The highly stable spin of neutron stars can be exploited for a variety of ( astro- ) physical investigations .
In particular arrays of pulsars with rotational periods of the order of milliseconds can be used to detect correlated signals such as those caused by gravitational waves .
Three such “ Pulsar Timing Arrays ” ( PTAs ) have been set up around the world over the past decades and collectively form the “ International ” PTA ( IPTA ) .
In this paper , we describe the first joint analysis of the data from the three regional PTAs , i.e .
of the first IPTA data set .
We describe the available PTA data , the approach presently followed for its combination and suggest improvements for future PTA research .
Particular attention is paid to subtle details ( such as underestimation of measurement uncertainty and long-period noise ) that have often been ignored but which become important in this unprecedentedly large and inhomogeneous data set .
We identify and describe in detail several factors that complicate IPTA research and provide recommendations for future pulsar timing efforts .
The first IPTA data release presented here ( and available online ) is used to demonstrate the IPTA ’ s potential of improving upon gravitational-wave limits placed by individual PTAs by a factor of \sim 2 and provides a 2 - \sigma limit on the dimensionless amplitude of a stochastic GWB of 1.7 \times 10 ^ { -15 } at a frequency of 1 { yr } ^ { -1 } .
This is 1.7 times less constraining than the limit placed by ( ) , due mostly to the more recent , high-quality data they used .