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Three Binary Millisecond Pulsars in NGC 6266

We present rotational and astrometric parameters of three millisecond pulsars located near the center of the globular cluster NGC 6266 (M62) resulting from timing observations with the Parkes radio telescope. Their accelerations toward the cluster center yield values of the cluster central density and mass-to-light ratio consistent with those derived from optical data. The three pulsars are in binary systems. One (spin period P=5.24 ms) is in a 3.5-day orbit around a companion of minimum mass 0.2 Msun. The other two millisecond pulsars (P=3.59 ms and 3.81 ms) have shorter orbital periods (3.4 hr and 5.0 hr) and lighter companions (minimum mass 0.12 Msun and 0.07 Msun respectively). The pulsar in the closest system is the fifth member of an emerging class of millisecond pulsars displaying irregular radio eclipses and having a relatively massive companion. This system is a good candidate for optical identification of the companion star. The lack of known isolated pulsars in NGC 6266 is also discussed.


INTRODUCTION
Recycled pulsars are old neutron stars revived through transfer of matter and angular momentum from a mass-donor companion in a binary system (e.g. Alpar et al. 1982;Smarr & Blandford 1976;Bhattacharya & van den Heuvel 1991). They are point-like objects and can be considered as test masses for probing gravitational effects. Most of them are also extremely stable clocks, allowing for accurate measurements of their rotational parameters, position and apparent motion in the sky. Because of these characteristics, recycled pulsars found in globular clusters (GCs) have proven to be valuable tools for studying the GC potential well (e.g. Phinney 1992;Camilo et al. 2000;D'Amico et al. 2002), the dynamical interactions in GC cores (e.g. Phinney & Sigurdsson 1991;Colpi, Possenti & Gualandris 2002) and neutron star retention in GCs (e.g. Rappaport et al. 2001). In the case of 47 Tucanae they allowed also the first detection of gas in a GC (Freire et al. 2001).
Globular clusters are a fertile environment for the formation of recycled pulsars: besides evolution from a primordial system, exchange interactions in the ultra-dense core of the cluster favor the formation of various types of binary systems suitable for spinning up the neutron stars they host (Davies & Hansen 1998). Because of this, about 60% of all known millisecond pulsars (MSPs) are in GCs. Unfortunately, pulsars in GCs are elusive sources since they are often distant and in close binary systems. Their large distances make their flux density typically very small and their signals strongly distorted by propagation through the dispersive interstellar medium. In addition, they frequently are members of close binary systems, causing large changes in the apparent spin period and sometimes periodic eclipsing of the radio signal.
The Parkes Globular Cluster (PKSGC) survey is a search for pulsars in the system of southern GCs using the Parkes 64-m radio telescope which commenced in 2000. It exploits the high sensitivity of the central beam of the Parkes multibeam receiver (Staveley-Smith et al. 1996), the efficiency of a modern data acquisition system (e.g., Manchester et al. 2001, D'Amico et al. 2001a) and the high resolution of a new filterbank designed and assembled at Jodrell Bank and Bologna, with the aim of improving the capability for probing distant clusters. Time series data are analyzed with a modern algorithm for the incoherent search of periodicities over a range of dispersion measures (DMs) and accelerations resulting from orbital motion.
This project has already resulted in the discovery of 12 millisecond pulsars in six globular clusters which had no previously associated pulsar (D'Amico et al. 2001a;D'Amico et al. 2001b;Possenti et al. 2001;D'Amico et al. 2002). These detections reversed the declining trend in discoveries of additional clusters hosting these objects; in the seven years from 1987 (when the first pulsar in a GC, B1821−24 in M28, was discovered at Jodrell Bank by Lyne et al. 1987) to 1994 (B1820-30A and B in NGC 6624: Biggs et al. 1994) 13 globular clusters were shown to contain at least one pulsar, whereas no new cluster joined the list in the following six years. More recently, pulsars have been detected in a further three GCs (Ransom 2003a;Ransom 2003b;Jacoby 2003), bringing the current total to 73 pulsars in 22 clusters. 1 This paper discusses results from the PKSGC survey of NGC 6266 (M62). The discovery of the first pulsar in this cluster, PSR J1701−3006A, was presented by D' Amico et al. (2001a). A preliminary announcement of the discovery of two more MSPs was also made by D' Amico et al. (2001b), while three further millisecond pulsars were later detected at the Green Bank Telescope (Jacoby et al. 2002). Here we report details of the discovery of the second and third pulsars, PSRs J1701−3006B and J1701−3006C, both members of shortperiod binary systems, and discuss timing results obtained over a 3-yr interval for all three MSPs discovered at Parkes. Based on these results we investigate the properties of the pulsars and of the host cluster. We particularly discuss the MSP in the tightest of the three systems, which belongs to the rare class of eclipsing radio pulsars.

