Previous XMM-Newton observations of the thermally emitting isolated neutron star RX~J1605.3+3249 provided a candidate for a shallow periodic signal and evidence of a fast spin down , which suggested a high dipolar magnetic field and an evolution from a magnetar .
We obtained a large programme with XMM-Newtonto confirm its candidate timing solution , understand the energy-dependent amplitude of the modulation , and investigate the spectral features of the source .
We performed extensive high-resolution and broadband periodicity searches in the new observations , using the combined photons of the three EPIC cameras and allowing for moderate changes of pulsed fraction and the optimal energy range for detection .
We also investigated the EPIC and RGS spectra of the source with unprecedented statistics and detail .
A deep 4 \sigma upper limit of 1.33 ( 6 ) \% for modulations in the relevant frequency range conservatively rules out the candidate period previously reported .
Blind searches revealed no other periodic signal above the 1.5 \% level ( 3 \sigma ; P > 0.15 s ; 0.3 - 1.35 keV ) in any of the four new observations .
While theoretical models fall short at physically describing the complex energy distribution of the source , best-fit X-ray spectral parameters are obtained for a fully or partially ionized neutron star hydrogen atmosphere model with B = 10 ^ { 13 } G , modified by a broad Gaussian absorption line at energy \epsilon = 385 \pm 10 eV .
The deep limits from the timing analysis disfavour equally well-fit double temperature blackbody models where both the neutron star surface and small hotspots contribute to the X-ray flux of the source .
We identified a low significance ( 1 \sigma ) temporal trend on the parameters of the source in the analysis of RGS data dating back to 2002 , which may be explained by unaccounted calibration issues and spectral model uncertainties .
The new dataset also shows no evidence of the previously reported narrow absorption feature at \epsilon \sim 570 eV , whose possible transient nature disfavours an atmospheric origin .