arXiv subjects

Shaon Ghosh

Publications and source records attributed to Shaon Ghosh.

24 records · Page 2Linked to original sources

arXiv2017

Hunting electromagnetic counterparts of gravitational-wave events using the Zwicky Transient Facility

Detections of coalescing binary black holes by LIGO have opened a new window of transient astronomy. With increasing sensitivity of LIGO and participation of the Virgo detector in Cascina, Italy, we expect to soon detect coalescence of compact binary systems with one or more neutron stars. These are the prime targets for electromagnetic follow-up of gravitational wave triggers, which holds enormous promise of rich science. However, hunting for electromagnetic counterparts of gravitational wave events is a non-trivial task due to the sheer size of the error regions, which could span hundreds of square degrees. The Zwicky Transient facility (ZTF), scheduled to begin operation in 2017, is designed to cover such large sky-localization areas. In this work, we present the strategies of efficiently tiling the sky to facilitate the observation of the gravitational wave error regions using ZTF. To do this we used simulations consisting of 475 binary neutron star coalescences detected using a mix of two- and three-detector networks. Our studies reveal that, using two overlapping sets of ZTF tiles and a (modified) ranked-tiling algorithm, we can cover the gravitational-wave sky-localization regions with half as many pointings as a simple contour-covering algorithm. We then incorporated the ranked-tiling strategy to study our ability to observe the counterparts. This requires optimization of observation depth and localization area coverage. Our results show that observation in r-band with ~600 seconds of integration time per pointing seems to be optimum for typical assumed brightnesses of electromagnetic counterparts, if we plan to spend equal amount of time per pointing. However, our results also reveal that we can gain by as much as 50% in detection efficiency if we linearly scale our integration time per pointing based on the tile probability.

astro-ph.IM↗

arXiv2016

Tiling strategies for optical follow-up of gravitational wave triggers by wide field of view telescopes

Binary neutron stars are among the most promising candidates for joint gravitational-wave and electromagnetic astronomy. The goal of this work is to investigate the strategy of using gravitational wave sky-localizations for binary neutron star systems, to search for electromagnetic counterparts using wide field of view optical telescopes. We examine various strategies of scanning the gravitational wave sky-localizations on the mock 2015-16 gravitational-wave events. We propose an optimal tiling-strategy that would ensure the most economical coverage of the gravitational wave sky-localization, while keeping in mind the realistic constrains of transient optical astronomy. Our analysis reveals that the proposed tiling strategy improves the sky-localization coverage over naive contour-covering method. The improvement is more significant for observations conducted using larger field of view telescopes, or for observations conducted over smaller confidence interval of gravitational wave sky-localization probability distribution. Next, we investigate the performance of the tiling strategy for telescope arrays and compare their performance against monolithic giant field of view telescopes. We observed that distributing the field of view of the telescopes into arrays of multiple telescopes significantly improves the coverage efficiency by as much as 50% over a single large FOV telescope in 2016 localizations while scanning around 100 sq. degrees. Finally, we studied the ability of optical counterpart detection by various types of telescopes. In Our analysis for a range of wide field-of-view telescopes we found improvement in detection upon sacrificing coverage of localization in order to achieve greater observation depth for very large field-of-view - small aperture telescopes, especially if the intrinsic brightness of the optical counterparts are weak.

astro-ph.IM↗

arXiv2014

Localizing gravitational wave sources with optical telescopes and combining electromagnetic and gravitational wave data

Neutron star binaries, which are among the most promising sources for the direct detection of gravitational waves (GW) by ground based detectors, are also potential electromagnetic (EM) emitters. Gravitational waves will provide a new window to observe these events and hopefully give us glimpses of new astrophysics. In this paper, we discuss how EM information of these events can considerably improve GW parameter estimation both in terms of accuracy and computational power requirement. And then in return how GW sky localization can help EM astronomers in follow-up studies of sources which did not yield any prompt emission. We discuss how both EM source information and GW source localization can be used in a framework of multi-messenger astronomy. We illustrate how the large error regions in GW sky localizations can be handled in conducting optical astronomy in the advance detector era. We show some preliminary results in the context of an array of optical telescopes called BlackGEM, dedicated for optical follow-up of GW triggers, that is being constructed in La Silla, Chile and is expected to operate concurrent to the advanced GW detectors.

