The X-ray Pulsar M82 X-2 on its Propeller Line

Abstract

NuSTAR has detected pulsations from the ultraluminous X-ray source X-2 in M82 and archival Chandra observations have given us a good idea of its duty cycle. The newly discovered pulsar exhibited at least 4 super-Eddington outbursts in the past 15 years but, in its lowest-power state, it radiates just below the Eddington limit and its properties appear to be typical of high-mass X-ray binaries. M82 X-2 has been described as a common neutron star with a 1 TG magnetic field that occasionally accretes above the Eddington rate and as a magnetar-like pulsar with a 10-100 TG magnetic field that reaches above the quantum limit. We argue in favor of the former interpretation. Using standard accretion theory and the available observations, we calculate the stellar magnetic field of this pulsar in two independent ways and we show that it cannot exceed 3 TG in either case. We discuss the implications of our results for other ultraluminous X-ray sources that occasionally exhibit similar powerful outbursts.

Lense-Thirring precession in ULXs as a possible means to constrain the neutron star equation of state

“…However, in our Lense-Thirring model, we require that rm < r sph (where rm is Rm in units of the gravitational radius) which would be broadly consistent with the sub-magnetar field strengths invoked by several authors (e.g. Kluzniak & Lasota 2015; King & Lasota 2016;Christodoulou et al 2016) for ULPs observed to-date. In the specific case of P13, for the mass accretion rates we infer (based on a sensible mass range for the neutron star of < 2.5 M⊙) and based on the above formula for the magnetospheric radius, we restrict our analysis to field strengths <10 13 G to ensure rm < r sph (although see also Israel et al 2017b and the prospects of strong quadrupole field components but weaker dipole fields).…”

“…In our analysis we have chosen to focus on sub-magnetar field strengths (< 10 13 G), consistent with the findings of multiple authors across the three identified ULPs to-date (e.g. Kluzniak & Lasota 2015; King & Lasota 2016;Christodoulou et al 2016;Fuerst et al 2016). In addition, Walton et al (2017) show explicitly the phase-averaged spectrum of P13 which appears consistent with that of other non-pulsed ULXs (see also Motch et al 2014;Pintore et al 2017) whilst we have tentative evidence for the presence of a radiatively driven outflow as seen in archetypal ULXs (Middleton et al 2015b) -P13 would therefore appear to show all the hallmarks of a canonical ULX.…”

“…Although we cannot rule out a scenario where a radiative warp is generated in the outer disc, this would seem to demand that r sph < rm which would require extremely high (magnetar) surface dipole field strengths which can explain much of the details of ULPs including their spectra and smooth pulse profile (Dall'Osso 2016; Mushtukov et al 2017) yet is disputed by a number of authors (e.g. Kluzniak & Lasota 2015; King & Lasota 2016;Christodoulou et al 2016). Assuming that a large component of the ULX population contains neutron stars (although these may not be pulsing: King et al 2017;Middleton & King 2017), the coupled spectral-variability would also appear to argue for a more classical super-critical flow (Middleton et al 2015a).…”

Constraining the dipolar magnetic field of M82 X-2 by the accretion model

“…Recently, based on the observations in the lowest observed states (Brightman et al 2016;Tsygankov et al 2016), Christodoulou, Kazanas & Laycock (2016) used two independent methods to calculate the magnetic field of the pulsar in M82 X-2, and obtain two similar results of 3.1 × 10 12 , and 2.3 × 10 12 G. Following their assumption, if the pulsar is in the propeller line, and is accreting at Eddington accretion rate, our equation ( 6) derives a magnetic field of 1.6×10 12 G (The main difference originates from different magnetic momentum (µ) formulas: we use µ = BR 3 , while they took µ = BR 3 /2). However, in our scenario such an accretion rate and a magnetic field can not provide an accretion torque fitting the observation.…”

How to make a mature accreting magnetar

“…Individual estimates of magnetic field can be very different for a given source as several models (and considerations) can be applied. For example, in the case of M82 X-2, which is the most famous source in the list, estimates range from standard fields ∼ 10 12 G (Christodoulou et al 2016) up to ∼ 10 14 G (Tsygankov et al 2016), including the case of normal dipole (∼ 10 12 G) but strong multipole fields (∼ 10 14 G) (Tong 2015).…”