23 research outputs found
Multi-timescale X-ray reverberation mapping of accreting black holes
Accreting black holes show characteristic reflection features in their X-ray
spectrum, including an iron K line, resulting from hard X-ray continuum
photons illuminating the accretion disk. The reverberation lag resulting from
the path length difference between direct and reflected emission provides a
powerful tool to probe the innermost regions around both stellar-mass and
supermassive black holes. Here, we present for the first time a reverberation
mapping formalism that enables modeling of energy dependent time lags and
variability amplitude for a wide range of variability timescales, taking the
complete information of the cross-spectrum into account. We use a pivoting
power-law model to account for the spectral variability of the continuum that
dominates over the reverberation lags for longer time scale variability. We use
an analytic approximation to self-consistently account for the non-linear
effects caused by this continuum spectral variability, which have been ignored
by all previous reverberation studies. We find that ignoring these non-linear
effects can bias measurements of the reverberation lags, particularly at low
frequencies. Since our model is analytic, we are able to fit simultaneously for
a wide range of Fourier frequencies without prohibitive computational expense.
We also introduce a formalism of fitting to real and imaginary parts of our
cross-spectrum statistic, which naturally avoids some mistakes/inaccuracies
previously common in the literature. We perform proof-of-principle fits to
Rossi X-ray Timing Explorer data of Cygnus X-1.Comment: Accepted for publication in MNRA
MAXI and NuSTAR observations of the faint X-ray transient MAXI J1848-015 in the GLIMPSE-C01 Cluster
We present the results of MAXI monitoring and two NuSTAR observations of the
recently discovered faint X-ray transient MAXI J1848-015. Analysis of the MAXI
light-curve shows that the source underwent a rapid flux increase beginning on
2020 December 20, followed by a rapid decrease in flux after only days.
NuSTAR observations reveal that the source transitioned from a bright soft
state with unabsorbed, bolometric (- keV) flux , to a low hard state with flux . Given a distance of
kpc, inferred via association of the source with the GLIMPSE-C01 cluster,
these fluxes correspond to an Eddington fraction of order for an
accreting neutron star of mass , or even lower for a more massive
accretor. However, the source spectra exhibit strong relativistic reflection
features, indicating the presence of an accretion disk which extends close to
the accretor, for which we measure a high spin, . In addition
to a change in flux and spectral shape, we find evidence for other changes
between the soft and hard states, including moderate disk truncation with the
inner disk radius increasing from to
, narrow Fe emission whose centroid
decreases from keV to keV, and an increase in
low-frequency (- Hz) variability. Due to the high spin we
conclude that the source is likely to be a black hole rather than a neutron
star, and we discuss physical interpretations of the low apparent luminosity as
well as the narrow Fe emission.Comment: 19 pages, 9 figures, 3 tables. Accepted for publication in Ap
Disk, Corona, Jet Connection in the Intermediate State of MAXI J1820+070 Revealed by NICER Spectral-timing Analysis
We analyze five epochs of Neutron star Interior Composition Explorer (NICER) data of the black hole X-ray binary MAXI J1820+070 during the bright hard-to-soft state transition in its 2018 outburst with both reflection spectroscopy and Fourier-resolved timing analysis. We confirm the previous discovery of reverberation lags in the hard state, and find that the frequency range where the (soft) reverberation lag dominates decreases with the reverberation lag amplitude increasing during the transition, suggesting an increasing X-ray emitting region, possibly due to an expanding corona. By jointly fitting the lag-energy spectra in a number of broad frequency ranges with the reverberation model reltrans, we find the increase in reverberation lag is best described by an increase in the X-ray coronal height. This result, along with the finding that the corona contracts in the hard state, suggests a close relationship between spatial extent of the X-ray corona and the radio jet. We find the corona expansion (as probed by reverberation) precedes a radio flare by ~5 days, which may suggest that the hard-to-soft transition is marked by the corona expanding vertically and launching a jet knot that propagates along the jet stream at relativistic velocities
