Pulsar In NGC Research Articles

Absence of nebular He ii λ4686 constrains the UV emission from the Ultraluminous X-ray pulsar NGC 1313 X–2

Abstract While much has been learned in recent decades about the X-ray emission of the extragalactic Ultraluminous X-ray sources (ULXs), their radiative output in the UV band remains poorly constrained. Understanding of the full ULX spectral energy distribution (SED) is imperative to constrain the accretion flow geometry powering them, as well as their radiative power. Here we present constraints on the UV emission of the pulsating ULX (PULX) NGC 1313 X–2 based on the absence of nebular He ii λ4686 emission in its immediate environment. To this end, we first perform multi-band spectroscopy of the ULX to derive three realistic extrapolations of the SED into the unaccessible UV, each predicting varying levels of UV luminosity. We then perform photo-ionization modelling of the bubble nebula and predict the He ii λ4686 fluxes that should have been observed based on each of the derived SEDs. We then compare these predictions with the derived upper limit on He ii λ4686 from MUSE data, which allows us to infer a UV luminosity LUV ≲ 1 × 1039 erg/s in the PULX NGC 1313 X–2. Comparing the UV luminosity inferred with other ULXs, our work suggests there may be an intrinsic difference between hard and soft ULXs, either related to different mass-transfer rates and/or the nature of the accretor. However, a statistical sample of ULXs with inferred UV luminosities is needed to fully determine the distinguishing features between hard and soft ULXs. Finally, we discuss ULXs ionising role in the context of the nebular He ii λ4686 line observed in star-forming, metal-poor galaxies.

Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1

NGC 5907 ULX1 is the most luminous ultra-luminous X-ray pulsar (ULXP) known to date, reaching luminosities in excess of 1041 erg s−1. The pulsar is known for its fast spin-up during the on-state. Here, we present a long-term monitoring of the X-ray flux and the pulse period between 2003 and 2022. We find that the source was in an off- or low-state between mid-2017 to mid-2020. During this state, our pulse period monitoring shows that the source had spun down considerably. We interpret this spin-down as likely being due to the propeller effect, whereby accretion onto the neutron star surface is inhibited. Using state-of-the-art accretion and torque models, we use the spin-up and spin-down episodes to constrain the magnetic field. For the spin-up episode, we find solutions for magnetic field strengths of either around 1012 G or 1013 G, however, the strong spin-down during the off-state seems only to be consistent with a very high magnetic field, namely, > 1013 G. This is the first time a strong spin-down is seen during a low flux state in a ULXP. Based on the assumption that the source entered the propeller regime, this gives us the best estimate so far for the magnetic field of NGC 5907 ULX1.

Spectral Evolution of Ultraluminous X-Ray Pulsar NGC 300 ULX-1

We report on results from a 1 yr soft X-ray observing campaign of the ultraluminous X-ray pulsar NGC 300 ULX-1 by the Neutron star Interior Composition Explorer (NICER) during 2018–2019. Our analysis also made use of data from Swift/XRT and XMM-Newton in order to model and remove contamination from the nearby eclipsing X-ray binary NGC 300 X-1. We constructed and fitted a series of 5 day averaged NICER spectra of NGC 300 ULX-1 in the 0.4–4.0 keV range to evaluate the long-term spectral evolution of the source, and we found that an absorbed power-law model provided the best fit overall. Over the course of our observations, the source flux (0.4–4.0 keV; absorbed) dimmed from 2 × 10−12 to below 10−13 erg s−1 cm−2 and the spectrum softened, with the photon index going from Γ ≈ 1.6 to Γ ≈ 2.6. We interpret the spectral softening as reprocessed emission from the accretion disk edge coming into view while the pulsar was obscured by the possibly precessing disk. Some spectral fits were significantly improved by the inclusion of a disk blackbody component, and we surmise that this could be due to the pulsar emerging in between obscuration episodes by partial covering absorbers. We posit that we observed a low-flux state of the system (due to line-of-sight absorption) punctuated by the occasional appearance of the pulsar, indicating short-term source variability nested in longer-term accretion disk precession timescales.

