GRB follow-up and science with THESEUS/IRT

The aim of the space mission concept THESEUS is to continue to collect and study the GRB events like Swift. It will allow us to study the early Universe. Moreover, it will offer us to study with unprecedented sensitivity GRB emission and to measure the redshift for the bursts with z>5. In this work, we investigate the advantages of a optical and near-infrared telescope mounted on the same satellite that is triggered by the GRB like THESEUS/IRT. Afterwards, we investigate the possible future developments in the GRB science, first for the prompt phase and the for afterglow phase. We find that more than half of the sources detected by THESEUS, and will never be visible from a a ground-based telescope. Moreover, only ∼50% of all observable sources are visible within one hour, i.e. <30% of all THESEUS transient sources. A higher number of observable sources can only be achieved with a network of telescopes. THESEUS will permit to detect the NIR prompt phase of the longest GRBs, increasing the number of events studied from gamma-rays to the near-infrared from a handful of events studied up to now to ≳10 GRBs per year

Synergy between THESEUS and E-ELT

The Transient High Energy Sky and Early Universe Surveyor (THESEUS) is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. A fundamental contribution to achieve this goal will be provided by the powerful synergy between THESEUS and the extremely large ground-based telescopes which will operate in the next decade, like E-ELT. We discuss great improvements coming from this joint effort and describe some possible observing scenarios

GW170817: implications for the local kilonova rate and for surveys from ground-based facilities

We compute the local rate of events similar to GRB 170817A, which has been recently found to be associated with a kilonova (KN) outburst. Our analysis finds an observed rate of such events of R_KN∼ 352^{+810}_{-281} Gpc-3 yr-1. After comparing at their face values this density of sGRB outbursts with the much higher density of binary neutron star (BNS) mergers of 1540^{+3200}_{-1220} Gpc-3 yr-1, estimated by LIGO-Virgo collaboration, one can conclude, admittedly with large uncertainty that either only a minor fraction of BNS mergers produces short gamma-ray bursts (sGRB)/KN events or the sGRBs associated with BNS mergers are beamed and observable under viewing angles as large as θ ≲ 40°. Finally, we provide preliminary estimates of the number of sGRB/KN events detected by future surveys carried out with present/future ground-based/space facilities, such as LSST, VST, ZTF, SKA, and THESEUS

Detection methods for the Cherenkov Telescope Array at very-short exposure times

The Cherenkov Telescope Array (CTA) will be the next generation ground-based observatory for very-high-energy (VHE) gamma-ray astronomy, with the deployment of tens of highly sensitive and fast-reacting Cherenkov telescopes. It will cover a wide energy range (20 GeV - 300 TeV) with unprecedented sensitivity. To maximize the scientific return, the observatory will be provided with an online software system that will perform the first analysis of scientific data in real-time. This study investigates the precision and accuracy of available science tools and analysis techniques for the short-term detection of gamma-ray sources, in terms of sky localization, detection significance and, if significant detection is achieved, a first estimation of the integral photon flux. The scope is to evaluate the feasibility of the algorithms' implementation in the real-time analysis of CTA. In this contribution we present a general overview of the methods and some of the results for the test case of the short term detection of a gamma-ray burst afterglow, as the VHE counterpart of a gravitational wave event

What's Next for VST: Electromagnetic Follow-Up of Gravitational Waves Events

A big step forward in the long-standing quest for gravitational waves (GWs) will be made next year when the LIGO and VIRGO collaborations will start regular operations of their sensitive, upgraded interferometers. It is crucial that the electromagnetic counterparts of GW events are securely identified, a difficult task because of the large size of error box expected to be returned by the interferometers (dozens to hundreds of square degrees). Our group is tackling the challenge by organizing a follow-up campaign covering the widest possible range of the electromagnetic spectrum. The optical counterpart will be covered by the VST thanks to its characteristics. The sensitivity and optical quality of the telescope will allow us to probe faint transients (e.g. kilonovae and short GRBs) that are among the most promising GW source candidates

Supplement: "Localization and broadband follow-up of the gravitational-wave transient GW150914" (2016, ApJL, 826, L13)

This Supplement provides supporting material for Abbott et al. (2016a). We briefly summarize past electromagnetic (EM) follow-up efforts as well as the organization and policy of the current EM follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the EM follow-up observations that were performed in the different bands

