title: An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907.
abstract: Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any x-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 seconds in 2003 to 1.13 seconds in 2014. It has an isotropic peak luminosity of [Formula: see text]1000 times the Eddington limit for a NS at 17.1 megaparsec. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity [Formula: see text] 1041erg second[Formula: see text]) might harbor NSs.
authors.affiliation: Osservatorio Astronomico di Roma, Istituto Nazionale di Astrofisica (INAF), via Frascati 33, I-00078 Monteporzio Catone, Italy. gianluca@oa-roma.inaf.it.
authors.forename: Gian Luca
authors.identifier: 0000-0001-5480-6438
authors.initials: GL
authors.lastname: Israel
authors.affiliation: Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via E. Bassini 15, I-20133 Milano, Italy.
authors.forename: Andrea
authors.identifier: 0000-0002-2526-1309
authors.initials: A
authors.lastname: Belfiore
authors.affiliation: Osservatorio Astronomico di Roma, Istituto Nazionale di Astrofisica (INAF), via Frascati 33, I-00078 Monteporzio Catone, Italy.
authors.forename: Luigi
authors.identifier: 0000-0002-0018-1687
authors.initials: L
authors.lastname: Stella
authors.affiliation: Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090GE Amsterdam, Netherlands.|Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via E. Bassini 15, I-20133 Milano, Italy.
authors.forename: Paolo
authors.identifier: 0000-0003-4849-5092
authors.initials: P
authors.lastname: Esposito
authors.affiliation: Osservatorio Astronomico di Roma, Istituto Nazionale di Astrofisica (INAF), via Frascati 33, I-00078 Monteporzio Catone, Italy.
authors.forename: Piergiorgio
authors.identifier: 0000-0002-0752-3301
authors.initials: P
authors.lastname: Casella
authors.affiliation: Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via E. Bassini 15, I-20133 Milano, Italy.|Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Via Bassi 6, 27100 Pavia, Italy.
authors.forename: Andrea
authors.identifier: 0000-0001-6739-687X
authors.initials: A
authors.lastname: De Luca
authors.affiliation: Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via E. Bassini 15, I-20133 Milano, Italy.
authors.forename: Martino
authors.identifier: 0000-0002-8017-0338
authors.initials: M
authors.lastname: Marelli
authors.affiliation: Osservatorio Astronomico di Roma, Istituto Nazionale di Astrofisica (INAF), via Frascati 33, I-00078 Monteporzio Catone, Italy.
authors.forename: Alessandro
authors.identifier: 0000-0001-6289-7413
authors.initials: A
authors.lastname: Papitto
authors.affiliation: Agenzia Spaziale Italiana Science Data Center, via del Politecnico snc, I-00133 Roma, Italy.|Osservatorio Astronomico di Roma, Istituto Nazionale di Astrofisica (INAF), via Frascati 33, I-00078 Monteporzio Catone, Italy.
authors.forename: Matteo
authors.identifier: 0000-0003-3613-4409
authors.initials: M
authors.lastname: Perri
authors.affiliation: Agenzia Spaziale Italiana Science Data Center, via del Politecnico snc, I-00133 Roma, Italy.|Osservatorio Astronomico di Roma, Istituto Nazionale di Astrofisica (INAF), via Frascati 33, I-00078 Monteporzio Catone, Italy.
authors.forename: Simonetta
authors.identifier: 0000-0002-2734-7835
authors.initials: S
authors.lastname: Puccetti
authors.affiliation: Osservatorio Astronomico di Roma, Istituto Nazionale di Astrofisica (INAF), via Frascati 33, I-00078 Monteporzio Catone, Italy.
authors.forename: Guillermo A Rodríguez
authors.identifier: 0000-0003-3952-7291
authors.initials: GA
authors.lastname: Castillo
authors.affiliation: Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via E. Bassini 15, I-20133 Milano, Italy.
authors.forename: David
authors.identifier: 0000-0002-3853-5110
authors.initials: D
authors.lastname: Salvetti
authors.affiliation: Scuola Universitaria Superiore IUSS Pavia, piazza della Vittoria 15, I-27100 Pavia, Italy.|Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via E. Bassini 15, I-20133 Milano, Italy.|Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Via Bassi 6, 27100 Pavia, Italy.
authors.forename: Andrea
authors.identifier: 0000-0002-6038-1090
authors.initials: A
authors.lastname: Tiengo
authors.affiliation: Osservatorio Astronomico di Padova, INAF, vicolo dell'Osservatorio 5, I-35122 Padova, Italy.
authors.forename: Luca
authors.identifier: 0000-0002-6516-1329
authors.initials: L
authors.lastname: Zampieri
authors.affiliation: Istituto di Matematica Applicata e Tecnologie Informatiche "E. Magenes," CNR, via de Marini 6, I-16149 Genova, Italy.
authors.forename: Daniele
authors.identifier: 0000-0003-2649-0071
authors.initials: D
authors.lastname: D'Agostino
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching, Germany.
authors.forename: Jochen
authors.initials: J
authors.lastname: Greiner
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching, Germany.
authors.forename: Frank
authors.identifier: 0000-0002-0107-5237
authors.initials: F
authors.lastname: Haberl
authors.affiliation: Scuola Universitaria Superiore IUSS Pavia, piazza della Vittoria 15, I-27100 Pavia, Italy.|Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via E. Bassini 15, I-20133 Milano, Italy.
authors.forename: Giovanni
authors.identifier: 0000-0001-7304-9858
authors.initials: G
authors.lastname: Novara
authors.affiliation: Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via E. Bassini 15, I-20133 Milano, Italy.
authors.forename: Ruben
authors.identifier: 0000-0002-9393-8078
authors.initials: R
authors.lastname: Salvaterra
authors.affiliation: Dipartimento di Fisica e Astronomia, Università di Padova, via F. Marzolo 8, I-35131 Padova, Italy.
authors.forename: Roberto
authors.identifier: 0000-0003-3977-8760
authors.initials: R
authors.lastname: Turolla
authors.affiliation: Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH , UK.
authors.forename: Mike
authors.initials: M
authors.lastname: Watson
authors.affiliation: Dr. Karl-Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Sternwartstrasse 7, D-96049 Bamberg, Germany.
authors.forename: Joern
authors.identifier: 0000-0003-2065-5410
authors.initials: J
authors.lastname: Wilms
authors.affiliation: Osservatorio Astronomico di Brera, INAF, via Brera 28, I-20121 Milano, Italy.
authors.forename: Anna
authors.identifier: 0000-0001-5840-9835
authors.initials: A
authors.lastname: Wolter