PSR J1738+0333: a new gravitational laboratory
aa r X i v : . [ a s t r o - ph ] N ov PSR J1738+0333: a new gravitational laboratory
Paulo C. C. Freire ∗ , Bryan A. Jacoby † and Matthew Bailes ∗∗ ∗ N.A.I.C., Arecibo Observatory, HC3 Box 53995, PR 00612, U.S.A.; [email protected] † Naval Research Laboratory, Washington, DC; [email protected] ∗∗ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Australia; [email protected]
Abstract.
We describe in this paper a new binary millisecond pulsar, PSR J1738+0333. Using Arecibo, we have achievedgood timing accuracy for this object, about 220 ns for 1-hour integrations over 100 MHz. This allowed us to measure a preciseproper motion, parallax and orbital parameters for this system. We highlight the system’s potential for constraining alternativetheories of gravitation.
Keywords:
Millisecond Pulsars; Binary Pulsars; Precision Timing, Precision Astrometry; Gravitation
PACS:
INTRODUCTION
PSR J1738+0333 is a 5.85-ms pulsar in a binary systemwith an orbital period of 8.5 hours and a companionwhite dwarf (WD) with a mass of about 0.2 M ⊙ . Thismillisecond pulsar (MSP) was found with the Parkes 64-m Radio Telescope in a 20-cm Multi-Beam search forpulsars in intermediate Galactic latitudes ( 5 ◦ < | b | < ◦ ) [1]; we have been timing it with Arecibo for the last4 years using the Wide-band Arecibo Pulsar Processors(WAPPs, [2]). We have obtained a TOA residual rms of220 ns per WAPP per hour. This pulsar will be usedin the array that is being used to search for nano-Hertzgravitational waves. TIMING OF PSR J1738+0333
Initially we sought to determine the companion and pul-sar masses from a measurement of the Shapiro delay. De-spite the high timing precision, the measurement was notpossible given the system’s low orbital inclination.Fortunately, it is possible to determine the massesof the components independently. This comes from re-cent optical work of Marten van Kerkwijk and one ofus (BAJ). Using the Magellan telescope on Las Cam-panas, Chile, they detected the companion star and foundits spectrum to be similar to that of the companion ofPSR J1909 − ⊙ , mea-sured by Shapiro delay [3]. For this reason, and from hereon, we assume that the companion of PSR J1738+0333has mass ( m = . ± . M ⊙ ), but note that a preciseestimate of this mass has not yet been made. This impliesan orbital inclination of about 30 ◦ .Introducing the Shapiro delay that corresponds to thiscompanion mass and inclination, we obtain an eccen- FIGURE 1.
Radial velocity measurements of the companionof PSR J1738+0333 as a function of orbital phase. tricity of ( ± ) × − , the lowest ever measuredfor any binary system. This opens up the possibilityof greatly improved tests of the fundamental nature ofspacetime, introducing the most stringent constraintsever on preferred-frame effects and non-conservation ofmomentum [4].More recently, the radial-velocity curve was measuredusing Gemini South on Cerro Pachón (see Fig. 1). Fromthis we can derive the mass ratio of the system, R = . ± . m = ( . ± . ) M ⊙ . The error es-timate for the companion mass assumed above is veryconservative, it admits a wide possible range of compan-ion and pulsar masses (see Fig. 2). It will certainly bemeasurable with better precision in the near future, as inthe case of PSR J1911 − = 30i = 60i = 90 PP bb (+1 sigma) bP (−1 sigma) FIGURE 2.