X-ray Reverberation Mapping of Ark 564 using Gaussian Process Regression
Ark 564 is an extreme high-Eddington Narrow-line Seyfert 1 galaxy, known for
being one of the brightest, most rapidly variable soft X-ray AGN, and for
having one of the lowest temperature coronae. Here we present a 410-ks NuSTAR
observation and two 115-ks XMM-Newton observations of this unique source, which
reveal a very strong, relativistically broadened iron line. We compute the
Fourier-resolved time lags by first using Gaussian processes to interpolate the
NuSTAR gaps, implementing the first employment of multi-task learning for
application in AGN timing. By fitting simultaneously the time lags and the flux
spectra with the relativistic reverberation model RELTRANS, we constrain the
mass at , although additional components
are required to describe the prominent soft excess in this source. These
results motivate future combinations of machine learning, Fourier-resolved
timing, and the development of reverberation models.Comment: 19 pages, 9 figures. Accepted for publication in The Astrophysical
Journa
Tracking the X-Ray Polarization of the Black Hole Transient Swift J1727.8â1613 during a State Transition
We report on an observational campaign on the bright black hole (BH) X-ray binary Swift J1727.8â1613 centered around five observations by the Imaging X-ray Polarimetry Explorer. These observations track for the first time the evolution of the X-ray polarization of a BH X-ray binary across a hard to soft state transition. The 2â8 keV polarization degree decreased from âŒ4% to âŒ3% across the five observations, but the polarization angle remained oriented in the northâsouth direction throughout. Based on observations with the Australia Telescope Compact Array, we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher-spatial-resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long (âł10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8â1613 is a member of a hitherto undersampled subpopulation
Extending the Baseline for SMC X-1's Spin and Orbital Behavior with NuSTAR Stray Light
StrayCats, the catalog of NuSTAR stray light observations, contains data from bright X-ray sources that fall within crowded source regions. These observations offer unique additional data with which to monitor sources such as X-ray binaries that show variable timing behavior. In this work, we present a timing analysis of stray light data of the high-mass X-ray binary SMC X-1, the first scientific analysis of a single source from the StrayCats project. We describe the process of screening stray light data for scientific analysis, verify the orbital ephemeris, and create both time- and energy-resolved pulse profiles. We find that the orbital ephemeris of SMC X-1 is unchanged and confirm a long-term spin-up rate of ÎœË=(2.52±0.03)Ă10â11 Hz s-1. We also note that the shape of SMC X-1's pulse profile, while remaining double peaked, varies significantly with time and only slightly with energy
First X-ray polarization measurement confirms the low black-hole spin in LMC X-3
X-ray polarization is a powerful tool to investigate the geometry of
accreting material around black holes, allowing independent measurements of the
black hole spin and orientation of the innermost parts of the accretion disk.
We perform the X-ray spectro-polarimetric analysis of an X-ray binary system in
the Large Magellanic Cloud, LMC X-3, that hosts a stellar-mass black hole,
known to be persistently accreting since its discovery. We report the first
detection of the X-ray polarization in LMC X-3 with the Imaging X-ray
Polarimetry Explorer, and find the average polarization degree of 3.2% +- 0.6%
and a constant polarization angle -42 deg +- 6 deg over the 2-8 keV range.
Using accompanying spectroscopic observations by NICER, NuSTAR, and the Neil
Gehrels Swift observatories, we confirm previous measurements of the black hole
spin via the X-ray continuum method, a ~ 0.2. From polarization analysis only,
we found consistent results with low black-hole spin, with an upper limit of a
< 0.7 at a 90% confidence level. A slight increase of the polarization degree
with energy, similar to other black-hole X-ray binaries in the soft state, is
suggested from the data but with a low statistical significance.Comment: 14 pages, 8 figures, submitted to Ap