© Serena Vinciguerra

In 2019 the NICER collaboration published the first mass and radius inferred for PSR J0030+0451, thanks to NICER observations, and consequent constraints on the equation of state characterising dense matter. Two independent analyses found a mass of ~ 1.3-1.4 Msun and a radius of ∼13km. They also both found that the hot spots were all located on the same hemisphere, opposite to the observer, and that at least one of them had a significantly elongated shape. Here we reanalyse the same NICER data set, incorporating the effects of an updated NICER response matrix and using an upgraded analysis framework. We expand the adopted models and jointly analyse also XMM-Newton data, which enables us to better constrain the fraction of observed counts coming from PSR J0030+0451. We find a multi-modal structure in the posterior surface, which becomes crucial especially when XMM-Newton data is accounted for. Including the corresponding constraints disfavors the main solutions found previously, in favor of new and more complex models. These have inferred masses and radii of ~[1.4 Msun, 11.5 km] and ~[1.7 Msun, 14.5 km], depending on the assumed model. They display configurations that do not require the two hot spots generating the observed X-rays to be on the same hemisphere, nor to show very elongated features, and may point instead to the presence of temperature gradients and the need to account for them.