  The power-law parametrization for the energy density spectrum of
gravitational wave (GW) background is a useful tool to study its physics and
origin. While scalar induced secondary gravitational waves (SIGWs) from some
particular models fit the signal detected by NANOGrav, Parkers Pulsar Timing
Array, European Pulsar Timing Array, and Chinese Pulsar Timing Array
collaborations better than GWs from supermassive black hole binaries (SMBHBs),
we test the consistency of the data with the infrared part of SIGWs which is
somewhat independent of models. Through Bayesian analysis, we show that the
infrared parts of SIGWs fit the data better than GW background from SMBHBs. The
results give tentative evidence for SIGWs.