DATA COLLECTION AND PROCESSING
The PKSGC survey uses the dual polarization central beam of the 20-cm multibeam receiver of the Parkes radio telescope. The two channels have a system temperature of ∼ 22 K and a central frequency of 1390 MHz. A high-resolution filterbank system consisting of 512 × 0.5 MHz adjacent channels per polarization is used to minimize dispersion smearing, preserving significant sensitivity to a 3 ms pulsar with dispersion measure up to 300 cm −3 pc. In this case, the limiting sensitivity is ∼ 0.15 mJy for a signal-to-noise ratio (s/n) of 8 and a standard 2-h observation (assuming a typical duty cycle of ∼ 20% and negligible scattering). After adding the outputs in polarization pairs, the resulting 512 data streams are each high-pass-filtered, integrated and 1-bit digitized every 125 µs. Each observation typically produces 2 − 4 Gbytes of data; a cluster of 10 Alpha-500MHz CPUs at the Astronomical Observatory of Bologna has been used for offline processing.
The processing pipeline first splits each data stream into nonoverlapping segments of 1050, 2100, 4200 or 8400 s, which are processed separately. When no pulsar is known in a GC (and the DM is therefore unknown) the data are dedispersed over a wide range of ∼ 500 − 1000 trial DMs, spanning the interval (1.0 ± 0.4) DM exp , where DM exp is the DM expected for the cluster according to a model for the Galactic distribution of the ionized gas (Taylor & Cordes 1993). Each dedispersed series is then transformed using a Fast Fourier Transform, and the resulting spectra are searched for significant peaks. The process is repeated for spectra obtained from summing 2, 4, 8 and 16 harmonics. This produces a large number of candidate periods above a threshold. The time-domain data are then folded in sub-integrations at each of these periods in turn and searched for both a linear and a parabolic shift in pulse phase. A linear shift corresponds to a correction in the candidate period, whereas a parabolic correction is a signature of acceleration of the pulsar due to its orbital motion. Parameters for final pulse profiles with significant s/n are displayed for visual inspection. This processing scheme led to the discovery of the first pulsar in NGC 6266 (D'Amico et al. 2001a).
For an MSP in a very close orbit and with relatively high minimum companion mass, the acceleration may undergo significant variations during an observation. As a consequence, weak sources can be missed at the confirmation stage if a constant acceleration term is applied to the data. Hence code has been developed at Bologna for searching both the acceleration and the derivative of the acceleration in the sub-integration ar-rays of interesting candidates. Spanning a cubic space, the code searches also for the period in a small interval of values around the nominal candidate period. Using this code we were able to confirm two more binary millisecond pulsars in NGC 6266.
