GMRT discovery of a 1.69 ms radio pulsar associated with XSS J12270-4859

ATel #5890; J. Roy (NCRA-TIFR), B. Bhattacharyya (NCRA-TIFR), P. S. Ray (NRL)
on 15 Feb 2014; 17:02 UT
Credential Certification: Paul S. Ray (paul.ray@nrl.navy.mil)

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Following the reported state change observed in the low-mass X-ray binary XSS J12270-4859 (ATel #5647; Bassa et al. 2014, arXiv:1402.0765), we were granted Director's Discretionary Time to search for radio pulsations using the GMRT. We observed at 607 MHz with the GMRT coherent phased-array mode utilizing 70% of the array resulting in a beam width of 30 arcsec. We recorded 3 scans, each of 1-hour beginning on 2014 Feb 12 at 20:46:15 UTC, producing filter-bank outputs of 512 x 0.0651 MHz sampled at 61.44 microsec. There were 5-10 minute gaps between the scans for phase calibrations.

We searched the data in 6 half-hour intervals using the acceleration search code in PRESTO (by S. Ransom) with the newly commissioned 512-core NCRA IBM cluster over a dispersion range of 0-500 pc/cm^3. In the first interval, we discovered a highly-accelerated 1.69 ms pulsar that is only strongly visible in the first portion of the observation. The significance of the detection in the first 700 seconds is >19 sigma. During this interval, the measured flux density is 0.7 mJy at 607 MHz. For a typical spectral index of -1.7, this is consistent with the Parkes non-detection (upper limit of 0.2 mJy at 1.4 GHz) reported by Bassa et al. 2014. This GMRT discovery of millisecond radio pulsations from XSS J12270-4859 confirms that the system has transitioned from a low-mass X-ray binary state to a millisecond pulsar state.

The observed dispersion measure is 43.5 pc/cm^3. The NE2001 distance estimate is 1.4 kpc, consistent with the lower end of the 1.4-3.6 kpc range inferred from optical studies by de Martino et al. (2013, A&A 550, A89).

We searched the other 5 30-minute intervals and found only one marginal detection in the 4th interval at a similar period and DM. In that interval, the acceleration was observed to be of opposite sign, indicating that this pulsar is in a short period orbit, as expected from the 6.913(2) hour orbital period measured in the optical by Bassa et al. 2014. We don't have enough information to fit a full orbital solution, but assuming the optical orbital period, the observed accelerations are consistent with a minimum companion mass of about 0.7 solar masses.

Previously, we had observed the 2FGL J1227.7-4953, which is positionally coincident with XSS J12270-4859, using the GMRT at 322 MHz on 2012 July 23 at 10:20:25 UTC in search for millisecond pulsations. In that observations, pulsations were not seen, which is consistent with the system being in an accreting state prior to the state change in 2012 December.

Further GMRT observations have been proposed to measure the full orbital parameters of the binary.

We thank the staff of the GMRT who have made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. We acknowledge the NCRA HPC facility. This work was partially supported by the Fermi Guest Observer Program (Proposal #061103).