black hole accretion is the most efficient process which can address the issue of power generated in the neighbourhood of a black hole. Historically, the study of black hole accretion has been based on two extreme cases of accretion process: radiatively inefficient flow called Bondi flow (Bondi (1952), Michel (1972)) and radiatively efficient one called Keplerian disk (Shakura & Sunyaev (1973), Novikov & Thorne (1973)).

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Section: Astrophysical Significancesupporting

confidence: 90%

Section: Basic Equationsmentioning

confidence: 99%

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confidence: 99%

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Effects of resistivity on standing shocks in low angular momentum flows around black holes

Singh,

Okuda,

Aktar

2021

Preprint

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“…Moreover, Molteni, Ryu & Chakrabarti (1996) showed through numerical simulation that when infall time scale matches with the cooling time scale of the accreting matter, resonance oscillation of PSC takes place. As PSC modulates, emergent hard radiations also exhibit non-steady variations which are in general quasi-periodic in nature (Lee, Ryu & Chattopadhyay 2011;Das et al 2014;Suková & Janiuk 2015;Suková, Charzyński & Janiuk 2017;Okuda et al 2019). Hence, the modulation of PSC perhaps be potentially viable to account for the QPO phenomena commonly observed in Galactic Black hole sources (Belloni, Psaltis & van der Klis 2002;Remillard et al 2006;Nandi et al 2012;Iyer, Nandi & Mandal 2015;Nandi et al 2018).…”

Section: Introductionmentioning

confidence: 99%

Shocks in relativistic viscous accretion flows around Kerr black holes

Dihingia,

Das,

Maity

et al. 2019

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We study the relativistic viscous accretion flows around the Kerr black holes. We present the governing equations that describe the steady state flow motion in full general relativity and solve them in 1.5D to obtain the complete set of global transonic solutions in terms of the flow parameters, namely specific energy (E ), specific angular momentum (L ) and viscosity (α). We obtain a new type of accretion solution which was not reported earlier. Further, we show for the first time to the best of our knowledge that viscous accretion solutions may contain shock waves particularly when flow simultaneously passes through both inner critical point (r in ) and outer critical point (r out ) before entering into the Kerr black holes. We examine the shock properties, namely shock location (r s ) and compression ratio (R, the measure of density compression across the shock front) and show that shock can form for a large region of parameter space in L − E plane. We study the effect of viscous dissipation on the shock parameter space and find that parameter space shrinks as α is increased. We also calculate the critical viscosity parameter (α cri ) beyond which standing shock solutions disappear and examine the correlation between the black hole spin (a k ) and α cri . Finally, the relevance of our work is conferred where, using r s and R, we empirically estimate the oscillation frequency of the shock front (ν QP O ) when it exhibits Quasiperiodic (QP) variations. The obtained results indicate that the present formalism seems to be potentially viable to account for the QPO frequency in the range starting from milli-Hz to kilo-Hz as 0.386 Hz ≤ ν QP O 10M⊙ MBH ≤ 1312 Hz for a k = 0.99, where M BH stands for the black hole mass.

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“…Accretion solutions of this kind contain more than one critical point, and hence, they are known as multi-transonic advective accretion solutions (Fukue 1987;Chakrabarti 1989;Das, Pendharkar & Mitra 2003;Das 2007). Needless to mention that numerous groups of researchers extensively studied the shock induced advective accretion solutions around black holes both theoretically as well as numerically (Fukue 1987;Chakrabarti 1989;Yang & Kafatos 1995;Molteni, Ryu & Chakrabarti 1996;Ryu, Chakrabarti, & Molteni 1997;Lu, Gu, & Yuan 1999;Becker & Kazanas 2001;Fukumura & Tsuruta 2004;Nishikawa et al 2005;Das 2007;Kumar, et al 2013;Das, et al 2014;Okuda & Das 2015;Suková & Janiuk 2015;Fukumura et al 2016;Sarkar & Das 2016;Aktar et al 2017;Dihingia et al 2018c;Kim et al 2018;Okuda, et al 2019).…”

Section: Introductionmentioning

confidence: 99%

Properties of two-temperature magnetized advective accretion flow around rotating black hole

Dihingia,

Das,

Prabhakar

et al. 2019

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We study the two-temperature magnetized advective accretion flow around the Kerr black holes. During accretion, ions are heated up due to viscous dissipation, and when Coulomb coupling becomes effective, they transfer a part of their energy to the electrons. On the contrary, electrons lose energy due to various radiative cooling processes, namely bremsstrahlung, synchrotron, and Comtonization processes, respectively. To account for the magnetic contribution inside the disc, we consider the toroidal magnetic fields which are assumed to be dominant over other components. Moreover, we adopt the relativistic equation of state to describe the thermal characteristics of the flow. With this, we calculate the global transonic accretion solutions around the rotating black holes. We find that accretion solution containing multiple critical points may harbor shock wave provided the standing shock conditions are satisfied. Further, we investigate the shock properties, such as shock location (x s ) and compression ratio (R) that delineate the post-shock corona (hereafter PSC) and find that the dynamics of PSC is controlled by the flow parameters, such as accretion rate ( ṁ) and magnetic fields (β, defined as the ratio of gas pressure to the magnetic pressure), etc. Finally, we calculate the emission spectra of the accretion flows containing PSC and indicate that both ṁ and β play the pivotal roles in explaining the spectral state transitions commonly observed for black hole X-ray binaries.

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A possible model for the long-term flares of Sgr A*

Cited by 17 publications

References 79 publications

Effects of resistivity on standing shocks in low angular momentum flows around black holes

Effects of resistivity on standing shocks in low angular momentum flows around black holes

Shocks in relativistic viscous accretion flows around Kerr black holes

Properties of two-temperature magnetized advective accretion flow around rotating black hole

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