We present the results obtained from follow-up observations of the MUSE Ultra
Deep Field (MUDF) at X-ray energies with XMM-Newton. The MUDF is centred on a
unique field with two bright, physically associated quasars at z≃3.23,
separated by ∼500 kpc in projection. Both quasars are embedded within
extended Lyα nebulae (≳100 kpc at a surface brightness flux
level of ≈6×10−19erg s−1 cm−2 arcsec−2), whose
elongated morphology is suggestive of an extended filament connecting the
quasar haloes. The new X-ray observations presented here allow us to
characterise the physical properties (e.g. X-ray slope, luminosities, gas
column densities) in the innermost region of the MUDF quasars. We find that
both quasars are X-ray underluminous compared to objects at similar ultraviolet
luminosities. Based on our X-ray spectral analysis, absorbing columns of
NH​(z)≳ 1023 cm−2 appear unlikely, therefore such a weakness
is possibly intrinsic. When also including literature data, we do not observe
any detectable trend between the area of the nebulae and nuclear luminosities
at both the rest-frame 2 keV and 2500 AËš. The area is also not
correlated with the X-ray photon index nor with the integrated band flux in the
hard band (2−10 keV). We also do not find any trend between the extended
Lyα emission of the nebulae and the nuclear X-ray luminosity. Finally,
the properties of the MUDF quasars' nebulae are consistent with the observed
relation between the Lyα integrated luminosity of the nebulae and their
area. Our results suggest that the quasar ionization power is not a strong
driver of the morphology and size of the nebulae.Comment: 15 pages, 9 figures, reference added, published in MNRA