gr-qc↗

arXiv2013

Toward finding gravitational-wave signals from progenitors of short hard gamma-ray bursts and orphaned afterglows

With multiple observatories and missions being planned for detecting orphaned afterglows associated with gamma-ray bursts (GRBs) we emphasize the importance of developing data analysis strategies for searching their possible counterpart signals in the data of gravitational wave (GW) detectors in the advanced detector era. This is especially attractive since short hard gamma-ray bursts (SGRBs) may have compact binary coalescences involving neutron stars (CBCNSs) as their progenitors, which emit gravitational waves. Joint electromagnetic (EM) and GW observations of these objects will enrich our understanding of their beaming, energetics, galactic environment, and shed light on a host of other outstanding questions related to them. Here we recognize some of the astrophysical factors that determine what fraction of CBCNS sources can generate orphaned afterglows. Pipelines already exist that target the sky-position and time of occurrence of SGRBs, known from EM observations, to search for their counterparts in GW detector data. Modifying them to analyze extended periods of time in the GW data in the past of the afterglow detection, while targeting a single sky-position, can search for GWs from the common progenitor. We assess the improvement in GW detectability to be had from utilizing the sky-position information. We also propose a method for improving the detection efficiency of targeted searches of GW signals from the putative CBCNS sources of afterglows and short gamma ray bursts in the presence of errors in detector calibration or CBCNS waveform models used in the search. The improvement arises from searching in a wider patch of the sky even when the sky-position is known accurately from EM observations and utilizes the covariance of the errors in waveform parameters with those in the sky position.

astro-ph.HE↗

arXiv2012

Systematic errors in measuring parameters of non-spinning compact binary coalescences with post-Newtonian templates

We study the astrophysical impact of inaccurate and incomplete modeling of the gravitational waveforms from compact binary coalescences (CBCs). We do so by the matched filtering of complete inspiral-merger-ringdown (IMR) signals with a bank of inspiral-phase templates modeled after the 3.5 post-Newtonian TaylorT1 approximant. The rationale for the choice of the templates is threefold: (1) The inspiral phase of the Phenomenological signals, which are an example of complete IMR signals, is modeled on the same TaylorT1 approximant. (2) In the low-mass limit, where the merger and ringdown phases last much shorter than the inspiral phase, the errors should tend to vanishingly small values and, thus, provide an important check on the numerical aspects of our simulations. (3) Since the binary black hole (BBH) signals are not yet known for mass-ratios above ten and since signals from CBCs involving neutron stars are affected by uncertainties in the knowledge of their equation of state, inspiral templates are still in use in searches for those signals. The results from our numerical simulations are compared with analytical calculations of the systematic errors using the Fisher matrix on the template parameter space. We find that the loss in signal-to-noise ratio (SNR) can be as large as 60% even for binary black holes with component masses m1 = 13M\odot and m2 = 20M\odot. Also, the estimated total-mass for the same pair can be off by as much as 20%. Both of these are worse for some higher-mass combinations. Even the estimation of the symmetric mass-ratio η suffers a nearly 20% error for this example, and can be worse than 50% for the mass ranges studied here. These errors significantly dominate their statistical counterparts (at a nominal SNR of 10). It may, however, be possible to mitigate the loss in SNR by allowing for templates with unphysical values of η.

gr-qc↗

arXiv2011

A blind hierarchical coherent search for gravitational-wave signals from coalescing compact binaries in a network of interferometric detectors

We describe a hierarchical data analysis pipeline for coherently searching for gravitational wave (GW) signals from non-spinning compact binary coalescences (CBCs) in the data of multiple earth-based detectors. It assumes no prior information on the sky position of the source or the time of occurrence of its transient signals and, hence, is termed "blind". The pipeline computes the coherent network search statistic that is optimal in stationary, Gaussian noise, and allows for the computation of a suite of alternative statistics and signal-based discriminators that can improve its performance in real data. Unlike the coincident multi-detector search statistics employed so far, the coherent statistics are different in the sense that they check for the consistency of the signal amplitudes and phases in the different detectors with their different orientations and with the signal arrival times in them. The first stage of the hierarchical pipeline constructs coincidences of triggers from the multiple interferometers, by requiring their proximity in time and component masses. The second stage follows up on these coincident triggers by computing the coherent statistics. The performance of the hierarchical coherent pipeline on Gaussian data is shown to be better than the pipeline with just the first (coincidence) stage.

astro-ph.IM↗