Once a pulsar is detected and confirmed in a cluster, the data are reprocessed with dedispersion at the DM value of the newly discovered pulsar. The resulting time series is then subject to a fully coherent search for Doppler-distorted signals over a large range of acceleration values. Applying this extremely CPU-intensive procedure to NGC 6266, exploring accelerations in the interval |a| < 35 m s −2 for 35-min long segments (and |a| < 17.5 m s −2 for 70-min long segments), resulted in no further discoveries. Regular pulsar timing observations at the Parkes 64-m radio telescope began shortly after the discovery of these pulsars, using the same observing system as the search observations. Timing observations, typically of 30 to 60 minutes duration, are dedispersed and synchronously folded at the predicted topocentric pulsar spin period in an off-line process, forming pulse profiles every few minutes of integration. Topocentric pulse times of arrival (TOAs) are determined by convolving a standard high s/n pulse template with the observed pulse profiles and then analyzed using the program TEMPO 2 . TEMPO converts the topocentric TOAs to solar-system barycentric TOAs at infinite frequency (using the DE200 solar-system ephemeris, Standish 1982) and then performs a multi-parameter fit to determine the pulsar parameters. Table 1 lists the timing parameters obtained for the three pulsars, including precise positions. Values of the dispersion measure (DM) were obtained for each pulsar by splitting the total bandwidth into four adjacent 64-MHz wide sub-bands and computing the differential delays. The available data do not yet allow a constraining fit for the orbital eccentricity e for any of the three pulsars (see the footnotes to Table 1 for details of the fitting procedure.) The mean flux densities at 1400 MHz (S 1400 ) in Table 1 are average values, derived from the system sensitivity estimate and the observed s/n. In the case of PSR J1701−3006B, the quoted flux density refers only to epochs away from the eclipse (see below). As expected from the relatively high DMs, interstellar scintillation does not significantly affect the detectability of these sources; observed variations are within 30% of the nominal flux density reported in Table 1. Post-fit timing residuals as a function of orbital phase for the three millisecond pulsars in NGC 6266 discussed in this paper. All the orbits have been uniformly sampled, with the exception of PSR J1701−3006B for which we have excluded from the fit the TOAs in the region of the eclipse.
The inferred radio luminosities of the three millisecond pulsars (∼ 10 − 20 mJy kpc 2 at 1400 MHz, corresponding to a luminosity at 400 MHz L 400 > ∼ 100 mJy kpc 2 for a typical spectral index −1.7, see Table 1) places all these sources in the bright tail of the luminosity function of millisecond pulsars in the Galactic disk (Lyne et al. 1998) and in 47 Tucanae (Camilo et. al 2000). If we assume a luminosity distribution dN ∝ L −1 d log L (Lorimer 2001), NGC 6266 would contain a few hundred pulsars with L 400 > ∼ 1 mJy kpc 2 , the approximate limiting luminosity observed for Galactic disk pulsars. Unfortunately, the cluster distance and the lack of any strong signal enhancement due to scintillation will make difficult detecting the fainter pulsar population, probably preventing a direct investigation of the shape of the pulsar luminosity function in this cluster.
The pulsar PSR J1701−3006A has the largest flux density and the longest orbital period among the three and was first detected in an observation during 1999 December (D'Amico et al. 2001a). PSR J1701−3006B and J1701−3006C are weaker pulsars in closer binary systems and were confirmed in 2000 November. Once the orbits were determined, signals from these two pulsars were recovered in all the observations performed prior to their confirmation. Therefore the timing solutions reported in Table 1 (and whose residuals are displayed in Fig. 1) cover the same time-span for all three MSPs, from 2000 June to 2003 May. Inspection of Figure 2 shows that all the orbits have been uniformly sampled (excepting PSR J1701−3006B for which we have excluded TOAs in the region of the eclipse; see below) and that there are no systematic trends in the residuals as a function of binary phase.
The three millisecond pulsars discussed here are all located close to the center of mass of the cluster, at least in projection, with projected distances < ∼ 1.8 Θ c , where Θ c = 10. ′′ 8 is the core radius of NGC 6266 (Harris 1996). This is consistent with the hypothesis that the cluster has reached thermal equilibrium, in which energy equipartition gives less velocity to the most massive species, constraining them to reside deep in the cluster potential well.
The spin period derivativesṖ are all negative, implying that the line-of-sight acceleration a l imparted to the pulsars is directed toward the observer and that it overcomes the (positive) P i due to intrinsic spin-down (see e.g. Phinney 1993). The probability that a nearby passing star in the crowded cluster core is significantly accelerating at least one of the three MSPs is < 1% (Phinney 1993). Moreover, given the position and the kinematics of the globular cluster NGC 6266, the centrifugal acceleration of the system (Shklovskii 1970) and the vertical acceleration in the Galactic potential (Kuijken & Gilmore 1989) produce only negligible effects on the measured a l = |cṖ/P|. The differential Galactic rotation (Damour & Taylor 1991) can contribute at most a positive ∼10%, ∼2% and ∼25% to the ob-servedṖ/P of the pulsars A, B and C respectively; hence, we conclude that the sign of the line-of-sight accelerations is dominated by the radially symmetric mean gravitational field of the globular cluster and that the three MSPs are located behind the plane of the sky through the cluster center.