Upgraded GMRT survey for pulsars in globular clusters

Context. Globular clusters (GCs) contain a unique pulsar population, with many exotic systems that can form only in their dense stellar environments. Such systems are potentially very interesting for new tests of gravity theories and neutron-star mass measurements. Aims. The leap in sensitivity of the upgraded Giant Metrewave Radio Telescope (uGMRT) in India, especially at low radio frequencies (< 1 GHz), motivated a new search for radio pulsars in a group of eight southern GCs. We aim to image these clusters in order to have independent measurements of the radio fluxes of known pulsars and the identification of bright radio sources that could be pulsars missed by pulsation search pipelines due to their inherent limitations. Methods. The observations were conducted at 650 MHz (Band 4 receivers) on Terzan 5, NGC 6441, NGC 6440, and NGC 6544, and at 400 MHz (Band 3 receivers) on NGC 6652, NGC 6539, NGC 1851, and M 30. Segmented acceleration and jerk searches were performed on the data. Simultaneously, we obtained interferometric data on these clusters, which were later converted into radio images. Results. We discovered PSR J1835−3259B, a 1.83-ms pulsar in NGC 6652; this is in a near-circular wide orbit of 28.7-h with an unidentified low-mass (∼0.2 M⊙) companion, likely a helium white dwarf. We derived a ten-year timing solution for this system. We also present measurements of scattering, flux densities, and spectral indices for some of the previously known pulsars in these GCs. Conclusions. A significant fraction of the pulsars in these clusters have steep spectral indices. Additionally, we detected eight radio point sources not associated with any known pulsar positions in the radio images. There are four newly identified sources, three in NGC 6652 and one in NGC 6539, as well as one previously identified source in NGC 1851, NGC 6440, NGC 6544, and Terzan 5. Surprisingly, our images show that our newly discovered pulsar, PSR J1835−3259B, is the brightest pulsar in all GCs we have imaged; like other pulsars with broad profiles (Terzan 5 C and O), its flux density in the radio images is much larger than in its pulsations. This indicates that their pulsed emission is only a fraction of their total emission. The detection of radio sources outside the core radii but well within the tidal radii of these clusters show that future GC surveys should complement the search analysis by using the imaging capability of interferometers, and preferentially synthesise large number of search beams in order to obtain a larger field of view.

Antiglitches in accreting pulsars from superfluid vortex avalanches

ABSTRACT Three sudden spin-down events, termed ‘antiglitches’, were recently discovered in the accreting pulsar NGC 300 ULX-1 by the Neutron Star Interior Composition Explorer mission. Unlike previous antiglitches detected in decelerating magnetars, these are the first antiglitches recorded in an accelerating pulsar. One standard theory is that pulsar spin-up glitches are caused by avalanches of collectively unpinning vortices that transfer angular momentum from the superfluid interior to the crust of a neutron star. Here, we test whether vortex avalanches are also consistent with the antiglitches in NGC 300 ULX-1, with the angular momentum transfer reversed. We perform N-body simulations of up to 5 × 103 pinned vortices in two dimensions in secularly accelerating and decelerating containers. Vortex avalanches routinely occur in both scenarios, propagating inwards and outwards, respectively. The implications for observables, such as size and waiting time statistics, are considered briefly.

Discoveries and timing of pulsars in NGC 6440

ABSTRACT Using the MeerKAT radio telescope, a series of observations have been conducted to time the known pulsars and search for new pulsars in the globular cluster NGC 6440. As a result, two pulsars have been discovered, NGC 6440G and NGC 6440H, one of which is isolated and the other a non-eclipsing (at frequencies above 962 MHz) ‘Black Widow’, with a very low mass companion (Mc > 0.006 M⊙). It joins the other binary pulsars discovered so far in this cluster that all have low companion masses (Mc < 0.30 M⊙). We present the results of long-term timing solutions obtained using data from both Green Bank and MeerKAT telescopes for these two new pulsars and an analysis of the pulsars NGC 6440C and NGC 6440D. For the isolated pulsar NGC 6440C, we searched for planets using a Markov chain Monte Carlo technique. We find evidence for significant unmodelled variations but they cannot be well modelled as planets nor as part of a power-law red-noise process. Studies of the eclipses of the ‘Redback’ pulsar NGC 6440D at two different frequency bands reveal a frequency dependence with longer and asymmetric eclipses at lower frequencies (962–1283 MHz).

