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Register a new userThe Timing Noise of Magnetars
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D. Cerri-Serim
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We represent noise strength analysis of Anomalous X-Ray Pulsars (AXPs) 4U 0142+61, 1RXS J170849.9-400910, 1E 1841-045, 1E 2259+586 and Soft Gamma Repeaters (SGRs) SGR J1833-0832, SWIFT J1822.3-1606 and SWIFT J1834.9-0846 together with the X-Ray binaries GX 1+4 and 4U 1907+09 for comparison with accreting sources. Using our timing solutions, we extracted residuals of pulse arrival times after removal of spin down trends and we calculated assoicated noise strength of each source. Our preliminary results indicate that the noise strength is scaling up with spin-down rate. This indicates that, increase in spin-down rate leads to more torque noise on the magnetars. In addition, we present our analysis with Bayesian statistics on the previously reported transient QPO feature of 4U 1907+09.
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While pulsars possess exceptional rotational stability, large scale timing studies have revealed at least two distinct types of irregularities in their rotation: red timing noise and glitches. Using modern Bayesian techniques, we investigated the timing noise properties of 300 bright southern-sky radio pulsars that have been observed over 1.0-4.8 years by the upgraded Molonglo Observatory Synthesis Telescope (MOST). We reanalysed the spin and spin-down changes associated with nine previously reported pulsar glitches, report the discovery of three new glitches and four unusual glitch-like events in the rotational evolution of PSR J1825$-$0935. We develop a refined Bayesian framework for determining how red noise strength scales with pulsar spin frequency ($ u$) and spin-down frequency ($dot{ u}$), which we apply to a sample of 280 non-recycled pulsars. With this new method and a simple power-law scaling relation, we show that red noise strength scales across the non-recycled pulsar population as $ u^{a} |dot{ u}|^{b}$, where $a = -0.84^{+0.47}_{-0.49}$ and $b = 0.97^{+0.16}_{-0.19}$. This method can be easily adapted to utilise more complex, astrophysically motivated red noise models. Lastly, we highlight our timing of the double neutron star PSR J0737$-$3039, and the rediscovery of a bright radio pulsar originally found during the first Molonglo pulsar surveys with an incorrectly catalogued position.
Radio pulsars are often used as clocks in a wide variety of experiments. Imperfections in the clock, known as timing noise, have the potential to reduce the significance of, or even thwart e.g. the attempt to find a stochastic gravitational wave (GW) background. We measure the timing noise in a group of 129 mostly middle-aged pulsars (i.e. characterstic ages near 1~Myr) observed with the Parkes radio telescope on a monthly basis since 2014. We examine four different metrics for timing noise, but it remains unclear which, if any, provides the best determination. In spite of this, it is evident that these pulsars have significantly less timing noise than their younger counterparts, but significantly more than the (much older) millisecond pulsars (MSPs). As with previous authors, we find a strong correlation between timing noise and the pulsar spin-down rate, $dot{ u}$. However, for a given $dot{ u}$ there is a spread of about a factor 30 in the strength of the timing noise likely indicating that nuclear conditions in the interior of the stars differs between objects. We briefly comment on the implications for GW detection through pulsar timing arrays as the level of timing noise in MSPs may be less than predicted.
We analyze timing noise from five years of Arecibo and Green Bank observations of the seventeen millisecond pulsars of the North-American Nanohertz Observatory for Gravitational Waves (NANOGrav) pulsar timing array. The weighted autocovariance of the timing residuals was computed for each pulsar and compared against two possible models for the underlying noise process. The first model includes red noise and predicts the autocovariance to be a decaying exponential as a function of time lag. The second model is Gaussian white noise whose autocovariance would be a delta function. We also perform a ``nearest-neighbor correlation analysis. We find that the exponential process does not accurately describe the data. Two pulsars, J1643-1224 and J1910+1256, exhibit weak red noise, but the rest are well described as white noise. The overall lack of evidence for red noise implies that sensitivity to a (red) gravitational wave background signal is limited by statistical rather than systematic uncertainty. In all pulsars, the ratio of non-white noise to white noise is low, so that we can increase the cadence or integration times of our observations and still expect the root-mean-square of timing residual averages to decrease by the square-root of observation time, which is key to improving the sensitivity of the pulsar timing array.
We study the possibility that the long term red timing-noise in pulsars originates from the evolution of the magnetic inclination angle $chi$. The braking torque under consideration is a combination of the dipole radiation and the current loss. We find that the evolution of $chi$ can give rise to extra cubic and fourth-order polynomial terms in the timing residuals. These two terms are determined by the efficiency of the dipole radiation, the relative electric-current density in the pulsar tube and $chi$. The following observation facts can be explained with this model: a) young pulsars have positive $ddot{ u}$; b) old pulsars can have both positive and negative $ddot{ u}$; c) the absolute values of $ddot{ u}$ are proportional to $-dot{ u}$; d) the absolute values of the braking indices are proportional to the characteristic ages of pulsars. If the evolution of $chi$ is purely due to rotation kinematics, then it can not explain the pulsars with braking index less than 3, and thus the intrinsic change of the magnetic field is needed in this case. Comparing the model with observations, we conclude that the drift direction of $chi$ might oscillate many times during the lifetime of a pulsar. The evolution of $chi$ is not sufficient to explain the rotation behavior of the Crab pulsar, because the observed $chi$ and $dot{chi}$ are inconsistent with the values indicated from the timing residuals using this model.
We consider the current observed ensemble of pulsing ultraluminous X-ray sources (PULXs). We show that all of their observed properties (luminosity, spin period, and spinup rate) are consistent with emission from magnetic neutron stars with fields in the usual range $10^{11} - 10^{13}, {rm G}$, which is collimated (`beamed) by the outflow from an accretion disc supplied with mass at a super-Eddington rate, but ejecting the excess, in the way familiar for other (non-pulsing) ULXs. The observed properties are inconsistent with magnetar-strength fields in all cases. We point out that all proposed pictures of magnetar formation suggest that they are unlikely to be members of binary systems, in agreement with the observation that all confirmed magnetars are single. The presence of magnetars in ULXs is therefore improbable, in line with our conclusions above.
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