The maximum possible a l due to the mean gravitational field in NGC 6266 is given by the following relation (accurate at the 10% level for Θ ⊥ < ∼ 2Θ c , Phinney 1992) where σ l = 14.3 ± 0.4 km s −1 is the line-of-sight velocity dispersion (Dubath et al. 1997) and D = 6.9 ± 1.0 kpc is the distance (Brocato et al. 1996). Θ c and Θ ⊥ are the angular core radius and the angular displacement with respect to the globular cluster center, located at R.  (Harris 1996(Harris , catalog revision 2003. In particular, for a pulsar with negativeṖ the following inequality must hold: where c is the speed of light. The observed lower limit on the magnitude of the line-ofsight acceleration of PSR J1701−3006B, a l = 2.9 × 10 −6 cm s −2 , is the third largest after those of PSR B2127+11A and D in M15 (Anderson et al. 1990) and is almost identical to those of the two MSPs with negativeṖ recently discovered in the central regions of NGC 6752 (D'Amico et al. 2002). For NGC 6752, the high values ofṖ imply a central mass-to-light ratio larger than that from optical estimates (D'Amico et al. 2002). For NGC 6266 on the other hand, the upper panel in Figure 3 shows that the parameters derived from optical observations can entirely account for the largeṖ/P of PSR J1701−3006B (the vertical size of the dots in Figure 3 represents the contribution to a l /c due to the differential Galactic rotation). In particular, applying equation (1)  Similarly, using the observedṖ/P of J1701−3006A (corrected for the Galactic contribution) and equation (7) of Camilo et al. (2000) the inferred lower limit ρ 0 = 2.1 × 10 5 M ⊙ pc −3 of the central mass density of NGC 6266 is within the limits obtained from optical data (Pryor & Meylan 1993). These results suggest that, even though all three clusters display a compact core and very high line-of-sight accelerations for the embedded pulsars, the dynamics in the inner region of NGC 6266 probably more resemble those of M15, for which 2 < M/L < 3 was inferred by Phinney (1993). The satisfactory match between the dynamical parameters of NGC 6266 constrained from pulsar timing observations and de-rived from optical data allows use of the latter for deriving reliable constraints on the age and surface magnetic field of the millisecond pulsars. For instance, the lower panel in Figure 3 shows that the intrinsic characteristic age of PSR J1701−3006B should be greater than ∼ 1.3 Gyr to be consistent with the cluster's distance and velocity dispersion (including their 1σ uncertainties). This in turn implies an upper limit on the surface magnetic field B s = 3.2 × 10 19 (PṖ) 1/2 = 4.0 × 10 8 G. Less stringent limits can be similarly derived for PSR J1701−3006A (τ i > ∼ 0.15 Gyr and B s < ∼ 17 × 10 8 Gauss) and PSR J1701−3006C (τ i > ∼ 25 Myr and B s < ∼ 31 × 10 8 Gauss). These values are typical for MSPs, both in GCs and in the Galactic disk.

RANGE OF DISPERSION MEASURES
The MSPs in NGC 6266 show the second largest range in DM (a maximum deviation ∆DM = 0.9 cm −3 pc with respect to the average DM ave = 114.34 cm −3 pc) after PSR B1744−24A and PSR J1748−2446C in Terzan 5 . This large range is probably due to a significant gradient in the Galactic electron column density across different lines-of-sight toward the cluster, an interpretation supported by the strong variations in reddening observed across this cluster: δE = ∆E(B − V)/E(B − V) = 0.19/0.48 for an angular displacement of ∆θ E ∼ 7 ′ (as derived from Fig. 3 of Minniti, Coyne & Claria 1992). Alternately, the variations may have a local origin as in the case of 47 Tucanae, where pulsar timing observations show that they are due to a plasma permeating the cluster (Freire et al. 2001). The same explanation has been proposed in the case of M15 (Freire et al. 2001) and NGC 6752 (D'Amico et al. 2002). For NGC 6266, the electron number density of a uniform fully ionized gas would be surprisingly high, n e = 1. 6±0.4 cm −3 , at least an order of magnitude larger than that estimated for the other clusters. The determination of positions, accelerations and precise DMs of the additional MSPs discovered in this cluster (Jacoby et al. 2002) will help in determining the origin of the scatter in DM.