Mass constraints for PSR J1738+0333 and its white dwarf companion. The closed solid lines (contours) include 66.3,95.4 and 99.7% of the probability, this is dictated by the information regarding the companion mass (horizontal dotted lines, with − , + s limits indicated) and the mass ratio (inclined dotted lines, with − , + s limits also indicated. If the observedorbital decay, corrected for kinematic effects ˙ P b (dashed lines) is due to emission of quadrupolar gravitational waves, then the tworegions should overlap, as observed. The vertical gray bar indicates the range of precisely measured neutron star masses. al. these proceedings). A test of the Strong Equivalence Principle
The measurement of the masses of the components ofimportant because it allows a (low-precision) estimate ofthe expected rate of orbital decay due to the emissionof quadrupolar gravitational waves, as predicted by gen-eral relativity (GR): − ( . + . − . ) × − s/s. This periodderivative is about 60 times smaller than what was mea-sured for the Hulse-Taylor binary pulsar [7].Fortunately, the timing precision for PSR J1738+0333is such that we can already measure this value after fouryears of timing, although not with much significance: itis − ( . ± . ) × − s/s. What is more important, the difference between the predicted value and the observedvalue (after acorrection for kinematic effects) is verysmall: △ ( ˙ P b ) = ( . ± . ) × − ( < . × − ) . This is the tightest limit ever on a possible contribu-tion to the orbital decay by the emission of dipolar grav-itational waves predicted by alternative theories of grav-itation. As an example, in Brans-Dicke gravity, the emis-sion of dipolar gravitational waves is given by: (cid:18) ˙ P b P b (cid:19) D = − + w BD ( s − s ) (cid:18) p P b (cid:19) m RR + T ⊙ , (1)where T ⊙ is the solar mass in time units, and w BD is theBrans-Dicke constant; for GR this is infinite. The vari-able s n is the fractional change of the gravitational bind-ing energy (mass) of object n with a variation of the grav-itational constant G ( s n = ¶ ln m n / ¶ ln G ) N at a constanttotal number of baryons N (see e.g. [8]). For neutronstars, s n depends on the equation of state, but generallyit is of the order of 0.2. In double neutron star systems,we have s ≃ s , and therefore ( s − s ) ≃
0. This meansthat (cid:16) ˙ P b P b (cid:17) D might be zero even if w BD is finite. In the caseof MSP-WD binaries like PSR J1738+0333, the bindingnergy of the WD is many orders of magnitude smallerthan the binding energy of the MSP, therefore s ≃ ( s − s ) = s =
0; for this reason they are called“asymmetric” binaries. This means that if (cid:16) ˙ P b P b (cid:17) D is verysmall (or zero), then w BD must very large (or infinite).Using △ ( ˙ P b ) / P b as an experimental upper limit on (cid:12)(cid:12)(cid:12)(cid:16) ˙ P b P b (cid:17) D (cid:12)(cid:12)(cid:12) , we obtain w > ∼ ( s / . ) (85% C.L.).This is very similar to the limits derived from Arecibotiming of PSR J0751+1807 [9]. This is not as good asthe result from the Cassini spacecraft ( w > , po-tential for further improvement of this test. Over the next5(10) years, the precision in the measurement of ˙ P b willincrease by a factor of 10(40). If the component massesare determined from the optical studies to a precision of10%, or better, then the prediction of ˙ P b will be accu-rate to 6 × − s/s or better. This will be the limit-ing factor in the precision of this test. If the measuredvalue conforms to the prediction, that will be equivalentto w > , ( s / . ) , an order of magnitude improve-ment on all previous pulsar tests.One of the advantages of the high timing precisionof PSR J1738+0333 has been a precise measurementof the proper motion (7.106 ± ± ± ACKNOWLEDGMENTS
The Parkes Radio Telescope is part of the Australia Tele-scope, which is funded by the Commonwealth of Aus-tralia for operation as a National Facility managed byCSIRO. The Arecibo Observatory, a facility of the Na-tional Astronomy and Ionosphere Center, is operated byCornell University under a cooperative agreement withthe National Science Foundation. This research was per-formed while BAJ held a National Research Council Re- search Associateship Award at the Naval Research Labo-ratory (NRL). Basic research in radio astronomy at NRLis supported by the Office of Naval Research.
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