Search of the Orion spur for continuous gravitational waves using a loosely coherent algorithm on data from LIGO interferometers

We report results of a wideband search for periodic gravitational waves from isolated neutron stars within the Orion spur towards both the inner and outer regions of our Galaxy. As gravitational waves interact very weakly with matter, the search is unimpeded by dust and concentrations of stars. One search disk (A) is 6.87° in diameter and centered on $20^{\mathrm{h}}10^{\mathrm{m}}54.71^{\mathrm{s}}+33°33^{\prime }25.29^{\prime \prime }$, and the other (B) is 7.45° in diameter and centered on $8^{\mathrm{h}}35^{\mathrm{m}}20.61^{\mathrm{s}}-46°49^{\prime }25.151^{\prime \prime }$. We explored the frequency range of 50–1500 Hz and frequency derivative from 0 to $-5\times {10}^{-9}\mathrm{Hz}/\mathrm{s}$. A multistage, loosely coherent search program allowed probing more deeply than before in these two regions, while increasing coherence length with every stage. Rigorous follow-up parameters have winnowed the initial coincidence set to only 70 candidates, to be examined manually. None of those 70 candidates proved to be consistent with an isolated gravitational-wave emitter, and 95% confidence level upper limits were placed on continuous-wave strain amplitudes. Near 169 Hz we achieve our lowest 95% C.L. upper limit on the worst-case linearly polarized strain amplitude $h_0$ of $6.3\times {10}^{-25}$, while at the high end of our frequency range we achieve a worst-case upper limit of $3.4\times {10}^{-24}$ for all polarizations and sky locations.

Figure 1

Distribution of known pulsars in the Milky Way galaxy. The Orion spur region (marked by dashed rectangle) connects the Perseus and Sagittarius galactic arms and includes regions marked A and B. The ranges shown for gravitational-wave searches correspond to a 1500 Hz frequency and an ellipticity of $10^{-6}$. The arc shown for the PARKES survey [1] shows the search area, not the range. The green stars show locations of pulsars from the ATNF database (retrieved on January 29, 2015 [20]) with Galactic latitude Gb below 0.06 radians. The background image is due to R. Hurt [22].

Figure 2

S6 95% C.L. upper limits on the signal strain amplitude. The upper (green) curve shows worst-case upper limits in the analyzed 0.25 Hz bands (see Table 2 for list of excluded bands). The lower (grey) curve shows upper limits assuming a circularly polarized source. The values of solid points and circles (marking power line harmonics for circularly and linearly polarized sources) are not considered reliable. They are shown to indicate contaminated bands.

Figure 3

Range of the PowerFlux search for neutron stars spinning down solely due to gravitational waves. This is a superposition of two contour plots. The grey and red solid lines are contours of the maximum distance at which a neutron star in optimum orientation could be detected as a function of gravitational-wave frequency $f$ and its derivative $\dot{f}$. The dashed lines are contours of the corresponding ellipticity $\epsilon (f,\dot{f})$. The fine dotted line marks the maximum spindown searched. Together these quantities tell us the maximum range of the search in terms of various populations (see text for details). In particular, at 1500 Hz we are sensitive to stars with an ellipticity of $5\times {10}^{-7}$ up to 1 kpc away.

Figure 4

SNR sky map for outlier A 14. The disk (0.025 rad radius) is centered on the location of the signal. Each pixel (0.555 mrad) on the sky map shows the SNR maximized over a $5\times 5$ template sub-grid and all polarizations. The high frequency of the signal (1404 Hz) allows good localization.