THE ABSENCE OF ISOLATED PULSARS IN NGC 6266
In contrast to other globular clusters in which at least five pulsars have been discovered (in order of decreasing number of pulsars: 47 Tuc, M15 and NGC 6752), all the MSPs known in NGC 6266 are in binary systems (including the three detected by Jacoby et al. 2002). The absence of known isolated pulsars in NGC 6266 cannot simply be ascribed to a selection effect since, for a given spin period and flux density, an isolated MSP is easier to detect than a binary MSP. Unfortunately, the observational biases affecting the fraction F is of isolated pulsars discovered in a given cluster (with respect to the total observed MSP population) are difficult to quantify precisely. Considering all the other clusters, F is > ∼ 2/5. If this ratio applies to NGC 6266, the probability of having the first six detected pulsars be all binary is < ∼ 5%. If this absence of isolated pulsars in NGC 6266 is not a statistical fluctuation, it must relate to the mechanisms of formation of these objects and their interplay with the dynamical state of the cluster. The few isolated millisecond pulsars observed in the Galactic disk (where F is ∼ 1/3) are thought to be endpoints of a rare process of ablation and eventually evaporation of the companion star by the energetic flux of particles and electromagnetic waves emitted by the pulsar (e.g. Ruderman, Shaham & Tavani 1989). Besides this formation channel, the isolated MSPs seen in globular clusters can also result from close stel-lar encounters disrupting a binary system which had previously been through the recycling process (e.g. Sigurdsson & Phinney 1993).
This suggests that NGC 6266 is now in a dynamical state where the rate R form of formation (and of hardening) of binary systems containing a neutron star (and suitable for producing new MSPs) is much larger than the rate of disruption R disr of such systems. This idea is supported by comparison of the relevant rates with other clusters. Table 2 summarizes the values of R form and R disr for the four clusters containing at least five known pulsars. R form scales as the rate of close encounters in the cluster, in turn proportional to ρ 1.5 0 r 2 c , where ρ 0 is the central luminosity density and r c the core radius of the cluster . Inspection of Table 2 shows that the expected frequency of close encounters in the core of NGC 6266 3 is 40% larger than that of M15 and seven times that of NGC 6752. Although the numbers of known pulsars in these clusters are similar, the comparison of their radio luminosities (see discussion in § 2) indicates that NGC 6266 hosts many more pulsars than NGC 6752, in accordance with the trend suggested by the values of R form .
On the other hand, the probability that a binary, once formed, will experience a further encounter, which may change or split it (sometimes creating an isolated millisecond pulsar), scales as R disr ∝ ρ 0.5 0 r −1 c . Hence large values of the ratio R form /R disr ∝ ρ 0 r 3 c should indicate that more neutron stars are in binary systems than are isolated. As Table 2 shows, this prediction roughly conforms with the numbers for the three globular clusters having collapsed cores: NGC 6266 (six binary pulsars) has a ratio R form /R disr a few times larger than M15 (one binary and seven single pulsars) and NGC 6752 (one binary and four single pulsars).
Despite these encouraging agreements, the scaling relations may miss many factors which could strongly differentiate the pulsar population in the clusters, e.g., the mass-to-light ratio, the mass function in the core, the neutron-star retention fraction, the period distribution of the binary systems and the effects of the collapse of the core. The last point could be especially relevant, as the only non-collapsed cluster containing more than five known pulsars, 47 Tucanae, fits with the predictions based on R form , but does not satisfy those related to R disr (see Table 2) -its binary disruption rate should be less than half that of NGC 6266, but it hosts several isolated millisecond pulsars, with a value of F is similar to that seen in the Galactic field, where dynamical encounters are unimportant in the formation of isolated millisecond pulsars.
