Limits on anisotropy in the nanohertz stochastic gravitational-wave background

S. R. Taylor (1, 2) , C. M. F. Mingarelli (3, 2) , J. R. Gair (4) , A. Sesana (5, 3) , Gilles Theureau (6, 7) , S. Babak (3) , C. G. Bassa (8) , P. Brem (3) , M. Burgay (9) , R. N. Caballero (10) , D. J. Champion (10) , Ismaël Cognard (7, 6) , G. Desvignes (10) , Lucas Guillemot (6, 7) , J. W. T. Hessels (11, 12) , G. H. Janssen (8) , R. Karuppusamy (10) , M. Kramer (10, 13) , A. Lassus (7) , P. Lazarus (3) , L. Lentati (4) , K. Liu (14) , S. Osłowski (15) , D. Perrodin (9) , A. Petiteau (16) , A. Possenti (9) , M. B. Purver (8) , P. A. Rosado (17, 18) , S. A. Sanidas (8) , R. Smits (11) , B. Stappers (8) , C. Tiburzi (9) , R. van Haasteren (2) , A. Vecchio (5) , J. P. W. Verbiest (10)

1 CAM - University of Cambridge [UK]
2 JPL - Jet Propulsion Laboratory
3 AEI - Max-Planck-Institut für Gravitationsphysik ( Albert-Einstein-Institut )
4 Cavendish Laboratory
5 BCU - Birmingham City University
6 USN - Unité Scientifique de la Station de Nançay
7 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
8 Jodrell Bank Centre for Astrophysics
9 OAC - INAF - Osservatorio Astronomico di Cagliari
10 MPIFR - Max-Planck-Institut für Radioastronomie
11 ASTRON - Netherlands Institute for Radio Astronomy
12 UvA - University of Amsterdam [Amsterdam]
13 CAL - Columbia Astrophysics Laboratory
14 CPPM - Centre de Physique des Particules de Marseille
15 Technische Fakultät, Universität Bielefeld
16 APC (UMR_7164) - AstroParticule et Cosmologie
17 Centre for Astrophysics and Supercomputing - Centre for Astrophysics and Supercomputing
18 AEI - Max Planck Institute for Gravitational Physics (Albert Einstein Institute)

The paucity of observed supermassive black hole binaries (SMBHBs) may imply that the gravitational wave background (GWB) from this population is anisotropic, rendering existing analyses sub-optimal. We present the first constraints on the angular distribution of a nanohertz stochastic GWB from circular, inspiral-driven SMBHBs using the $2015$ European Pulsar Timing Array data [Desvignes et al. (in prep.)]. Our analysis of the GWB in the $\sim 2 - 90$ nHz band shows consistency with isotropy, with the strain amplitude in $l>0$ spherical harmonic multipoles $\lesssim 40\%$ of the monopole value. We expect that these more general techniques will become standard tools to probe the angular distribution of source populations.

Keywords: Gravitational waves: theory; Gravitational wave detectors and experiments; Pulsars; Galaxy mergers collisions and tidal interactions

Domains: General Relativity and Quantum Cosmology [gr-qc]; Astrophysics [astro-ph]

S. R. Taylor, C. M. F. Mingarelli, J. R. Gair, A. Sesana, Gilles Theureau, et al.. Limits on anisotropy in the nanohertz stochastic gravitational-wave background. Physical Review Letters, 2015, 115 (4), pp.041101. ⟨10.1103/PhysRevLett.115.041101⟩. ⟨insu-01265421⟩