Investigation of the Timing and Spectral Properties of an Ultraluminous X-Ray Pulsar NGC 7793 P13

Abstract We perform both timing and spectral analyses using the archival X-ray data taken with Swift, XMM-Newton, NICER, and NuSTAR from 2016 to 2020 to study an ultraluminous pulsar, NGC 7793 P13, that showed a long period of super-Eddington accretion. We use the Rayleigh test to investigate the pulsation at different epochs, and confirm the variation of the pulse profile with finite Gaussian mixture modeling and a two-sample Kuiper test. Taking into account the periodic variation of the spin periods caused by the orbital Doppler effect, we further determine an orbital period of ∼65 days and show that no significant correlation can be detected between the orbital phase and the pulsed fraction. The pulsed spectrum of NGC 7793 P13 in the 0.5–20 keV range can be simply described using a power law with a high-energy exponential cutoff, while the broadband phase-averaged spectrum of the same energy range requires two additional components to account for the contribution of a thermal accretion disk and the Comptonization photons scattered into the hard X-rays. We find that NGC 7793 P13 stayed in the hard ultraluminous state and the pulsed spectrum was relatively soft when the source was faint at the end of 2019. Moreover, an absorption feature close to 1.3 keV is marginally detected from the pulsed spectra and it is possibly associated with a cyclotron resonant scattering feature.

Anti-glitches in the Ultraluminous Accreting Pulsar NGC 300 ULX-1 Observed with NICER

Abstract We present evidence for three spin-down glitches (or “anti-glitches”) in the ultraluminous accreting X-ray pulsar NGC 300 ULX-1, in timing observations made with the Neutron Star Interior Composition Explorer. Our timing analysis reveals three sudden spin-down events of magnitudes Δν = −23, −30, and −43 μHz (fractional amplitudes Δν/ν = −4.4, −5.5, and −7.7 × 10−4). We determined fully phase-coherent timing solutions through the first two glitches, giving us high confidence in their detection, while the third candidate glitch is somewhat less secure. These are larger in magnitude (and opposite in sign) than any known radio pulsar glitch. This may be caused by the prolonged rapid spin up of the pulsar, causing a sudden transfer of angular momentum between the superfluid and non-superfluid components of the star. We find no evidence for profile or spectral changes at the epochs of the glitches, supporting the conclusion that these are due to the same process as in normal pulsar glitches, but in reverse.

Chandra reveals a possible ultrafast outflow in the super-Eddington Be/X-ray binary Swift J0243.6+6124

Accretion at super-Eddington rates is expected to be accompanied by strong outflows. Such outflows are observed in Galactic X-ray binaries and extragalactic Ultra-luminous X-ray sources (ULXs). However, due to their large source distances, ULX outflows are challenging to detect and study in detail. Galactic neutron stars accreting from a Be-star companion at super-Eddington rates show many similarities to ULX pulsars, and therefore offer an alternative approach to study outflows in this accretion regime. Here, we present Chandra high-resolution spectroscopy of such a super-Eddington accreting neutron star, Swift J0243.6+6124, to search for wind outflow signatures during the peak of its 2017/2018 giant outburst. We detect narrow emission features at rest from Ne, Mg, S, Si, and Fe. In addition, we detect a collection of absorption features which can be identified in two ways: either as all Fe transitions at rest (with a possible contribution from Mg), or a combination of three blue-shifted Ne and Mg lines at $\sim 0.22c$, while the remaining lines are at rest. The second scenario would imply an outflow with a velocity similar to those seen in ULXs, including the ULX pulsar NGC 300 ULX-1. This result would also imply that Swift J0243.6+6124 launches both a jet, detected in radio and reported previously, and an ultra-fast wind outflow simultaneously at super-Eddington accretion rates.

A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13

We present a timing analysis of multiple XMM-Newton and NuSTAR observations of the ultra-luminous pulsar NGC 7793 P13 spread over its 65 d variability period. We use the measured pulse periods to determine the orbital ephemeris, confirm a long orbital period with Porb = 63.9+0.5−0.6 d, and find an eccentricity of e ≤ 0.15. The orbital signature is imprinted on top of a secular spin-up, which seems to get faster as the source becomes brighter. We also analyze data from dense monitoring of the source with Swift and find an optical photometric period of 63.9 ± 0.5 d and an X-ray flux period of 66.8 ± 0.4 d. The optical period is consistent with the orbital period, while the X-ray flux period is significantly longer. We discuss possible reasons for this discrepancy, which could be due to a super-orbital period caused by a precessing accretion disk or an orbital resonance. We put the orbital period of P13 into context with the orbital periods implied for two other ultra-luminous pulsars, M82 X-2 and NGC 5907 ULX, and discuss possible implications for the system parameters.