Detailed numerical simulations are required to investigate if trapping of almost all the neutron stars in close binary systems can really occur, for instance, during the phase immediately preceding the core collapse or its reversal. 6. THE ECLIPSES IN PSR J1701−3006B PSR J1701−3006B displays partial or total eclipses of the radio signal at 1.4 GHz near superior conjunction, i.e. at orbital phase 0.25 (see Fig. 4), clearly due to gas streaming off the companion. A typical event starts at orbital phases in the range 0.15 − 0.20 and ends at orbital phase ∼ 0.35, hence sometimes displaying a slight asymmetry with respect to the expected nominal center of the eclipse at phase 0.25. At both eclipse ingress and egress, the pulses usually exhibit excess propaga-tion delays (see Figs. 4 and 5). The eclipse region covers up to 20% of the entire orbit but, as illustrated in Figure 4, unpredictable irregularities affect both the duration and the appearance of the eclipses. Sometimes the pulsation remains barely visible (see e.g. the case of Fig. 4a), while on other occasions the pulse is totally eclipsed for a large portion of the event (e.g. the cases of Fig. 4e). In a favorable case (Fig. 4b), it has been possible to measure a slight reduction of the s/n of the pulse (although at the 1σ level only: see caption of Fig. 5) as the pulsar signal crosses the region of interaction with the matter released by the companion. FIG Pulse broadening and reduction of the s/n prevent investigation of the frequency-dependent behavior of the delays in our 256−MHz bandwidth. However, assuming that they are completely due to dispersion in an ionized gas (as shown for other eclipsing pulsars, e.g. Fruchter et al. 1990;D'Amico et al. 2001c), the corresponding electron column density variations ∆N e may represent a first viable explanation of the eclipse phenomenology. With ∆N e ∼ 1.5 × 10 18 ∆t −3 cm −2 , where ∆t −3 is the delay at 1.4 GHz in ms, whenever ∆t −3 < ∼ 2 (which could be the case for the entire events in Figs. 4a and 4b), the implied pulse broadening over the receiver bandwidth ∆P −3 = 0.36∆ t −3 ms is at most 80% of the intrinsic pulse width (∼ 0.50 P at 10% of the peak). Hence the pulse may be only largely broadened (with an implied reduction of s/n), but not disappear completely. On other occasions, the delays may increase much more rapidly, possibly growing well beyond ∆t −3 = 2. In this case, the DM variations alone could completely smear the signal, causing a total disappearance of the pulsations.
Alternatively, free-free absorption of the radio-waves in an ionized envelope of matter released from the companion and expanding adiabatically can explain both the weakening and the total disappearance of the radio signal. The optical depth for this process can be written (see Spitzer 1978 andRasio, Shapiro &Teukolsky 1989) as where the orbital separation a and the radius of the eclipse, R E , defined to be the chord at radius a subtended by the angle between the orbital phase of eclipse ingress and orbital phase 0.25, are scaled for PSR J1701−3006B (assuming an orbital inclination of 60 degrees, see later), T is the temperature of the fully ionized gas and ∆t −3 is the observed delay in milliseconds at the border of the event. Relatively small delays (∆t −3 < ∼ 0.4) imply only a small reduction in the observed flux density (τ ff [∆t −3 ] < ∼ 0.1), whereas ∆t −3 > ∼ 1 would be accompanied by significant or complete absorption of the signal. Interferometric observations of the unpulsed continuum and observations at other wavelengths will help to clarify the nature of the eclipses. The occurrence of eclipses suggests that the orbital inclination i is not small. For i = 60 • , the median of all possible inclination angles, and an assumed pulsar mass of 1.40 M ⊙ , M c,60 = 0.14 M ⊙ . For i > ∼ 30 • the companion mass spans the interval 0.12 − 0.26 M ⊙ , corresponding to a Roche lobe radius in the range R L = 0.26 − 0.34 R ⊙ . Hence, independent of the eclipse mechanism, the extension of the eclipsing cloud, > ∼ 0.8 R ⊙ , is larger than R L and the cloud must be continuously refilled with matter released from the companion. The plasma density in the eclipse region is ρ E ∼ 1.6 × 10 −17 ∆t −3 g cm −3 and, assuming isotropic emission, mass continuity implies that the donor star loses gas at a rateṀ c = 4πR 2 where v f ,8 is the wind velocity at R E in units of 10 8 cm s −1 (the order of magnitude of the escape velocity from the surface of the companion).
