GCN Circular 26243

David Kaplan and John Friedman (University of Wisconsin-Milwaukee),
and Jocelyn Read (CSU Fullerton, Caltech) report on behalf of the
GROWTH collaboration:

The unmodelled GW burst S191110af (LVC, GCN 26222
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 ) consisted of a
narrow-band signal near 1781.72 Hz with duration 0.1 s.  This
frequency is consistent with the frequency of the fundamental
quadrupole mode of a star with mass in the range of 1.2 to 1.45 Msun
and radii within current estimates. We use Andersson & Kokkotas (1998,
MNRAS, 299, 10591068) Eqs. (1), (5) and (8) for the frequency, damping
time, and effective amplitude of the quadrupole f mode.  In
particular, for a neutron star with mass 1.25 Msun, a frequency of
1782 Hz corresponds to radius 13.3 km, and the damping time is then
0.2 s, consistent with observations.  Similarly, the range of EOS
considered in Chirenti et al. (2015, Phys. Rev. D 91, 044034) generate
f-modes along a band in frequency and decay time that seems compatible
with this candidate.

While we do not know the strain amplitude associated with this burst,
we can estimate a minimum detectable strain based on the observed
frequency, duration, and the LIGO noise properties.  At the observed
frequency we estimate a noise level of 1.5e-23 Hz**-0.5
(https://www.gw-openscience.org/detector_status/day/20191110/).
Following Eqs. (3) and (4) of Kokkotas et al. (2001, MNRAS, 320, 307)
we estimate that a signal with f=1782Hz and duration 0.15s will have
amplitude 5e-22, and SNR~10 in a detector with noise 1.5e-23 Hz**-0.5,
for a source at 1kpc emitting 2.5e-9 Msun c**2 of energy in
gravitational waves. Comparably, in a physical model where similar
oscillations come from a pulsar triggered by a significant glitch,
Keer & Jones (2015, MNRAS, 446, 865) estimate a stain of 3e-22 at a
distance of 1 kpc.

Motivated by this, we searched for potential counterparts among known
pulsars (note that if such a signal originates with a neutron star it
need not be visible as a pulsar). Based on the amplitude estimates
referenced above, we restrict our search to Galactic pulsars and do
not consider significantly more distant objects.

We looked at the latest version (1.61) of the ATNF pulsar catalog
(Manchester et al. 2005, AJ, 129, 1993;
http://www.atnf.csiro.au/research/pulsar/psrcat) and computed the
probability of the GW sky map at the position of each pulsar.  The top
3 pulsars ranked by probability are:

PSR J0045-7042 1.6e-3
PSR J0101-6422 2.4e-4
PSR B2045-16   4.9e-5

All other pulsars have probabilities <3e-5, and do not stand out from
the rest of the population, with 394 other sources at probabilities
>1e-6.

For these three pulsars:

PSR J0045-7042 is a slow (0.6s period) pulsar in the Small Magellanic
Cloud, and would not appear to be sufficiently energetic or close
enough to give rise to a significant GW signal.

PSR J0101-6422 is a nearby (~1 kpc), energetic (rotational energy loss
1.2e34 erg/s) millisecond pulsar in a 1.8d orbit, which is also a
Fermi gamma-ray source (Kerr et al. 2012, ApJ, 748, 2; Nolan et
al. 2012, ApJS, 199, 31).

PSR J2045-16 is a nearby (~0.8 kpc) slow (2.0s period) pulsar, which
does not appear otherwise notable.

Unfortunately, PSRs J0045-7042 and B2045-16 do not have sufficient
timing data to search for a recent glitch.  PSR J0101-6422 is timed
regularly with the Fermi Large Area Telescope, but detecting a
putative glitch will take weeks to months, depending on the magnitude
of the glitch (M. Kerr and P.S. Ray, private communication).  As
mentioned above, any or none of these sources could be the origin of
the possible gravitational wave emission, or the candidate could be
terrestrial in origin.  It also may be worth searching for short GW
bursts associated with known glitches, especially those from the Vela
pulsar (as in Abadie et al. 2011, PRD 83, 042001).

We thank Joe Swiggum, Sinead Walsh, Nils Andersson, and Cecilia
Chirenti for helpful conversations.  GROWTH is a worldwide
collaboration comprising Caltech, USA; IPAC, USA, WIS, Israel; OKC,
Sweden; JSI/UMd, USA; U Washington, USA; DESY, Germany; MOST, Taiwan;
UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan; IIT-B, India; IIA,
India; LJMU, UK; TTU USA and USyd, Australia. GROWTH acknowledges
generous support of the NSF under PIRE Grant No 1545949.