The origin of RX J1856.5−3754 and RX J0720.4−3125 - updated using new parallax measurements

Abstract

RX J1856.5−3754 and RX J0720.4−3125 are the only young isolated radio‐quiet neutron stars (NSs) for which trigonometric parallaxes were measured. Due to detection of their thermal emission in X‐rays, they are important to study NS cooling and to probe theoretical cooling models. Hence, a precise determination of their age is essential. Recently, new parallax measurements of RX J1856.5−3754 and RX J0720.4−3125 were obtained. Considering that NSs may originate from binary systems that got disrupted due to an asymmetric supernova, we attempt to identify runaway stars which may have been former companions to the NS progenitors. Such an identification would strongly support a particular birth scenario with time and place. We trace back each NS, runaway star and the centres of possible birth associations (assuming that most NSs are ejected directly from their parent association) to find close encounters. The kinematic age is then given by the time since the encounter. We use Monte Carlo simulations to account for observational uncertainties and evaluate the outcome statistically. Using the most recent parallax measurement of 8.16 ± 0.80 mas for RX J1856.5−3754 by Walter et al., we find that it originated in the Upper Scorpius association 0.46 ± 0.05 Myr ago. This kinematic age is slightly larger than the value we reported earlier (0.3 Myr) using the old parallax value of 5.6 ± 0.6 mas by Kaplan. Our result is strongly supported by its current radial velocity which we predict to be 6+19− 20 km s−1. This implies an inclination angle to the line of sight of 88°± 6° consistent with estimates by van Kerkwijk & Kulkarni from the bow shock. No suitable runaway star was found to be a potential former companion of RX J1856.5−3754. Making use of a recent parallax measurement for RX J0720.4−3125 of 3.6 ± 1.6 mas by Eisenbeiss, we find that this NS was possibly born in Trumpler 10 0.85 ± 0.15 Myr ago. This kinematic age is somewhat larger than the one obtained using the old parallax value of 2.77 ± 1.29 mas by Kaplan et al. (0.5 Myr). We suggest the B0 runaway supergiant HIP 43158 as a candidate for a former companion of the progenitor star. Then, the current distance of RX J0720.4−3125 to the Sun should be 286+27− 23 pc, in agreement with recent measurements. We then expect the radial velocity of RX J0720.4−3125 to be −76+34− 17 km s−1.

New radial velocities for 30 candidate runaway stars and a possible binary supernova origin for HIP 9470 and PSR J0152–1637

“…Neutron star parameters: heliocentric radial velocity v r , distance d or parallax π, proper motion µ * α , µ δ , peculiar space velocity v sp ; Predicted supernova position: distance of the supernova to Earth at the time of the supernova (d ,SN ) and as seen today (d ,today ), Galactic coordinates (Galactic longitude l, Galactic latitude b, J2000.0) as seen from the Earth today; Predicted time of the supernova in the past τ . For the derivation of the parameters we refer the reader to the work of Tetzlaff et al 2010 previously by Hoogerwerf et al (2001), Bobylev & Bajkova (2009), and by ourselves (Tetzlaff et al 2014(Tetzlaff et al , 2011a(Tetzlaff et al , 2009(Tetzlaff et al , 2010(Tetzlaff et al , 2012, to account for all errors on the observables. Among all young nearby neutron stars 5 in the ATNF pulsar database 6 with available distance and proper motion (105 neutron stars in total, see Tetzlaff 2013), we find two candidates for a common origin with HIP 9470: PSR J0358+5413 and PSR J0152−1637.…”

“…We note, however, that only about 35 per cent of former runaway star/neutron star pairs can be recovered due to the large uncertainties on the observables (Tetzlaff 2013). Owing to the unknown radial velocity of neutron stars, their flight paths can only be traced back up to a maximum of a few Myr (we use 5 Myr, e. g. Tetzlaff 2013;Tetzlaff et al 2011aTetzlaff et al , 2010). This also sets a limit on the maximum age of the runaway star.…”

“…Past stellar trajectories can be used to identify the parent association or cluster of a star and its runaway origin. Some runaway stars have been proposed as former companion candidates to the progenitors of neutron stars (Hoogerwerf et al 2001;Tetzlaff 2013;Tetzlaff et al 2011aTetzlaff et al , 2012. Such an association may explain the spectral properties of a runaway star, such as a high Helium abundance or high rotational velocity or even enhanced α element abundances as supernova debris (Blaauw 1993;Przybilla et al 2008).…”

The compactness of the isolated neutron star RX J0720.4−3125

“…In Tetzlaff et al (2011), the direction of motion was determined to be 62 ± 9 • , by tracing back the motion of RX J0720.4−3125 to its possible birth association Trumpler−10. The direction of motion determined kinematically is thus roughly consistent with almost alignment of spin and orbit.…”

THE X-RAY COUNTERPART OF THE HIGH-BPULSAR PSR J0726–2612

“…Empirical evidence for this comes from our X-ray timing efforts (Kaplan & van Kerkwijk 2005a, 2005b, 2009a, 2009bvan Kerkwijk & Kaplan 2008), which showed that the current field strengths of the INSs are remarkably similar, in the range (1.0-3.5) × 10 13 G, and that their characteristic ages of several Myr are substantially in excess of true ages of ∼0.5 Myr inferred from cooling and kinematics (Walter 2001;Motch et al 2005;Kaplan et al 2007;van Kerkwijk & Kaplan 2008;Motch et al 2009;Tetzlaff et al 2010Tetzlaff et al , 2011. The long periods and characteristic ages follow naturally if the INSs initially had much stronger fields and thus faster spin-down, and the similar current field strengths can be understood if fields stop decaying at a common value.…”