4 research outputs found
An investigation of the "10 keV feature" in the spectra of Accretion Powered X-ray Pulsars with NuSTAR
Some of the accreting X-ray pulsars are reported to exhibit a peculiar
spectral feature at 10 keV, known as the "10 keV feature". The feature
has been characterized as either an emission line or an absorption line, and
its origin is unknown. It has been found in multiple observations of the same
source by different observatories, but not all the observations of any
particular source consistently showed the presence of it. In this work, we have
carried out a systematic investigation for the presence of the "10 keV feature"
using data from NuSTAR, a low background spectroscopic observatory having
uninterrupted wide band coverage on either side of 10 keV. We performed a
systematic spectral analysis on 58 archival NuSTAR observations of 30 bright
X-ray pulsars. The 379 keV spectral continua of these selected sources were
fitted with a model chosen on the basis of its fitting quality in 315 keV
and model simplicity, and then inspected for the presence of the "10 keV
feature". Our analysis indicates the presence of such a feature in 16 out of 58
the NuSTAR observations of 11 different sources and is fitted with a Gaussian
absorption model centered around 10 keV. Our analysis also suggests that such a
feature could be wrongly detected if flare data is not analyzed separately from
persistent emission.Comment: 28 pages, 32 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks
The conventional accretion disk lore is that magnetized turbulence is the
principal angular momentum transport process that drives accretion. However,
when dynamically important magnetic fields thread an accretion disk, they can
produce mass and angular momentum outflows that also drive accretion. Yet, the
relative importance of turbulent and wind-driven angular momentum transport is
still poorly understood. To probe this question, we analyze a long-duration
() simulation of a rapidly rotating ()
black hole (BH) feeding from a thick (), adiabatic, magnetically
arrested disk (MAD), whose dynamically-important magnetic field regulates mass
inflow and drives both uncollimated and collimated outflows (e.g., "winds" and
"jets", respectively). By carefully disentangling the various angular momentum
transport processes occurring within the system, we demonstrate the novel
result that both disk winds and disk turbulence extract roughly equal amounts
of angular momentum from the disk. We find cumulative angular momentum and mass
accretion outflow rates of and , respectively. This result suggests that understanding both turbulent
and laminar stresses is key to understanding the evolution of systems where
geometrically thick MADs can occur, such as the hard state of X-ray binaries,
low-luminosity active galactic nuclei, some tidal disruption events, and
possibly gamma ray bursts.Comment: 15 pages, 6 figures. Submitted to ApJ. Comments welcom
Science with the Daksha High Energy Transients Mission
We present the science case for the proposed Daksha high energy transients
mission. Daksha will comprise of two satellites covering the entire sky from
1~keV to ~MeV. The primary objectives of the mission are to discover and
characterize electromagnetic counterparts to gravitational wave source; and to
study Gamma Ray Bursts (GRBs). Daksha is a versatile all-sky monitor that can
address a wide variety of science cases. With its broadband spectral response,
high sensitivity, and continuous all-sky coverage, it will discover fainter and
rarer sources than any other existing or proposed mission. Daksha can make key
strides in GRB research with polarization studies, prompt soft spectroscopy,
and fine time-resolved spectral studies. Daksha will provide continuous
monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy
counterparts to Fast Radio Bursts. Using Earth occultation to measure source
fluxes, the two satellites together will obtain daily flux measurements of
bright hard X-ray sources including active galactic nuclei, X-ray binaries, and
slow transients like Novae. Correlation studies between the two satellites can
be used to probe primordial black holes through lensing. Daksha will have a set
of detectors continuously pointing towards the Sun, providing excellent hard
X-ray monitoring data. Closer to home, the high sensitivity and time resolution
of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray
Flashes.Comment: 19 pages, 7 figures. Submitted to ApJ. More details about the mission
at https://www.dakshasat.in