Proper motions of thermally emitting isolated neutron stars measured with Chandra

Abstract

The remarkable astrometric capabilities of the Chandra Observatory offer the possibility to measure proper motions of X-ray sources with an unprecedented accuracy in this wavelength range. We recently completed a proper motion survey of three of the seven thermally emitting radio-quiet isolated neutron stars (INSs) discovered in the ROSAT all-sky survey. These INSs (RX J0420.0−5022, RX J0806.4−4123 and RX J1308.6+2127) either lack an optical counterpart or have one so faint that ground based or space born optical observations push the current possibilities of the instrumentation to the limit. Pairs of ACIS observations were acquired 3 to 5 years apart to measure the displacement of the sources on the X-ray sky using as a reference the background of extragalactic or remote Galactic X-ray sources. We derive 2σ upper limits of 123 mas yr −1 and 86 mas yr −1 on the proper motion of RX J0420.0−5022 and RX J0806.4−4123, respectively. RX J1308.6+2127 exhibits a very significant displacement (∼9σ) yielding μ = 220 ± 25 mas yr −1 , the second fastest measured among all ROSAT-discovered INSs. The source is probably moving away rapidly from the Galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of ∼740 (d/700 pc) km s −1 might be the largest of all ROSAT INSs and its corresponding spatial velocity lies among the fastest recorded for neutron stars. RX J1308.6+2127 is thus a middle-aged (age ∼1 My) high velocity cooling neutron star. We investigate its possible origin in nearby OB associations or from a field OB star. In most cases, the flight time from birth place appears significantly shorter than the characteristic age derived from spin down rate. Overall, the distribution in transverse velocity of the ROSAT INSs is not statistically different from that of normal radio pulsars.

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.…”

Galactic neutron stars

“…Popov et al 2005, and references therein). Recently Motch et al (2009) measured the proper motion of one of these objects and found a value of the 3D velocity of 600−1000 km s −1 . This is not uncommon in PSRs and hence they concluded that the velocity distribution of the Magnificent Seven is not statistically different from that of normal radio pulsars.…”

The isolated neutron star RBS1774 revisited

“…The excess flux was qualitatively explained as originating from cooler areas of the structured NS surface not detected in X-rays, see Schwope et al (2005) or Zane & Turolla (2006) for light curve and SED modeling of XDINS. Furthermore, optical observations are important to search for parallaxes and proper motions (Motch et al , 2009) and thus constrain the likely birth place of the stars.…”