Evidence for a variable Ultrafast Outflow in the newly discovered Ultraluminous Pulsar NGC 300 ULX-1

Ultraluminous pulsars are a definite proof that persistent super-Eddington accretion occurs in nature. They support the scenario according to which most Ultraluminous X-ray Sources (ULXs) are super-Eddington accretors of stellar mass rather than sub-Eddington intermediate mass black holes. An important prediction of theories of supercritical accretion is the existence of powerful outflows of moderately ionized gas at mildly relativistic speeds. In practice, the spectral resolution of X-ray gratings such as RGS onboard XMM-Newton is required to resolve their observational signatures in ULXs. Using RGS, outflows have been discovered in the spectra of 3 ULXs (none of which are currently known to be pulsars). Most recently, the fourth ultraluminous pulsar was discovered in NGC 300. Here we report detection of an ultrafast outflow (UFO) in the X-ray spectrum of the object, with a significance of more than 3{\sigma}, during one of the two simultaneous observations of the source by XMM-Newton and NuSTAR in December 2016. The outflow has a projected velocity of 65000 km/s (0.22c) and a high ionisation factor with a log value of 3.9. This is the first direct evidence for a UFO in a neutron star ULX and also the first time that this its evidence in a ULX spectrum is seen in both soft and hard X-ray data simultaneously. We find no evidence of the UFO during the other observation of the object, which could be explained by either clumpy nature of the absorber or a slight change in our viewing angle of the accretion flow.

A Potential Cyclotron Resonant Scattering Feature in the Ultraluminous X-Ray Source Pulsar NGC 300 ULX1 Seen by NuSTAR and XMM-Newton

Abstract Based on phase-resolved broadband spectroscopy using XMM-Newton and NuSTAR, we report on a potential cyclotron resonant scattering feature (CRSF) at E ∼ 13 keV in the pulsed spectrum of the recently discovered ultraluminous X-ray source (ULX) pulsar NGC 300 ULX1. If this interpretation is correct, the implied magnetic field of the central neutron star is B ∼ 1012 G (assuming scattering by electrons), similar to that estimated from the observed spin-up of the star, and also similar to known Galactic X-ray pulsars. We discuss the implications of this result for the connection between NGC 300 ULX1 and the other known ULX pulsars, particularly in light of the recent discovery of a likely proton cyclotron line in another ULX, M51 ULX-8.

Super-Eddington accretion on to the neutron star NGC 7793 P13: Broad-band X-ray spectroscopy and ultraluminous X-ray sources

We present a detailed, broadband X-ray spectral analysis of the ULX pulsar NGC 7793 P13, a known super-Eddington source, utilizing data from the $XMM$-$Newton$, $NuSTAR$ and $Chandra$ observatories. The broadband $XMM$-$Newton+NuSTAR$ spectrum of P13 is qualitatively similar to the rest of the ULX sample with broadband coverage, suggesting that additional ULXs in the known population may host neutron star accretors. Through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures $\sim$0.5 and $\sim$1.5 keV are required to fit the data below 10 keV, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. The characteristic radii of the thermal components appear to be similar, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form, but is terminated by the neutron star's magnetic field soon after its onset, implying a limit of $B \lesssim 6 \times 10^{12}$ G for the dipolar component of the central neutron star's magnetic field. Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs. Finally, we examine the spectrum exhibited by P13 during one of its unusual 'off' states. These data require both a hard powerlaw component, suggesting residual accretion onto the neutron star, and emission from a thermal plasma, which we argue is likely associated with the P13 system.

