Abstract for Proposal 096476
Searching for crust cooling signatures in highly-magnetised accreting NSs
Crusts of neutron stars (NSs) constitute 1%% of the total mass and extend
down 1km from the surface. The crust's large density gradient and diverse
composition make it essential for understanding how accretion modifies
the NS. A useful way to observationally probe the NS structure and crust
composition, and the behaviour of matter at high densities, is by studying
their cooling emission after outbursts. The aim of this XMM-Newton-Swift
joint ToO program is to monitor the next Be/X-ray transient to study the
post-outburst spectra that hold the key to understanding the physical
differences in cooling among NSs. For this we require Swift's flexibility
to monitor the target's behaviour, and XMM-Newton' sensitivity and large
effective area to study the low X-ray luminosity regime of the system.
Details on Observing Strategy and Trigger Criteria
We will trigger the joint XMM-Newton-Swift ToO proposal on a single
Be/X-ray transient that meets the following conditions: a) the source
needs to exhibit a brighter and longer than normal outburst (the more
accreted matter, the more heat released in the NS crust), b) the Pspin
of the source should be <100s (to guarantee the potential propeller
regime at the end of the outburst; the pulse period of these sources
can be easily measured with Swift, MAXI or Fermi), c) the source is
at a distance <9kpc (so that we obtain sufficient photons to perform
spectral modeling), d) the extinction is moderately low (nH<10^22 cm^-2;
to accurately constrain the spectra at soft energies), and e) during
the Swift monitoring, the source has entered the cooling phase. Once we
trigger the XMM-Newton+Swift joint proposal, we will follow the decay
of the giant outburst and transition to the crust cooling phase of the
target using Swift. For that, we request 1ks Swift/XRT observations with a
cadence of 3 days for a total of 65ks. Then, we plan to observe the source
within 2 XMM-Newton observational windows close to or during apastron:
one observation of 40ks after 45-75 days of the peak of the outburst,
and another after 140-200 days