Taking the beat of the UNSEEN: the Ultraluminous Neutron Star  population of ULXs


UNSEEN is the acronym of Ultraluminous Neutron Star Extreme Extragalactic
populatioN, and it is a  > 1Ms exposure XMM-Newton Large Program aimed at searching and assessing the occurrence of Ultraluminous X-ray Sources (ULXs) hosting a Neutron Star (NS).  In fact owing to their X-ray luminosity far in excess of the Eddington limit (L_Edd) for a stellar mass black hole, (BH, ~10Msun), ULXs have long been considered a very well suited population to look for and study BHs of intermediate mass (IMBHs). The recent discovery of four ULXs, showing fast (~1s) and rapidly evolving pulsations (PULXs; two of which discovered by our group within the EXTraS project, see Figure above for the case of NGC 5907 ULX-1) unambiguously associated them to NSs exceeding by more than 500 times their L_Edd. These discoveries challenge our understanding of accretion physics and pose a key question about the nature of the ULXs as a class: are ULXs a heterogeneous population hosting both accreting BHs and NSs, or is there a dominant new Ultraluminous NS extragalactic populatioN with extreme properties? The program is aimed at answering the question by tripling the number of ULXs over which sensitive searches for X-ray pulsations and cyclotron resonant scattering features (CRSFs) can be carried out PI: G.L. Israel (INAF- AO Roma).

The blind search is done by applying a number of corrections to the photon arrival times of each detected source in order to take into account for both an UNKNOWN Doppler effect (SOPA; Search for Orbital Parameters with Acceleration techniques), due to the motion of the NS in an orbit, and an UNKNOWN strong intrinsic first (spin) period derivative (PASTA; Pulsation Accelerated Search for Timing Analysis) observed by the few known PULXs (see below).  Owing to the large number of analysis to be carried out for each source,  the search is done by using High Performing/Throughput Computer time obtained on a competition basis (HPCs/HTCs; such as Galileo and Marconi @ CINECA, PI: G. Rodriguez)

Participants: G.L. Israel, G. Rodriguez, F. Bernardini, P. Casella, L. Stella, A. Papitto 



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