Research
[1] Magnetically-driven accretion-disk wind in X-ray
It's been well-known for the past decades that more than ~50% of active galactic nuclei (AGNs), both Seyfert galaxies and quasars, exhibit a series of rich absorption features in the observed UV/X-ray spectra (e.g. between ~1-9 keV; kilo-electron volts). These spectroscopic signatures are clearly blueshifted (in wavelength) in most of the sources indicating that the "absorbers" must be expelled from their central engine (i.e. supermassive black hole) outflowing towards us. In a number of data obtained with a number of space-based X-ray observatories (e.g. Chandra, Suzaku, XMM-Newton…etc.) many absorption lines from various ions have been identified and detected due to atomic processes (primarily electric photoionization). By measuring these line transitions of rich spectroscopic features, one can learn and constrain the physical nature of the X-ray absorbers both locally and globally. While one of the most promising scenarios suggests the presence of an accretion disk wind around a black hole, the exact geometry and their launching mechanism(s) are yet to be understood to date. Our group has been proposing a magnetically-driven disk-wind in which accreting plasma is eventually expelled by the action of a global magnetic field through the Lorentz force.
It's been well-known for the past decades that more than ~50% of active galactic nuclei (AGNs), both Seyfert galaxies and quasars, exhibit a series of rich absorption features in the observed UV/X-ray spectra (e.g. between ~1-9 keV; kilo-electron volts). These spectroscopic signatures are clearly blueshifted (in wavelength) in most of the sources indicating that the "absorbers" must be expelled from their central engine (i.e. supermassive black hole) outflowing towards us. In a number of data obtained with a number of space-based X-ray observatories (e.g. Chandra, Suzaku, XMM-Newton…etc.) many absorption lines from various ions have been identified and detected due to atomic processes (primarily electric photoionization). By measuring these line transitions of rich spectroscopic features, one can learn and constrain the physical nature of the X-ray absorbers both locally and globally. While one of the most promising scenarios suggests the presence of an accretion disk wind around a black hole, the exact geometry and their launching mechanism(s) are yet to be understood to date. Our group has been proposing a magnetically-driven disk-wind in which accreting plasma is eventually expelled by the action of a global magnetic field through the Lorentz force.
- Warm Absorbers (WAs) in AGNs
Magnetized Disk-Winds in NGC 3783
Simulated Correlations of UV C IV and Fe K Outflows in Quasars
Stratified AGN Disk-Winds and Jets
- Ultra-Fast Outflows (UFOs) in AGNs
Spectral Modeling of Fe K UFOs in PG 1211+143
[2] X-ray soft-excess from relativistic accreting plasma in black hole magnetosphere
The observed X-ray spectra of AGNs in general contain a number of rich spectral components. Among others, especially from the so called narrow-line Seyfert 1 galaxies — a sub-class of Seyfert AGNs, it's been noted that the soft X-ray band exhibits a pronounced level of "excess" photons" known as "soft excess" at around 0.1-0.2 keV. While obvious in X-ray data, it's been puzzling as to what causes this feature. Here in our research, we are proposing a scenario in which accreting plasma, plunging from an accretion disk, slows and gets compressed developing a shock front at some point before reaching the event horizon. The downstream flow is then sufficiently heated providing an ideal geometrical site where the electrons can be efficiently accelerated accordingly. These energetic electrons can then Compton up-scatter the incoming thermal photons from the accretion disk producing a "hot" spectral feature similar to the detected soft excess.
The observed X-ray spectra of AGNs in general contain a number of rich spectral components. Among others, especially from the so called narrow-line Seyfert 1 galaxies — a sub-class of Seyfert AGNs, it's been noted that the soft X-ray band exhibits a pronounced level of "excess" photons" known as "soft excess" at around 0.1-0.2 keV. While obvious in X-ray data, it's been puzzling as to what causes this feature. Here in our research, we are proposing a scenario in which accreting plasma, plunging from an accretion disk, slows and gets compressed developing a shock front at some point before reaching the event horizon. The downstream flow is then sufficiently heated providing an ideal geometrical site where the electrons can be efficiently accelerated accordingly. These energetic electrons can then Compton up-scatter the incoming thermal photons from the accretion disk producing a "hot" spectral feature similar to the detected soft excess.
[3] Broad Fe line reverberation from a black hole accretion disk
An accretion disk around a black hole (whether stellar-mass black holes in galactic binaries or supermassive black holes in AGNs) is believed to be the very physical site where a number of interesting spectroscopic features are observed. Among those is Fe (iron) fluorescent emission line. Iron atoms in the accretion disk is photoionized by some external hard X-ray source(s) with an electron being transitioned up to a higher-energy level. Since the configuration is energetically unstable, it is followed by emitting line photons at ~6.4 keV for neutral Fe atom and higher line photons for ionized Fe atom. This is called Fe fluorescence and it is known well that the process can imprint a well-defined spectroscopic signature. By studying the observed Fe line, one can probe the physics of the innermost accretion disk in the vicinity of the black hole via strong general relativistic effects under the curved spacetime.
An accretion disk around a black hole (whether stellar-mass black holes in galactic binaries or supermassive black holes in AGNs) is believed to be the very physical site where a number of interesting spectroscopic features are observed. Among those is Fe (iron) fluorescent emission line. Iron atoms in the accretion disk is photoionized by some external hard X-ray source(s) with an electron being transitioned up to a higher-energy level. Since the configuration is energetically unstable, it is followed by emitting line photons at ~6.4 keV for neutral Fe atom and higher line photons for ionized Fe atom. This is called Fe fluorescence and it is known well that the process can imprint a well-defined spectroscopic signature. By studying the observed Fe line, one can probe the physics of the innermost accretion disk in the vicinity of the black hole via strong general relativistic effects under the curved spacetime.
GR Accretion Disk Illumination around Black Holes
Irradiation of an Accretion Disk to Constrain Black Hole Spin
Irradiation of an Accretion Disk to Constrain Black Hole Spin
[4] Implementation of model spectra into xspec software package
With the calculated theoretical models in hand, we are also developing model spectra that can be implemented into the X-ray software tool, xspec, widely used in the X-ray astrophysics community originally developed at NASA/GSFC. This will provide an observational realization of what theoretical models predict and it is extremely useful for understanding the underlying physics that would otherwise be inaccessible from direct observations.
With the calculated theoretical models in hand, we are also developing model spectra that can be implemented into the X-ray software tool, xspec, widely used in the X-ray astrophysics community originally developed at NASA/GSFC. This will provide an observational realization of what theoretical models predict and it is extremely useful for understanding the underlying physics that would otherwise be inaccessible from direct observations.
[5] Modeling Quasi-Periodic Oscillations by X-Ray Echoes
One of the generic general relativistic features of rapidly-spinning Kerr black holes (BHs) is frame-dragging within the ergosphere. For a sufficiently high BH spin (e.g. a/M~0.95), the innermost stable circular orbit (ISCO) extends well into the ergosphere. Our study is then focused on its natural consequence that radiation emitted from the ISCO is inevitably subject to strong frame-dragging effect allowing a maximum of ~15%-20% of the emitted photons are forced to orbit around the BH before reaching a detector at infinity. Such photon orbits thus produce a coherent signal (e.g. in X-ray) in the form of "echoes" yielding a well-defined quasi-periodic oscillation at high frequency (e.g. ~kHz QPOs for XRBs while ~mHz in AGNs). |