4 research outputs found
Multiphase Neutral Interstellar Medium: Analyzing Simulation with H I 21cm Observational Data Analysis Techniques
Several different methods are regularly used to infer the properties of the
neutral interstellar medium (ISM) using atomic hydrogen (H I) 21cm absorption
and emission spectra. In this work, we study various techniques used for
inferring ISM gas phase properties, namely the correlation between brightness
temperature and optical depth , at each channel velocity
, and decomposition into Gaussian components, by creating mock spectra
from a 3D magnetohydrodynamic simulation of a two-phase, turbulent ISM. We
propose a physically motivated model to explain the
distribution and relate the model parameters to properties like warm gas spin
temperature and cold cloud length scales. Two methods based on Gaussian
decomposition -- using only absorption spectra and both absorption and emission
spectra -- are used to infer the column density distribution as a function of
temperature. In observations, such analysis reveals the puzzle of large amounts
(significantly higher than in simulations) of gas with temperature in the
thermally unstable range of to and
a lack of the expected bimodal (two-phase) temperature distribution. We show
that, in simulation, both methods are able to recover the true gas distribution
till temperatures (and the two-phase distribution in
general) reasonably well. We find our results to be robust to a range of
effects such as noise, varying emission beam size, and simulation resolution.
This shows that the observational inferences are unlikely to be artifacts, thus
highlighting a tension between observations and simulations. We discuss
possible reasons for this tension and ways to resolve it.Comment: 21 pages (including appendixes), 15 figures, 3 tables, Submitted to
MNRAS, Comments are welcom
Joint gravitational wave-short GRB detection of Binary Neutron Star mergers with existing and future facilities
We explore the joint detection prospects of short gamma-ray bursts (sGRBs)
and their gravitational wave (GW) counterparts by the current and upcoming
high-energy GRB and GW facilities from binary neutron star (BNS) mergers. We
consider two GW detector networks: (1) A four-detector network comprising LIGO
Hanford, Livingston, Virgo, and Kagra, (IGWN4) and (2) a future five-detector
network including the same four detectors and LIGO India (IGWN5). For the sGRB
detection, we consider existing satellites Fermi and Swift and the proposed
all-sky satellite Daksha. Most of the events for the joint detection will be
off-axis, hence, we consider a broad range of sGRB jet models predicting the
off-axis emission. Also, to test the effect of the assumed sGRB luminosity
function, we consider two different functions for one of the emission models.
We find that for the different jet models, the joint sGRB and GW detection
rates for Fermi and Swift with IGWN4 (IGWN5) lie within 0.07-0.62$\mathrm{\
yr^{-1}}\mathrm{\ yr^{-1}}\mathrm{\ yr^{-1}}\mathrm{\ yr^{-1}}\mathrm{\ Gpc^{-3}~yr^{-1}}\mathrm{\ yr^{-1}}\mathrm{\ yr^{-1}}$), which is 2-9 times
higher than the existing satellites. We show that such a mission with higher
sensitivity will be ideal for detecting a higher number of fainter events
observed off-axis or at a larger distance. Thus, Daksha will boost the joint
detections of sGRB and GW, especially for the off-axis events. Finally, we find
that our detection rates with optimal SNRs are conservative, and noise in GW
detectors can increase the rates further.Comment: 9 pages, 6 figures, 2 tables; accepted for publication in MNRAS. The
definitive version will be available on the journal pag
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