45 research outputs found
Probing the Central Regions of Active Galactic Nuclei
Active Galactic Nuclei (AGN) are one of the key players in the Universe. Their energy output can strongly affect the growth of their host galaxy and can promote or suppress star formation on galactic scales. Most of the processes that determine the power of an AGN as well as the form in which that power is released take place in the immediate surroundings of its supermassive black hole, a region that is still not entirely understood. A comprehension of these inner regions is, however, crucial to any ultimate understanding of the AGN's vast influence. This dissertation explores these close-in environments of the black hole using two approaches: X-ray spectroscopy and variability studies.
We begin by summarizing our current understanding of why AGN play such a significant role in galaxy formation. This is followed by a discussion of why X-ray spectroscopy is one of the best means to investigate them. We point out that, in particular, the X-ray reflection spectrum is interesting as it can directly probe parameters such as the black hole spin or the inclination of the accretion disk. Since the reflection spectrum is a broad band component, that usually only contributes a fraction of the total observed X-ray flux, the entire X-ray spectrum requires careful modeling.
To perform such modeling and gain access to the parameters of the reflection spectrum, we first select a target in which the spectral decomposition is simplified by the absence of absorption - the Seyfert 1 galaxy Fairall 9. We apply a multi-epoch fitting method that uses more than one spectrum at a time to get the best possible results on the parameters of the reflection spectrum that are invariant on human timescales. This technique enables us to tightly constrain the reflection parameters and leads us to conclude that Fairall 9 most likely possesses a composite soft X-ray excess, consisting of blurred reflection and a separate component such as Comptonization.
The reflection spectrum also provides a way to enhance our knowledge of jet formation. We present a multi-wavelength study of the broad line radio galaxy 3C120 centered around a study of the reflection spectrum from two Suzaku and one XMM observation. Our results confirm that jet formation is linked to changes in (and possibly the destruction of) the inner accretion disk, and the high measured spin suggests that the rotational energy could very well be the energy source required to launch the jet.
Finally, we present results from variability studies, which present another window into the processes taking place close to the black hole. A 10 year RXTE monitoring of Fairall 9 allows us to discover very rapid flux dips in the X-ray band which only last 5-15 days. While we are unable to determine the exact nature of the dips, we discuss a range of possible models, including the idea that the accretion disk in this radio-quiet AGN may be undergoing sporadic disruptions (via some yet-to-be-determined global instability) in much the same manner as is inferred to occur in 3C120 and other broad-line galaxies. Lastly we turn to the UV variability of Fairall~9 and its connection to the X-ray variability. From 2.5 months of Swift monitoring, we find that Fairall~9 shows significant variability on 4 day timescales, and the analysis of XMM-OM data shows that variability is present even on the time scales of hours. Folding in the X-ray variability, we determined that this fast UV variability can be explained as reprocessing of X-rays. We conclude by explaining how these studies fit into the field of AGN science as a whole and how they can be followed up with future observations
The study of thermonuclear X-ray bursts in accreting millisecond pulsar MAXI J1816-195 with NuSTAR and NICER
The millisecond pulsar MAXI J1816--195 was recently discovered by MAXI in
2022 May. We have studied different properties of the pulsar using data from
NuSTAR and NICER observations. The position of the source is measured by NuSTAR
as RA = , Dec = . The unstable
burning of accreted material on the surface of neutron stars induces
thermonuclear (Type-I) bursts. Several thermonuclear bursts have been detected
from the source during the outburst. We study the evolution of burst profile
with flux and energy using NuSTAR and NICER observations. During the NuSTAR
observation, a total of four bursts were detected from the source. The duration
of each burst was around 30 s and the ratio of peak to persistent count
rate is 26 as seen from the NuSTAR data. The thermonuclear bursts are
modeled to determine the burst timing parameters using a sharp linear rise and
exponential decay function. The burst profiles show a relatively long tail in
lower energies. The hardness ratio during the thermonuclear bursts shows
significant variation as observed by NuSTAR. We successfully model the
broadband burst-resolved spectra with a combination of an absorbed blackbody
along with a non-thermal component to account for the persistent emission. The
burst-resolved spectral parameters show significant evolution during the burst.
During the peak of the burst, the Eddington luminosity is found to be erg s. The burst-resolved spectral parameters provide a
source distance of kpc for isotropic burst emission.Comment: 13 pages, 9 figures, 4 tables, comments are welcom
Broad-band X-ray observation of broad-line radio galaxy 3C 109
We present a study of the central engine in the broad-line radio galaxy 3C
109. To investigate the immediate surrounding of this accreting, supermassive
black hole, we perform a multi-epoch broad-band spectral analysis of a joint
NuSTAR/XMM observation (2017), an archival xmm observation (2005) and the
105-month averaged Swift-BAT data. We are able to clearly separate the spectrum
into a primary continuum, neutral and ionized absorption, and a reflection
component. The photon index of the primary continuum has changed since 2005
(), while
other components remain unchanged, indicative of minimal geometric changes to
the central engine. We constrain the high-energy cutoff of 3C 109
(E\,keV ) for the first time. The
reflector is found to be ionized (log = )
but no relativistic blurring is required by the data. SED analysis confirms the
super-Eddington nature of 3C 109 initially ( 2.09). However,
we do not find any evidence for strong reflection (R = ) or a steep power law index, as expected from a super-Eddington source.
This puts the existing virial mass estimate of 2 M
into question. We explore additional ways of estimating the Eddington ratio,
some of which we find to be inconsistent with our initial SED estimate. We
obtain a new black hole mass estimate of 9.3 M, which
brings all Eddington ratio estimates into agreement and does not require 3C 109
to be super-Eddington.Comment: 13 pages, 8 figure
The hard X-ray perspective on the soft X-ray excess
The X-ray spectra of many active galactic nuclei (AGN) exhibit a `soft
excess' below 1keV, whose physical origin remains unclear. Diverse models have
been suggested to account for it, including ionised reflection of X-rays from
the inner part of the accretion disc, ionised winds/absorbers, and
Comptonisation. The ionised reflection model suggests a natural link between
the prominence of the soft excess and the Compton reflection hump strength
above 10keV, but it has not been clear what hard X-ray signatures, if any, are
expected from the other soft X-ray candidate models. Additionally, it has not
been possible up until recently to obtain high-quality simultaneous
measurements of both soft and hard X-ray emission necessary to distinguish
these models, but upcoming joint XMM-NuSTAR programmes provide precisely this
opportunity. In this paper, we present an extensive analysis of simulations of
XMM+NuSTAR observations, using two candidate soft excess models as inputs, to
determine whether such campaigns can disambiguate between them by using hard
and soft X-ray observations in tandem. The simulated spectra are fit with the
simplest "observer's model" of a black body and neutral reflection to
characterise the strength of the soft and hard excesses. A plot of the strength
of the hard excess against the soft excess strength provides a diagnostic plot
which allows the soft excess production mechanism to be determined in
individual sources and samples using current state-of-the-art and next
generation hard X-ray enabled observatories. This approach can be
straightforwardly extended to other candidate models for the soft excess.Comment: 12 pages, 11 figures, accepted for publication in ApJ. Added
reference