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Abstract

The LOFAR radio telescope discovered the $707$-Hz binary millisecond pulsar
J0952$-$0607 in a targeted radio pulsation search of an unidentified
$\textit{Fermi}$ gamma-ray source. This source shows a weak energy flux of
$F_\gamma = 2.6 \times 10^{-12}\,\text{erg}\,\text{cm}^{-2}\,\text{s}^{-1}$ in
the energy range between $100\,\text{MeV} - 100\,\text{GeV}$. Here we report
the detection of pulsed gamma-ray emission from PSR$\,$J0952$-$0607 in a very
sensitive gamma-ray pulsation search. The pulsar's rotational, binary and
astrometric properties are measured over seven years of $\textit{Fermi}$-Large
Area Telescope data. For this we take into account the uncertainty on the shape
of the gamma-ray pulse profile. We present an updated radio timing solution now
spanning more than two years and show results from optical modeling of the
black-widow-type companion based on new multi-band photometric data taken with
HiPERCAM on the Gran Telescopio Canarias on La Palma and ULTRACAM on the New
Technology Telescope at ESO La Silla. PSR$\,$J0952$-$0607 is now the
fastest-spinning pulsar for which the intrinsic spin-down rate has been
reliably constrained ($\dot{P}_\text{int} < 4.6 \times
10^{-21}\,\text{s}\,\text{s}^{-1}$). The inferred surface magnetic field
strength of $B_\text{surf} \lesssim 8.2 \times 10^{7}\,\text{G}$ is among the
ten lowest of all known pulsars. This discovery is another example of an
extremely fast spinning black-widow pulsar hiding within an unidentified
$\textit{Fermi}$ gamma-ray source. In the future such systems might help to pin
down the maximum spin frequency and the minimum surface magnetic field strength
of millisecond pulsars.