ChandraandXMM‐NewtonDiscovery of Transient X‐Ray Pulsar in the Nearby Spiral Galaxy NGC 2403

Abstract

We report on the discovery and analysis of the transient X-ray pulsar CXOU J073709.1+653544 detected in the 2004 August-October Chandra and XMM-Newton observations of the nearby spiral galaxy NGC 2403. The X-ray source exhibits X-ray pulsations with a period P $ 18 s and a nearly sinusoidal pulse shape and pulsed fraction 46%-70% during the first three observations. The observed pulsation period decreased rapidly from 18.25 s on August 9 to 17.93 s on September 12 and possibly 17.56 s on 2004 October 3. The X-ray spectra of CXOU J073709.1+653544 are hard and are well fitted with an absorbed simple power law of photon index À $ 0:9 1:2 in the 0.3-7 keV energy band. The X-ray properties of the source and the absence of an optical /UV counterpart brighter than 20 mag allow us to identify CXOU J073709.1+653544 as an accreting X-ray pulsar in NGC 2403. The maximum unabsorbed luminosity of the source in the 0.3-7 keV range, L X $ 2:6 ; 10 38 ergs s À1 at 3.2 Mpc, is at least 260 times higher than its quiescent luminosity. The estimated source luminosity in the 0.3-100 keV energy range could be as high as $1:2 ; 10 39 ergs s À1 . The rate of decrease of the pulsation period of the source (Ṗ $ À10 À7 s s À1 ) is one of the fastest observed among accreting pulsars. The evolution of the pulsation period suggests that it is dominated by the intrinsic spin-up of the compact object, which is almost certainly a neutron star. The X-ray luminosity of CXOU J073709.1+653544 is high enough to account for the observed spin-up rate, assuming that the X-ray source is powered by disk accretion onto a highly magnetized neutron star. Based on the transient behavior and overall X-ray properties of the source, it could be an X-ray pulsar belonging to either a Be binary system or a low-mass system similar to the transient Galactic bursting pulsar GRO J1744À28.

Not an oxymoron: some X-ray binary pulsars with enormous spin-up rates reveal weak magnetic fields

“…A large spinup rate of . PS = −1.1 × 10 −7 was also reported for the transient X-ray 18 s pulsar CXOU J073709.1+653544 in NGC 2403 (Trudolyubov et al 2007). In its only detected ouburst by Chandra and XMM-Newton, the 0.3-7 keV luminosity reached a maximum of 1.5L Edd , where L Edd = 1.77 × 10 38 erg s −1 , for the canonical pulsar mass of 1.4M⊙.…”

Discovery of a 2.8 s Pulsar in a 2 Day Orbit High-mass X-Ray Binary Powering the Ultraluminous X-Ray Source ULX-7 in M51

“…Alternatively, standard magnetic fields (in the -10 10 11 13 G range, without any significant multipolar component) are envisaged by models in which the disk is fed at a super-Eddington rate, the excess supply is ejected away, and the emission is highly beamed in a geometrically thick inner disk funnel (see, e.g., King & Lasota 2016King et al 2017;Koliopanos et al 2017;Pintore et al 2017;Walton et al 2018). We note that two more extragalactic transient pulsators share striking similarities with the above group, i.e., super-Eddington luminosities and/or a large first derivative of the spin period: XMMU J031747.5-663010 in NGC 1313 (∼766 s; Trudolyubov 2008) and CXOU J073709.1+653544 in NGC 2403 (∼18 s; Trudolyubov et al 2007). We suggest here for the first time that they could be PULXs that have gone unnoticed so far.…”

XMM-Newton discovery of transient X-ray pulsar in NGC 1313

“…XMMU J031747.5−663010 is the second X‐ray pulsar detected outside the local group, after 18 s pulsating X‐ray source CXOU J073709.1+653544 in the nearby spiral galaxy NGC 2403 (Trudolyubov et al 2007). Both sources are extremely bright transient systems with total luminosities ≳10 39 erg s −1 , and probably belong to a rare class of most luminous Be binary X‐ray pulsars.…”