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)
Subjects: Radio, Binary, Neutron Star, Pulsar
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).