Modelling the light curves of ultraluminous X-ray sources as precession

We present a freely available xspec model for the modulations seen in the long-term light curves of multiple ultraluminous X-ray sources (ULXs). By incorporating the physics of multiple electron scatterings (ray traced with a Monte-Carlo routine), we go beyond analytical predictions and show that the geometrical beaming of radiation in the conical outflow can be more than a factor of 100 for opening angles smaller than $10^\circ$. We apply our new model to the long-term, well sampled Swift light curve of the recently confirmed ULX pulsar NGC 5907 X-1 with an established period of 78 days. Our results suggest that geometrical beaming together with a slight precession of the conical wind can describe the light curve with a consistent set of parameters for the wind. The small opening angle of roughly $10\mathrm{-}13^\circ$ implies a highly super-critical flow and boosting factors at the order of $\mathcal{B}=60\mathrm{-}90$ that would yield a fairly low surface magnetic field strength of $2\times 10^{10}\,$Gauss.

COHERENT TIMING OF THE ACCRETING MILLISECOND PULSAR NGC 6440 X-2

We study the 205.9 Hz pulsations of the accreting millisecond X-ray pulsar NGC 6440 X-2 across all outbursts observed with the Rossi X-ray Timing Explorer over a period of 800 days. We find the pulsations are highly sinusoidal with a fundamental amplitude of 5%-15% rms and a second harmonic that is only occasionally detected with amplitudes of <2% rms. By connecting the orbital phase across multiple outbursts, we obtain an accurate orbital ephemeris for this source and constrain its 57 min orbital period to sub-ms precision. We do not detect an orbital period derivative to an upper limit of $ | \dot{P} | \leq 8 \times 10^{-11}$ s/s. We investigate the possibility of coherently connecting the pulse phase across all observed outbursts, but find that due to the poorly constrained systematic uncertainties introduced by a flux-dependent bias in the pulse phase, multiple statistically acceptable phase-connected timing solutions exist.

1 Hz FLARING IN THE ACCRETING MILLISECOND PULSAR NGC 6440 X-2: DISK TRAPPING AND ACCRETION CYCLES

The dynamics of the plasma in the inner regions of an accretion disk around accreting millisecond X-ray pulsars is controlled by the magnetic field of the neutron star. The interaction between an accretion disk and a strong magnetic field is not well-understood, particularly at low accretion rates (the so-called ``propeller regime'). This is due in part to the lack of clear observational diagnostics to constrain the physics of the disk-field interaction. Here we associate the strong ~1 Hz modulation seen in the accreting millisecond X-ray pulsar NGC 6440 X-2 with an instability that arises when the inner edge of the accretion disk is close to the corotation radius (where the stellar rotation rate matches the Keplerian speed in the disk). A similar modulation has previously been observed in another accreting millisecond X-ray pulsar (SAX J1808.4-3658) and we suggest that the two phenomena are related and that this may be a common phenomenon among other magnetized systems. Detailed comparisons with theoretical models suggest that when the instability is observed, the interaction region between the disk and the field is very narrow -- of the order of 1 km. Modelling further suggests that there is a transition region (~1-10 km) around the corotation radius where the disk-field torque changes sign from spin up to spin down. This is the first time that a direct observational constraint has been placed on the width of the disk-magnetosphere interaction region, in the frame of the trapped-disk instability model.

SIX NEW RECYCLED GLOBULAR CLUSTER PULSARS DISCOVERED WITH THE GREEN BANK TELESCOPE

We have completed sensitive searches for new pulsars in seven globular clusters using the Robert C. Byrd Green Bank Telescope, and have discovered six new recycled pulsars (four in NGC 6517 and two in M22), five of which are fully recycled millisecond pulsars with P < 10 ms. We report full timing solutions for all six new pulsars and provide estimates of their flux densities and spectral indices. None of the pulsars are detected in archival Chandra data down to L_X~10^32 erg/s for NGC 6517 and L_X~10^31 erg/s for M22. One of the millisecond pulsars in M22 appears to have a very low mass companion, and is likely a new "black widow". A second binary pulsar in NGC 6517 is in a long-period, mildly eccentric orbit. We are able to set some lower limits on the age of the system, and find that it may be less than a few hundred million years old, which would indicate recent pulsar recycling in NGC 6517. An isolated pulsar in NGC 6517 that lies about 20 core radii from the cluster center appears to have been ejected from the core by interacting with a massive binary. By analyzing the luminosity function of the pulsars in NGC 6517, we predict the cluster to harbor roughly a dozen pulsars. We use the observed period derivatives of three pulsars to set lower limits on the mass-to-light ratios in the cores of their host clusters, and find no evidence for large amounts of low-luminosity matter. We also discuss reasons for non-detections in some of the clusters we searched.

