LP 600 SDSS z 0
5.5 Discussion
Using LGS adaptive-optics supported near-IR observations, we have measured the proper motions of SGR 1806−20 and SGR 1900+14 to be (µα, µδ) = (−4.5,−6.9)±(1.4,2.0) milli- arcsecond yr−1 and (µα, µδ) = (−2.1,−0.6)± (0.4,0.5) milli-arcsecond yr−1 respectively.
These correspond to a linear transverse velocity of 350±100 km s−1 and 130±30 km s−1 respectively at the measured distances of their putative associations. Previously, using Very Long Baseline Interferometry (VLBI) at radio wavelengths, transverse linear velocities have been measured only for two magnetars: the AXP 1E 1810−197: 212±35 km s−1 (Helfand et al., 2007) and the AXP PSR J1550−5418: 280±120 km s−1 (Deller et al., 2012). The radio counterpart for AXP PSR J1622−4950 has been recently identified by Levin et al.
(2010) and would lead to an accurate proper motion measurement with VLBI. With the transverse velocity measurements for two AXPs and two SGRs in the 100−400 km s−1 range, it is highly unlikely that each of these objects has an extremely high radial velocity component. Hence we conclude that magnetars as a family do not possess the high space velocities (∼1000 km s−1) that were expected earlier (cf. Rothschild & Lingenfelter 1996).
Consider the space velocities of other families of neutron stars in contrast with mag- netars. Canonical radio pulsars (B ∼ 1011G) have typical space velocities of ∼ 200− 300 km s−1 (Hobbs et al.,2005). Tetzlaff et al. (2010) traced the motions of 4 young, hot X-ray bright isolated neutron stars to associate them with progenitors and constrain their ages. They calculated the space velocities of these objects to be ∼350±180 km s−1. There are a few fast moving pulsars such as PSR J1357−6429, which is a Vela-like radio pulsar has a transverse velocity of 1600−2000 km s−1 (Kirichenko et al., 2012), but these seem to be outliers from the family. From the these data, we observe that perhaps velocities are not a good discriminator of different groups of neutron stars and their origins.
5.5.1 Association
Our measured proper motions provide very good evidence linking SGR 1806−20 to the cluster of massive stars. The time required for SGR 1806−20 to move from the cluster to its current position is 650±300 yr. It may not be a surprise that one of the younger supernovae in our galaxy resulted from the magnetar. However, SGR 1806−20 lies in the galactic plane behind dust clouds which create very high extinction in the visible wavelengths. Hence, the supernova associated with the magnetar may not have been visible to the naked eye.
For SGR 1900+14, we rule out any association with the supernova remnant G 42.8+0.6 and confirm that this magnetar is associated with the star cluster. The time to trace the magnetar back to the cluster is 6±1.8 kyr.
The turn-off masses for the clusters with which the magnetars are associated allow us to place lower limits on the progenitor masses of these magnetars. Currently, progenitor mass estimates exist for three of the magnetars:
SGR 1806−20: 48+20−8 M (Bibby et al.,2008),
CXO J1647−455: >40 M (Muno et al.,2006;Ritchie et al.,2010) and SGR 1900+14: 17±2 M (Davies et al.,2009).
We note that only the two youngest SGRs have a star cluster in their vicinity. The lack of a star cluster in the vicinity of the older SGRs (despite ages of 4 to 10 kyr) suggests that it is not essential that SGRs should be associated with star clusters. Furthermore, the inferred progenitor masses of SGR 1900+14 does not compel us to believe that SGRs arise from massive stars. We conclude that binarity likely has a bigger role in forming SGRs.
Table 5.15. List of all known magnetar proper motionsa.
Object Vtangent Assoc. Method Ref.
(km s−1)
AXP 1E 1810−197 212±35 – Radio; VLBI Helfand et al.(2007)
AXP 1E 1547.0−5408b 280±120 SNR G327.24−0.13 Radio; VLBI Deller et al.(2012)
SGR 1900+14 130±30 Cluster NIR; LGSAO Tendulkar et al.(2012)
SGR 1806−20 350±100 Cluster NIR; LGSAO Tendulkar et al.(2012)
AXP 1E 2259+586 157±17 SNR CTB 109 NIR; LGSAO Tendulkar et al.(2013)
AXP 4U 0142+61 102±26 – NIR; LGSAO Tendulkar et al.(2013)
SGR 0501++4516 ≈70 HB9 (disproved by this result) NIR; LGSAO Tendulkar et al, unpublished
AXP 1E 1841-045 .160 NIR; LGSAO (Tendulkar,2013)
aThese are the tangential components of the ejection velocities.
bAlso known as PSR J1550−5418
5.5.2 Braking Index
If the association of the SGRs with the star clusters is taken for granted, we can constrain the braking index of the magnetars. The braking index n is calculated from the following implicit equation:
n= 1 + P
TP˙(1−(P0/P)(n−1)).
Here, T is the kinematic age of the magnetar (time taken to move from cluster to present position) and P0 is the spin period at birth.