Astrophysical Implications of the Binary Black-Hole Merger GW150914

The discovery of the gravitational-wave (GW) source GW150914 with the Advanced LIGO detectors provides the first observational evidence for the existence of binary black hole (BH) systems that inspiral and merge within the age of the universe. Such BH mergers have been predicted in two main types of formation models, involving isolated binaries in galactic fields or dynamical interactions in young and old dense stellar environments. The measured masses robustly demonstrate that relatively "heavy" BHs (≳25 M⊙) can form in nature. This discovery implies relatively weak massive-star winds and thus the formation of GW150914 in an environment with a metallicity lower than about 1/2 of the solar value. The rate of binary-BH (BBH) mergers inferred from the observation of GW150914 is consistent with the higher end of rate predictions (≳1 Gpc-3 yr-1) from both types of formation models. The low measured redshift (z ≃ 0.1) of GW150914 and the low inferred metallicity of the stellar progenitor imply either BBH formation in a low-mass galaxy in the local universe and a prompt merger, or formation at high redshift with a time delay between formation and merger of several Gyr. This discovery motivates further studies of binary-BH formation astrophysics. It also has implications for future detections and studies by Advanced LIGO and Advanced Virgo, and GW detectors in space

An improved analysis of GW150914 using a fully spin-precessing waveform model

This paper presents updated estimates of source parameters for GW150914, a binary black-hole coalescence event detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) on September 14, 2015 [1]. Reference presented parameter estimation [2] of the source using a 13-dimensional, phenomenological precessing-spin model (precessing IMRPhenom) and a 11-dimensional nonprecessing effective-one-body (EOB) model calibrated to numerical-relativity simulations, which forces spin alignment (nonprecessing EOBNR). Here we present new results that include a 15-dimensional precessing-spin waveform model (precessing EOBNR) developed within the EOB formalism. We find good agreement with the parameters estimated previously [2], and we quote updated component masses of $35^{+5}_{-3}\mathrm{M}_\odot$ and $30^{+3}_{-4}\mathrm{M}_\odot$ (where errors correspond to 90% symmetric credible intervals). We also present slightly tighter constraints on the dimensionless spin magnitudes of the two black holes, with a primary spin estimate $0.65$ and a secondary spin estimate $0.75$ at 90% probability. Reference [2] estimated the systematic parameter-extraction errors due to waveform-model uncertainty by combining the posterior probability densities of precessing IMRPhenom and nonprecessing EOBNR. Here we find that the two precessing-spin models are in closer agreement, suggesting that these systematic errors are smaller than previously quoted

SAPREMO: a simplified algorithm for predicting detections of electromagnetic transients in surveys

The multiwavelength detection of GW170817 has inaugurated multimessenger astronomy. The next step consists in interpreting observations coming from population of gravitational wave sources. We introduce SAPREMO, a tool aimed at predicting the number of electromagnetic signals characterized by a specific light curve and spectrum, expected in a particular sky survey. By looking at past surveys, SAPREMO allows us to constrain models of electromagnetic emission or event rates. Applying SAPREMO to proposed astronomical missions/observing campaigns provides a perspective on their scientific impact and tests the effect of adopting different observational strategies. For our first case study, we adopt a model of spin-down-powered X-ray emission predicted for a binary neutron star merger producing a long-lived neutron star. We apply SAPREMO on data collected by XMM-Newton and Chandra and during 104 s of observations with the mission concept THESEUS. We demonstrate that our emission model and binary neutron star merger rate imply the presence of some signals in the XMM-Newton catalogues. We also show that the new class of X-ray transients found by Bauer et al. in the Chandra Deep Field-South is marginally consistent with the expected rate. Finally, by studying the mission concept THESEUS, we demonstrate the substantial impact of a much larger field of view in searches of X-ray transients

Gravitational Waves optical follow-up at VST

We report on the deep optical follow-up surveys of the first two gravitational-wave events, GW150914 and GW151226, accomplished by the GRAvitational Wave Inaf TeAm Collaboration (GRAWITA) using the VLT Survey Telescope (VST). We responded promptly to the gravitational-wave alerts sent by the LIGO and Virgo Collaborations, covering a region of 90 deg2 and 72 deg2 for GW150914 and GW151226, respectively, and kept observing the two areas over nearly two months. Both surveys reached an average limiting magnitude of about 21 in the r-band. The paper outlines the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. Numerous transients have been discovered, mostly variable stars and eclipsing binaries, but no candidates are identified as related to the gravitational-wave events. The work done let on to gain experience and tune the tools for next LVC runs and in general to exploit the synergies between wide field optical surveys and future multi-messenger programs including projects like Theseus