CONSTRAINTS ON THE PROPERTIES OF THE NEUTRON STAR XTE J1814–338 FROM PULSE-SHAPE MODELS

The accretion-powered (non-X-ray burst) pulsations of XTE J1814-338 are modeled to determine neutron star parameters and their uncertainties. The model is a rotating circular hot spot and includes: (1) an isotropic blackbody spectral component; (2) an anisotropic Comptonized spectral component; (3) relativistic time-delays and light-bending; and (4) the oblate shape of the star due to rotation. This model is the simplest possible model that is consistent with the data. The resulting best-fit parameters of the model favor stiff equations of state, as can be seen from the 3-sigma allowed regions in the mass-radius diagram. We analyzed all data combined from a 23 day period of the 2003 outburst, and separately analyzed data from 2 days of the outburst. The allowed mass-radius regions for both cases only allow equations of state (EOS) that are stiffer than EOS APR (Akmal et al. 1998), consistent with the large mass that has been inferred for the pulsar NGC 6440B (Freire et al. 2008). The stiff EOS inferred by this analysis is not compatible with the soft EOS inferred from a similar analysis of SAX J1808.

ChandraandXMM‐NewtonDiscovery of Transient X‐Ray Pulsar in the Nearby Spiral Galaxy NGC 2403

We report on the discovery and analysis of the transient X-ray pulsar CXOU J073709.1+653544 detected in the 2004 August-October Chandra and XMM-Newton observations of the nearby spiral galaxy NGC 2403. The X-ray source exhibits X-ray pulsations with a period P ~ 18 s and a nearly sinusoidal pulse shape and pulsed fraction 46%-70% during the first three observations. The observed pulsation period decreased rapidly from 18.25 s on August 9 to 17.93 s on September 12 and possibly 17.56 s on 2004 October 3. The X-ray spectra of CXOU J073709.1+653544 are hard and are well fitted with an absorbed simple power law of photon index Γ ~ 0.9-1.2 in the 0.3-7 keV energy band. The X-ray properties of the source and the absence of an optical/UV counterpart brighter than 20 mag allow us to identify CXOU J073709.1+653544 as an accreting X-ray pulsar in NGC 2403. The maximum unabsorbed luminosity of the source in the 0.3-7 keV range, LX ~ 2.6 × 1038 ergs s-1 at 3.2 Mpc, is at least 260 times higher than its quiescent luminosity. The estimated source luminosity in the 0.3-100 keV energy range could be as high as ~1.2 × 1039 ergs s-1. The rate of decrease of the pulsation period of the source ( ~ -10-7 s s-1) is one of the fastest observed among accreting pulsars. The evolution of the pulsation period suggests that it is dominated by the intrinsic spin-up of the compact object, which is almost certainly a neutron star. The X-ray luminosity of CXOU J073709.1+653544 is high enough to account for the observed spin-up rate, assuming that the X-ray source is powered by disk accretion onto a highly magnetized neutron star. Based on the transient behavior and overall X-ray properties of the source, it could be an X-ray pulsar belonging to either a Be binary system or a low-mass system similar to the transient Galactic bursting pulsar GRO J1744-28.

Timing the Eccentric Binary Millisecond Pulsar in NGC 1851

We have used the Green Bank Telescope to observe the millisecond pulsar PSR J0514-4002A on 43 occasions spread over 2 years. This 5-ms pulsar is located in the globular cluster NGC 1851; it belongs to a binary system and has a highly eccentric (e = 0.888) orbit. We have obtained a phase-coherent timing solution for this object, including very precise position, spin and orbital parameters. The pulsar is located 4.6" (about 1.3 core radii) from the center of the cluster, and is likely to lie on its more distant half. The non-detection of eclipses at superior conjunction can be used, given the peculiar geometry of this system, to rule out the possibility of an extended companion. We have measured the rate of advance of periastron for this binary system to be $\dot{\omega}$ = 0.01289(4) degrees per year, which if due completely to general relativity, implies a total system mass of 2.453(14) solar masses. Given the known mass function, the pulsar mass has to be < 1.5 solar masses, and the mass of the companion has to be > 0.96 solar masses, implying that it is a heavy white dwarf. The 350-MHz flux density of this pulsar varies between 0.2 and 1.4 mJy; the origin of these variations is not known.