**The NANOGrav Pulsar Timing Array Observing Program**

# Parallel Session: GRAVITATIONAL WAVES - SPACE BASED EXPERIMENTS (M)

## Location: Park 3.01

#### M3 - Tuesday 14:00-15:40 (Carlos Sopuerta)

**14:00-14:35**

**David Nice (Physics, Lafayette College)**

The North American Observatory for Nanohertz Gravitational Waves (NANOGrav) collaboration is fifteen years into a program of long-term, high-precision millisecond pulsar timing, undertaken with the goal of detecting and characterizing nanohertz gravitational waves (i.e., gravitational waves with periods of many years) by measuring their effect on observed pulse arrival times. We presently observe 79 pulsars at least once a month using Arecibo Observatory, the Green Bank Telescope, and the VLA. In addition, daily observations of these pulsars have recently begun with CHIME. We target pulsars for which we can achieve timing precision of 1 microsecond or better across a wide range of radio frequencies at each epoch; we achieve precision better than 100 nanoseconds in the best cases.

Observing a large number of pulsars will allow for robust measurements of gravitational waves by analyzing correlations in the timing of pairs of pulsars depending on their separation on the sky. Our data are pooled with data from telescopes worldwide via the International Pulsar Timing Array (IPTA) collaboration, further increasing our sensitivity to gravitational waves.

We will summarize the observing program and data releases. We will describe new timing wideband techniques that will allow for increased efficiency in gravitational wave searches. We will report on synergistic results from our data set, including new measurements of a massive neutron star.

**Gravitational Wave Astronomy with the NANOGrav Pulsar Timing Array**

**14:35-15:10**

**Jeffrey Hazboun (Physical Sciences Division, University of Washington)**

Pulsar timing arrays will detect gravitational waves from the super-massive black hole binaries at the centers of merged galaxies in the next few years. The strongest signal is expected to be the unresolvable background from these binaries out to z~2. Soon afterwards pulsar timing arrays will be able to resolve single sources and have already set limits on various candidate binaries. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is an NSF funded Physics Frontiers Center monitoring over 70 millisecond pulsars for the signature of these gravitational waves. The most recent gravitational wave results from the NANOGrav 11-year and 12.5-year datasets, including limits on the stochastic background, single sources and gravitational wave memory events, will be presented.

**Detecting circumbinary exoplanets orbiting white dwarf binaries with LISA **

**15:10-15:40**

**Nicola Tamanini (Astrophysical and Cosmological Relativity, Max Planck Institute for Gravitational Physics)**

In this talk I will review the prospects for the detection of circumbinary exoplanets orbiting Galactic double white dwarf binaries with LISA. I will first explain how LISA, through an induced Doppler modulation on the gravitational wave signal emitted by Galactic compact binaries, will be able to measure the period and estimate the mass of sufficiently massive objects orbiting these binary systems everywhere in the Milky Way. Then I will present the latest forecasts on the expected rate of exoplanet detections, computed using a simulated Galactic synthetic population of white dwarf binaries and a circumbinary planetary occurrence rate motivated by white dwarf pollution observations. I will show that LISA, over the 4 years of its nominal mission duration, will be able to detect from few to several tens of exoplanets more massive than Jupiter. These forecasts improve up to few hundreds of detected exoplanets if an extended mission of 10 years is considered. Finally I will outline the implications of these observations for exoplanetary searches, and I will discuss the prospects of detecting more massive circumbinary objects with LISA, such as brown dwarfs and other stars.

#### M4 - Tuesday 16:10-17:50 (Carlos Sopuerta)

**Mapping the Gravitational Wave Background with LISA**

**16:10-16:35**

**Arianna Renzini (Theoretical Physics, Imperial College)**

In this talk, I will analyse the ability of the LISA detector to reconstruct a stochastic, incoherent gravitational wave signal on the sky. I will illustrate the response of the detector as a function of both time and frequency, and describe the map-making method. I will then show preliminary results of map injection+reconstruction obtained within our ongoing project as a part of the LISA Cosmology Working Group.

**Multiple source detection in GW astronomy: the label switching problem.**

**16:35-17:00**

**Riccardo Buscicchio (Institute for Gravitational Wave, University of Birmingham )**

The label switching problem arises in the Bayesian analysis of models containing multiple indistinguishable parameters with arbitrary ordering. Any permutation of these parameters is equivalent, therefore models with many such parameters have extremely multi-modal posterior distributions. It is difficult to sample efficiently from such posteriors.

In this talk I will present a solution to this problem in the context of GW astronomy, well known in the broader astrophysics community. It involves carefully mapping the input parameter space to a high dimensional hypertriangle.

It is demonstrated that this solution is efficient even for large numbers of parameters and can be easily applied alongside any stochastic sampling algorithm. This method is illustrated using two example problems from the field of gravitational wave astronomy: population inference on the BBH mass spectrum using LIGO detections; simultaneous detection of White Dwarves using LISA.

**Interferometers constraints on the inflationary field content**

**17:00-17:25**

**Laura Iacconi (ICG, University of Portsmouth)**

Understanding the laws of inflation can shed light on the processes that govern physics at very high energy scales, beyond current experimental limits. In particular, the characterisation and detection of primordial gravitational waves produced during inflation can be an excellent test for the particle content of the very early universe. We consider an inflationary realisation whose tensor spectrum is sourced already at linear order. We show how this set-up supports a sufficient production of primordial gravitational waves to make the signal detectable at interferometer scales. We complement theoretical consistency checks on the model with stringent observational bounds on its parameter space stemming from CMB measurements, LIGO and Big Bang Nucleosynthesis bounds, as well as constraints from Primordial Black Holes production and UltraCompact MiniHalos.

**Violation of Chandrasekhar’s mass limit in presence of modified gravity and magnetic field and generation of continuous gravitational wave from compact objects**

**17:25-17:50**

**Surajit Kalita (Department of Physics, Indian Institute of Science, Bangalore)**

Einstein’s theory of general relativity (GR) is an incredible theory to explain various astrophysical phenomena and early universe cosmology. It provides an immense understanding of the physics of various compact objects e.g. black holes, neutron stars, white dwarfs, etc. However, some recent observations of cosmology and also of compact objects question the complete validity of GR in the high-density regime. Moreover, a white dwarf having a binary partner, pulls matter out from the companion star and beyond a certain mass, known as Chandrasekhar mass limit (~ 1.4 solar mass for a carbon-oxygen white dwarf), the white dwarf becomes unstable and it burns out to produce type Ia supernova (SNIa). Nevertheless, some recent observations of SNeIa argue that the value of the Chandrasekhar mass limit has to be violated. Such SNeIa are believed to be originating either from white dwarfs with mass much less than the Chandrasekhar mass limit (~ 0.5 solar mass), or much higher than it (~ 2.8 solar mass). In my prsentation, I will explain that these two classes of white dwarfs (sub- and super-Chandrasekhar limiting mass white dwarfs) can easily be explained using various forms of the f(R) gravity. I will show that the central density of the white dwarf is enough to explain sub- and super-Chandrasekhar limiting mass white dwarfs in f(R) gravity, keeping the parameters of the model fixed. It is also to be noted that all of these models are viable with respect to the solar system test. At last, I will briefly explain about the generation of continuous gravitational wave from rotating magnetized white dwarfs/neutron stars and these compact objects can be detected by the upcoming detectors e.g. LISA, BBO, DECIGO, Einstein Telescope, KAGRA, etc.

References:

1. S. Kalita, B. Mukhopadhyay, JCAP, 09 (2018) 007

2. S. Kalita, B. Mukhopadhyay, arXiv: 1905.02730