The Eccentric Binary Millisecond Pulsar in NGC 1851
aa r X i v : . [ a s t r o - ph ] N ov The Eccentric Binary Millisecond Pulsar in NGC 1851
Paulo C. C. Freire ∗ , Scott M. Ransom † and Yashwant Gupta ∗∗ ∗ N.A.I.C., Arecibo Observatory, HC3 Box 53995, PR 00612, U.S.A.; [email protected] † N.R.A.O., 520 Edgemont Road, Charlottesville, VA 22903, U.S.A.; [email protected] ∗∗ National Centre for Radio Astrophysics, P.O. Bag 3, Ganeshkhind, Pune 411007, India; [email protected]
Abstract.
PSR J0514 − e = . w = . ( ) ◦ yr − , which if due completelyto general relativity, implies a total system mass of 2 . ( ) M ⊙ ; we also derive m p < . M ⊙ and m c > . M ⊙ . Thecompanion is likely to be a massive white dwarf star. Keywords:
Millisecond Pulsars; Binary Pulsars; Precision Timing
PACS:
DISCOVERY
PSR J0514 − ∼ ⊙ companion and a highly eccen-tric orbit ( e = . P < e ∼ . A NEW POPULATION
NGC 1851A and PSR J2140-2310B (also known asM30B [5]) were the first objects of a significant popula-tion of eccentric ( e > .
15) binary MSPs recently discov-ered in globular clusters. Before 2004, no such systemswere known, but we now know of at least seven more:Terzan 5 I, Q, U, X and Z, M28 C and NGC 6440B (seeFig. 1). The companions of most of these objects seemto be significantly less massive than the companion ofNGC 1851A, with the possible exception of Terzan 5 Q.There are no known selection effects against the de-tection of this population, particularly considering thatthese objects have orbital periods of a few days or more.For that reason, the causes of the recent (and unexpected)surge in the number of eccentric binary MSP discoveriesis not entirely clear. We note, however, that these sys-tems tend to inhabit clusters with very dense cores, likeTerzan 5, NGC 6440 and M28; this makes sense consid-ering the formation mechanisms for this sort of binarysystem. Given their large distances and resulting DMs,many of these dense globular clusters have only recentlybeen surveyed with good sensitivity to MSPs, thanks ingreat part to the GBT/S-band/spigot observing system.
GC 1851AJ1903+0327Z UQ M28 C I M28 DNGC 6441AX M5 BH NGC 6440BNGC 6539A NGC 6342AM3 Dad M15 C JNGC 6440F
FIGURE 1. ∼ TIMING NGC 1851A
Using the Green Bank Telescope, we have observedNGC 1851A over the last two years, deriving precise or-bital and rotational parameters [4]. One relativistic effect,the precession of periastron, is now measured with highsignificance: ˙ w = . ( ) ◦ yr − .The companion of NGC 1851A is very likely to becompact [4]. In that case, ˙ w is due solely to the effectsof general relativity. This allows an estimate of the to-tal mass of the system: 2 . ( ) M ⊙ . Given the massfunction, the pulsar mass cannot be larger than 1.50 M ⊙ and the companion mass must be larger than 0.96 M ⊙ .For a median inclination of 60 ◦ , the mass of the pulsaris 1.350 M ⊙ , a value that is fairly typical of the neutronstars with well-determined masses. In this case, the com-panion mass would be about 1.105 M ⊙ .For 49 . ◦ < i < . ◦ , both components would havemasses within the present range of well-measured neu-tron star masses (see Fig. 2); however, given a flat proba-bility distribution in cos i , it is about 15 times more likely that 52 . ◦ < i < ◦ , where m c < . ⊙ . In the higherinclination range, there is the possibility that the com-panion is the lightest NS ever discovered. However, thisis probably more than compensated for in the lower in-clination range by the possibility that the companion is aWD or a stripped core of a giant star with m c > . ⊙ .We therefore believe that the probabilities are not farfrom 15 to 1 against the companion being a neutron star.Given the slight possibility, though, that the companionstar could be a MSP, we searched several of the 350, 820,and 1950 MHz observations for additional pulsations, butfound none.The mass limit of this highly recycled pulsar is ofparticular importance. It indicates that spinning up aneutron star to hundreds of Hz can be accomplished withmodest amounts of mass. This confirms previous studiesthat indicate similar ( m p < . M ⊙ ) masses for someMSPs, like PSR J1909 − − − m p < = 52.86 i = 90i = 49.76i = 22.95 FIGURE 2.
Constraints on the masses of NGC 1851A and its companion. The hatched region is excluded by knowledge of themass function and by sin i ≤
1. The diagonal dashed lines correspond to a total system mass that causes a general-relativistic ˙ w equalor within 2- s of the measured value. The four solid curves indicate constant inclinations. Between the two middle inclinations, bothcomponents could have masses from 1.2 to 1.44 solar masses, the range of precise neutron star mass measurements (gray bars). Wealso display the probability density function for the mass of the pulsar ( top ) and the mass of the companion ( right ), and mark therespective medians with vertical (horizontal) lines. . M ⊙ . Four binary systems (Terzan 5 I, J, M5B andNGC 6440B) seem to host neutron stars that are signifi-cantly more massive than 1 . M ⊙ (see Paulo Freire’s talkon massive neutron stars in these Proceedings). It appearsthat MSPs can be spun up with modest amounts of mat-ter, but some accrete significantly larger amounts. ACKNOWLEDGMENTS
The National Radio Astronomy Observatory is a facilityof the National Science Foundation operated under coop-erative agreement by Associated Universities, Incorpo-rated. The Giant Metrewave Radio Telescope is run bythe National Center for Radio Astrophysics of the TataInstitute of Fundamental research.
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