The instantaneous ˙P is known to vary by a factor of three to four corresponding to large variations of braking torque on the magnetar (Woods et al., 2002, 2007). We use the X- ray timing measurements from Kouveliotou et al.(1998);Mereghetti et al.(2005b);Woods et al. (2007); Marsden et al. (1999); Woods et al. (2002, 2003); Mereghetti et al. (2006);
Nakagawa et al.(2009) to calculate an average ˙P of 49×10−11s s−1 for SGR 1806−20 and 17 × 10−11s s−1 for SGR 1900+14 from 1996 to 2006.
Assuming P0/P 1, we estimate n to be 1.76+0.65−0.24 for SGR 1806−20 and 1.16+0.04−0.07 for SGR 1900+14. This is significantly smaller than the canonical value of n = 3 for the magnetic dipole spindown mechanism for pulsars. Low braking indices have been discussed in the context of twisted magnetospheres (eg. Thompson et al., 2002) and particle wind spindown (e.g. Tong et al., 2012). However, the large variations in ˙P over tens of years implies that these measurements cannot be taken at face value.
5.5.3 Proper Motions of the Magnetar Family
Table 5.15 lists the six available measurements of the tangential space velocities of mag- netars. Figure 5.21 combines the probability distributions (assumed to be gaussian) of all the six magnetar tangential velocities. The weighted average velocity is 200 km s−1 with
0 100 200 300 400 500 600 Tangential Velocity (km s−1)
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045
Probability
Figure 5.21. The probability distributions of the tangential velocities of six magnetars as detailed in Table5.15are plotted as filled curves (colored red in the online version). The dashed black line is the sum of the individual probability distributions. The mean and standard deviation of this distribution is 200 km s−1and 100 km s−1. This is very well consistent with the mean and standard deviation of the normal pulsar population (Hobbs et al., 2005).
a weighted standard deviation of 100 km s−1. This is in good agreement with the tangen- tial velocities of the pulsar population which is measured to be 211 km s−1 (Hobbs et al., 2005) with an standard deviation of ∼100 km s−1. Thus, the kinematics of magnetars are completely consistent with those of pulsars.
Given this velocity distribution, it is improbable that SGR 0526−66 has a ∼103km s−1 velocity. This adds to the growing body of evidence suggesting that SGR 0526−66 may not be associated with SNR N49 (Gaensler et al.,2001;Klose et al.,2004;Badenes et al.,2009;
Park et al.,2012). The original expectation of a large natal kick came from the idea that SGR 0526−66 had rapidly moved to the edge of SNR N49 and the since discredited idea that short hard GRBs are from the galactic halo (Rothschild & Lingenfelter,1996). With these measurements, the probability of finding a magnetar with a large (&1000 km s−1) space velocity is very low and NS kick mechanisms, as enumerated in Lai (2004), may be applicable in a very small fraction of supernovae. Along with the conclusion from Vink &
Kuiper(2006) that supernovae associated with magnetars show no evidence of milli-second proto-neutron stars or higher than typical energetics, the idea of what makes magnetar creating supernovae different from pulsar creating supernovae need to be revisited.
5.5.4 Age of CTB 109 and AXP 1E 2259+586
There has been much recent work to estimate the age of CTB 109. Sasaki et al. (2004) modeled the shell of CTB 109 as a Sedov-Taylor shock with data from deep XMM Newton observations. They estimate the age of CTB 109 to be 8.8±1 kyr. More recent work by (Sasaki et al., 2013, ; in review, personal communication) reports the age to be 14± 2 kyr. These estimates are in contrast with previous estimates from Wang et al. (1992) (3 kyr; hydrodynamical simulations of X-ray temperature),Rho & Petre (1997) (6 - 21 kyr;
ionization modeling) andParmar et al.(1998) (3 kyr; ionization modeling and spectra). The consensus put forth by these studies is that the supernova exploded at the eastern edge of a dense giant molecular cloud complex detected in CO by Israel(1980). The western edge of the expanding supernova shell has been slowed and quenched due to its collision with the molecular cloud and its eastern edge, expanding into a less dense interstellar medium is significantly less quenched. The apparent center of the expanding shell would be expected to move eastward. We observe this in our measurements.
The separation between the current center of CTB 109 and AXP 1E 2259+586 would correspond to a kinematic age of 24±5 kyr which is significantly larger than the estimated age of CTB 109. This discrepancy implies that the current center of CTB 109 has moved to the east, opposite to the movement of AXP 1E 2259+586. It is more worthwhile to reverse the calculation to find the actual center of the explosion. Assuming the age of the remnant to be 14±2 kyr, we can estimate that AXP 1E 2259+586 moved 20.4±0.40towards the west after the explosion and consequently the current center of CTB 109 would have moved by 10.6±0.40 to the east after the explosion. Back calculating from the current position of AXP 1E 2259+586, we estimate that the explosion occurred at (α, δ)J2000 = (23h00m50s,+58◦5200200). The error ellipse at this position has a semi-major axis ofσmaj = 2600, a semi-minor axis ofσmin= 1500 oriented at an angle of 17◦ south of west.