If the companion is a helium white dwarf (whose maximum radius is R wd = 0.04 R ⊙ for masses > 0.12 M ⊙ and T < ∼ 10 4 K, Driebe et al. 1998), and assuming isotropic emission of the pulsar flux, a significant fraction f = (4% − 20%) × (3.7 × 10 34 erg s −1 /Ė) of the energy deposited onto the companion surface is necessary for releasing the observedṀ c (wherė E is the spin-down power of the pulsar and 3.7 × 10 34 erg s −1 its upper limit derived using the arguments of § 3). However, the energy requirements are more easily satisfied for a non-degenerate bloated companion (as appears to be the case in most eclipsing binary pulsars, Applegate & Shaham 1994).
For example, f = (0.04% − 0.2%) × (3.7 × 10 34 erg s −1 /Ė) for a donor with the radius of a main-sequence star of the same mass, that is, 3-10 times larger than that of a white dwarf. Mass loss from the donor star can be sustained by ablation of its loosely bound surface layers by the relativistic wind emitted by the pulsar. This model has been successfully applied to explain the radio eclipses in close orbital systems having very light companions, e.g., the cases of PSRs B1957+20 (Fruchter et al. 1990) and J2051−0827 (Stappers et al. 2001). As with these other systems, the apparent mass-loss rate from the companion to PSR J1701−3006B is very small; the ablation time scale τ abl = χM c,60 /Ṁ c = χ140 Gyr, where χ is the ionized fraction, is longer than the upper limit on the pulsar age (i.e. the cluster age) unless χ < 0.09.
Following an alternate interpretation, the PSR J1701−3006B system may more resemble that of PSR J1740−5340, where the effects of the pulsar irradiation are negligible in triggering the eclipsing wind from the secondary star (D'Amico et al. 2001c) and the eclipses (or the excess propagation delays, sometimes seen far away the nominal phases of eclipse) are caused by matter overflowing the Roche lobe of the donor star due to the nuclear evolution of the companion (Ferraro et al. 2001). In that system, accretion of matter onto the neutron star is inhibited by the sweeping effect of the pulsar energetic wind, according to the so-called radio-ejection mechanism (Burderi, D'Antona & Burgay 2002). We note that J1701−3006B shares with PSR J1740−5340 (i) a companion significantly more massive than those of PSRs B1957+20 and J2051−0827, (ii) the occurrence of excess propagation delays at 1.4 GHz which are much larger (up to ∼ 1 ms vs few tens of µs) than those observed in any of the systems having very low mass companions 4 and (iii) the presence of irregularities in the eclipses.
A new class of eclipsing recycled pulsars having relatively massive companions (M c,60 = 0.10 − 0.22 M ⊙ ) is emerging from the globular cluster searches. Besides PSR J1701−3006B in NGC 6266 and PSR J1740−5340 in NGC 6397, there are PSR B1744−24A in Terzan 5 (Lyne et al. 1990), PSR J0024−7204W in 47 Tucanae (Camilo et. al. 2000) and PSR J2140−2310A in M30 (Ransom et. al. 2003a), whereas no similar system has been detected in the Galactic field to date. A simple explanation for the overabundance of evaporating "black widow" pulsars in globular clusters with respect to the galactic disk has been recently proposed by King, Davies & Beer (2003): namely, the current companion of most of the eclipsing pulsars in globulars would be the swelled descendent of a turn-off star which replaced the original white dwarf companion of the pulsar in an exchange interaction in the cluster core. This scenario posits that the radio-ejection mechanism (Burderi et al. 2001) is now operating in all the eclipsing millisecond pulsars and provides an evolutionary basis for separating the systems with very low mass companion with respect to those having more massive donor stars; in the former, the mass loss would be driven by angular momentum loss through gravitational radiation, whereas in the latter the mass loss rate would be determined by the nuclear evolution of the companion.
The relatively massive systems in globular clusters are good candidates for optical detection of the donor star 5 and follow-up observations. Unlike the Galactic eclipsing systems, their age, metallicity, extinction, distance and hence intrinsic luminosity and radius can be estimated from the parent cluster parameters (see, e.g., Edmonds et al. 2001a;Ferraro et al. 2001;Edmonds et al. 2002). In the case of J1740−5340 in NGC 6397, the companion is a red variable star of magnitude V ∼ 16.5 (Ferraro et al. 2001) and stringent constraints have been set on the effectiveness of the irradiation of the companion (Orosz & van Kerkwijk 2002), on the occurrence of the radio-ejection mechanism ) and on the evolutionary path of the system (e.g. Burderi, D'Antona & Burgay 2002;Grindlay et al. 2002;Ergma & Sarna 2002).
