Narrowband searches for continuous gravitational waves from known pulsars in the first two parts of the fourth LIGO--Virgo--KAGRA observing run

Abstract

Rotating non-axisymmetric neutron stars (NSs) are promising sources for continuous gravitational waves (CWs). Such CWs can, if detected, inform us about the internal structure and equation of state of NSs. Here, we present a narrowband search for CWs from known pulsars, for which an efficient and sensitive matched-filter search can be applied. Narrowband searches are designed to be robust to mismatches between the electromagnetic (EM) and gravitational emissions, in contrast to fully targeted searches where the CW emission is assumed to be phase-locked to the EM one. In this work, we search for the CW counterparts emitted by 34 pulsars using data from the first and second parts of the fourth LIGO--Virgo--KAGRA observing run. This is the largest number of pulsars so far targeted for narrowband searches in the advanced detector era. We use the 5n-vector narrowband pipeline, which applies frequency-domain matched filtering. In previous searches, it covered a narrow range in the frequency -- frequency time derivative ($f$ -- $\dot{f}$) space. Here, we also explore a range in the second time derivative of the frequency $\ddot{f}$ around the value indicated by EM observations. Additionally, for the first time, we target sources in a binary system with this kind of search. We find no evidence for CWs and therefore set upper limits on the strain amplitude emitted by each pulsar, using simulated signals added in real data. For 20 analyses, we report an upper limit below the theoretical spin-down limit. The tightest constraint is for pulsar PSR J0534+2200 (the Crab pulsar), for which our strain upper limit on the CW amplitude is $\lesssim 2\%$ of its spin-down limit, corresponding to less than $0.04\%$ of the spin-down power being radiated in the CW channel.

Figures (6)

• Figure 1: Sky location in equatorial coordinates of the selected targets. The color scale indicates the target's $f_{\rm gw}$.
• Figure 2: L1 spectrogram computed each 2048 seconds over the O4ab run. The PSRs J0117+5914 and J18261334 outlier tracks, calculated with Equation \ref{['eq:freq_track']}, are displayed in fuchsia and cyan, respectively. During the first part of the dataset, the spectrum is affected by a known noise transient reported in transient_noise_L1 whose strength reduces after the MJD 60159 (vertical black line), see text for more details.
• Figure 3: Expected sensitivity of the narrowband search using O4ab (shaded pink region) dataset from the two LIGO detectors. The curve is compared with the spin-down limits (triangles) and the 95% CL ULs (stars) averaged over all the $10^{-4}$ Hz bands for each source. ULs below the spin-down limit are highlighted with orange circles. Superimposed blue stars correspond to the multiple analyses of glitching pulsars.
• Figure 4: Sensitivity depths calculated with Equation \ref{['eq:sens_depth']} using the results in Figure \ref{['fig:NB_ULs']}. The two horizontal dotted lines highlight the theoretical sensitivity for the reported number of explored templates, calculated with Equations \ref{['eq:hsens']} and \ref{['eq:sens_depth']}.
• Figure 5: ULs in Figure \ref{['fig:NB_ULs']} of non-glitching pulsars divided by previous 5n-vector narrowband upper limits. More specifically, blue stars compare with the O4a-only analysis of LIGOScientific:2025kei, orange circles with O3 LIGOScientific:2021quq, grey triangles with O2 O2Narrowband, and O1 purple squares from 2017PhRvD..96l2006A. Note that multiple analyses might have targeted the same pulsar; we show all the comparisons to highlight the significant improvements from one observing run to the other.