Testing the relativistic Doppler boost hypothesis for the binary candidate quasar PG1302-102 with multi-band Swift data. (arXiv:1907.11246v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Xin_C/0/1/0/all/0/1">Chengcheng Xin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Charisi_M/0/1/0/all/0/1">Maria Charisi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Haiman_Z/0/1/0/all/0/1">Zoltan Haiman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Graham_M/0/1/0/all/0/1">Matthew J. Graham</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stern_D/0/1/0/all/0/1">Daniel Stern</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+DOrazio_D/0/1/0/all/0/1">Daniel J. D'Orazio</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schiminovich_D/0/1/0/all/0/1">David Schiminovich</a>
The bright quasar PG1302-102 has been identified as a candidate supermassive
black hole binary from its near-sinusoidal optical variability. While the
significance of its optical periodicity has been debated due to the stochastic
variability of quasars, its multi-wavelength variability in the ultraviolet
(UV) and optical bands is consistent with relativistic Doppler boost caused by
the orbital motion in a binary. However, this conclusion was based previously
on sparse UV data which were not taken simultaneously with the optical data.
Here we report simultaneous follow-up observations of PG1302-102 with the
Ultraviolet Optical Telescope on the Neil Gehrels Swift Observatory in six
optical + UV bands. We perform a statistical analysis to further test the
Doppler boost hypothesis, which predicts that UV variability should track the
optical, but with a $sim2.2$ times higher amplitude. We find that the
additional nine Swift observations produce light curves roughly consistent with
this trend. The data are consistent with the Doppler boost hypothesis when we
compare the optical B-band and UV light curves. However, the ratio of UV to
V-band variability is larger than expected and is consistent with the Doppler
model, only if either the UV/optical spectral slopes vary, the stochastic
variability makes a large contribution in the UV, or the sparse new optical
data underestimate the true optical variability. We have evidence for the
latter from comparison with the optical light curve from ASAS-SN. Additional,
simultaneous optical + UV observations tracing out another cycle of the
5.2-year proposed periodicity should lead to a definitive conclusion.
The bright quasar PG1302-102 has been identified as a candidate supermassive
black hole binary from its near-sinusoidal optical variability. While the
significance of its optical periodicity has been debated due to the stochastic
variability of quasars, its multi-wavelength variability in the ultraviolet
(UV) and optical bands is consistent with relativistic Doppler boost caused by
the orbital motion in a binary. However, this conclusion was based previously
on sparse UV data which were not taken simultaneously with the optical data.
Here we report simultaneous follow-up observations of PG1302-102 with the
Ultraviolet Optical Telescope on the Neil Gehrels Swift Observatory in six
optical + UV bands. We perform a statistical analysis to further test the
Doppler boost hypothesis, which predicts that UV variability should track the
optical, but with a $sim2.2$ times higher amplitude. We find that the
additional nine Swift observations produce light curves roughly consistent with
this trend. The data are consistent with the Doppler boost hypothesis when we
compare the optical B-band and UV light curves. However, the ratio of UV to
V-band variability is larger than expected and is consistent with the Doppler
model, only if either the UV/optical spectral slopes vary, the stochastic
variability makes a large contribution in the UV, or the sparse new optical
data underestimate the true optical variability. We have evidence for the
latter from comparison with the optical light curve from ASAS-SN. Additional,
simultaneous optical + UV observations tracing out another cycle of the
5.2-year proposed periodicity should lead to a definitive conclusion.
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