More recently, the companion of the millisecond pulsar J0024−7204W in 47 Tucanae has been optically identified with a blue variable star of mean magnitude V ∼ 22.3, probably a heated main sequence star close to the center of the cluster . Unfortunately, the pulsar is weak and only occasionally detectable, which makes the system difficult to characterize (Camilo et al. 2000). In the case of PSR B1744−24A in Terzan 5, the strong obscuration toward the Galactic center ( > ∼ 7 mag in V) prevents detection of the optical counterpart, even with deep HST observations ).
Consequently, PSR J1701−3006B is likely to be a primary candidate for improving the modeling of eclipsing millisecond pulsars with relatively massive companions. Indeed, PSR J1701−3006B seems to be a twin of PSR J0024−7204W in 47 Tucanae, with similar orbital parameters and hence minimum companion mass (Camilo et al. 2000). Also the pulsar periods are comparable, 3.6 ms versus 2.4 ms. Moreover, unlike PSRs J1740−5340 and B1744−24A, both PSRs J1701−3006B and J0024−7204W reside well within one core radius of the parent cluster center and hence are in more similar environments. Assuming that the companion to PSR J1701−3006B has the same luminosity and colors as the companion to PSR J0024−7204W, its photometry would be feasible with deep exposures reaching V-magnitude 24.5. Photometry would of course be much easier if the companion fills its Roche lobe as is believed to be the case in PSR J1740−5340.
The X-ray counterparts of two of the five eclipsing MSPs with relatively massive companions (namely PSRs J1740−5340 and J0024−7204W) have been identified using Chandra observations . Their spectra appear significantly harder than those of most other known X-ray counterparts to MSPs in globular clusters. That suggests ) that a non-thermal contribution to the X-ray emission, perhaps arising from shock interactions at the interface between the companion and pulsar winds, dominates over the thermal component seen in the other MSPs, which probably originates from heated magnetic polar caps on the neutron star. The identification of the X-ray counterpart of PSR J1701−3006B and a comparison of its X-ray hardness ratio with that of the other MSPs in NGC 6266 would test the above picture. Interestingly, a long Chandra pointing towards NGC 6266 shows that it hosts the largest number of X-ray sources (with luminosity > 4 × 10 30 erg s −1 in the 0.5-6.0 keV range) observed so far in a globular cluster (Pooley et al. 2003). Possibly among the 51 detected sources is a significant population of neutron stars, of which the 6 pulsars discovered so far are a manifestation. 7. CONCLUSION We have presented rotational and astrometric parameters of three binary millisecond pulsars located within 1.8 core radii of the center of the globular cluster NGC 6266. One of these systems, PSR J1701−3006B, displays eclipses for ∼ 20% of the orbit. In summary, we note that: 1. The derived lower limits on the central mass-density (2.1 × 10 5 M ⊙ pc −3 ) and the central mass-to-light ratio (M/L > 1.6 in solar units) of NGC 6266 are consistent with optical estimates.
2. The large spread in the dispersion measures of the three millisecond pulsars is probably due to a significant gradient in the Galactic electron column density across different lines-of-sight toward the cluster.
3. Even though the nature of the eclipses cannot yet be fully constrained, the relatively low mass-loss rate from the secondary star makes unlikely that PSR J1701−3006B will evaporate its companion. 4. The lack of known isolated pulsars in NGC 6266 is unlikely to be due to chance or observational bias and suggests that the cluster is in a dynamical phase favoring formation over disruption of binary systems containing a neutron star.
We thank the referee Scott Ransom for very helpful criticisms of the original manuscript. ND'A and AP received financial support from the Italian Space Agency (ASI) and the Italian Minister of Research (MIUR) under Cofin 2001 national program. FC acknowledges support from NSF grant AST-02-05853. The Parkes radio telescope is part of the Australia Telescope which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.  -R form is estimated as ∝ ρ 1.5 0 r 2 c , whereas R disr ∝ ρ 0.5 0 r −1 c (see text for details). All the values are normalized to the parameters of 47 Tucanae. Central luminosity density ρ 0 and core radius rc are obtained from the catalog of Harris (1996Harris ( , revision 2003.