ChandraStudy of a Complete Sample of Millisecond Pulsars in 47 Tucanae and NGC 6397

Abstract

We report Chandra observations of the complete sample of millisecond pulsars (MSPs) with precise radio positions in the globular clusters 47 Tuc (NGC 104) and NGC 6397. The x-ray luminosities and colors are derived or constrained and compared to x-ray MSPs previously detected in the field as well as one previously detected in a globular cluster (M28). The 47 Tuc MSPs are predominantly soft sources suggestive of thermal emission from small (r x < 0.6km) polar caps on the neutron star rather than magnetospheric emission and are a relatively homogeneous sample, with most x-ray luminosities in a surprisingly narrow range (L x ∼1-4 ×10 30 erg s −1 ). We use previously derived intrinsicṖ values and find a new relation between L x and spindown luminosity,Ė : L x ∝ E β , with β ∼0.5±0.2 vs. ∼1.0 for both pulsars and MSPs in the field. Adding the single MSP in NGC 6397 constrains β = 0.5 ± 0.15. This L x -Ė relation and also the L x /Ė vs. spindown age are each similar to that found by for thermal emission from polar cap heating. However, the cluster MSPs are relatively longer-lived (in thermal x-rays) than either the models or field MSPs, which may have additional magnetospheric (non-thermal) components. We suggest the cluster MSPs may have altered surface magnetic field topology (e.g. multipole) or their neutron stars are more massive from repeated accretion episodes due to encounters and repeated exchange interactions. MSP binary companions on or just off the main sequence (e.g. NGC 6397) are likely to have been re-exchanged and might show anomalousṖ andĖ values due to relaxation of misaligned core-crust spins. The radial distribution of ∼40 soft Chandra sources in 47 Tuc is consistent with a ∼ 1.4 M ⊙ component in a multi-mass King model and with the identified MSP sample. The implied total MSP population in 47 Tuc with L x > ∼ 10 30 erg s −1 is ∼35-90, and can constrain the relative beaming in radio vs. soft x-rays. NGC 6397 is relatively deficient in MSPs; its single detected example may have been re-exchanged out of the cluster core.

A Millisecond Pulsar Optical Counterpart with Large‐Amplitude Variability in the Globular Cluster 47 Tucanae

“…A smaller ratio between the V-band fluxes is given by a brighter, more massive secondary (requiring an inclination less than 60 ), but then the heating flux would be too small to cause the observed variations. These inconsistencies suggest that the higher _ E E value given earlier is closer to the true value, providing indirect support for the L X versus _ E E relationship of Grindlay et al (2002).…”

“…Using the observed X-ray luminosity of 7:8 Â 10 30 ergs s À1 and the L X versus _ E E relationship of Grindlay et al (2002), we estimate that _ E E for 47 Tuc W is $3 Â 10 35 ergs s À1 (the 1 range is $2 8 Â 10 35 ergs s À1 ). A much smaller value of _ E E (7:8 Â 10 33 ergs s À1 ) is derived using the linear relation of Becker & Trü mper (1999), L X $ 10 À3 _ E E, derived largely for field MSPs.…”

“…Unlike with W29 opt , there is no period or phase match for W34 opt with a known MSP (its period is similar to, but statistically different from, the 95:39 AE 0:1 minute period of 47 Tuc R, an eclipsing MSP with a very low mass companion; CLF00), but a significant fraction of the 47 Tuc MSPs have not yet been detected in the radio (CLF00; Grindlay et al 2002). Given the short period of this system, the heating effects (if it is an MSP) may be even more dramatic than for W29 opt , since the ratio between the heating flux and the stellar luminosity would be higher, assuming a similar luminosity companion star.…”

“…If the MSP was initially ejected from the cluster core in an exchange encounter, it may have had time to sink back into the cluster core, since the MS star could have been captured several Gyr ago, and the central relaxation time for 47 Tuc is 0.1 Gyr (increasing to 3 Gyr at the half-mass radius of 2<8; Harris 1996). The NGC 6397 MSP, by contrast, is found $30 00 from the cluster center (Ferraro et al 2001) and is therefore well outside the cluster core ðr c ¼ 3>0; Harris 1996), possibly showing evidence for ejection from the core Grindlay et al 2002). This ejection may have occurred quite recently, since the central relaxation time for the core-collapsed NGC 6397 is only 0.08 Myr, increasing to 0.3 Gyr at the half-mass radius of 2<3 (Harris 1996).…”

Helium Core White Dwarfs in Globular Clusters

“…Grindlay et al (2002) have examined the core of NGC 6397 with the Chandra satellite, identifying many sources (including a millisecond pulsar and several potential CVs), none of which are coincident with the NFs. This '' surprising '' (in their words) finding is consistent with our analysis.…”