The Timing Noise of Magnetars
D. Cerri-Serim, M.M. Serim, D. Yucalan, S. Sahiner, S.C. Inam, A. Baykal
TThe Timing Noise of Magnetars
D. Çerri-Serim ∗ Middle East Technical UniversityE-mail: [email protected]
M. M. Serim
Middle East Technical UniversityE-mail: [email protected]
D. Yücalan
Middle East Technical UniversityE-mail: [email protected] ¸S. ¸Sahiner
Middle East Technical UniversityE-mail: [email protected]
S. Ç. ˙Inam
Ba¸skent UniversityE-mail: [email protected]
A. Baykal
Middle East Technical UniversityE-mail: [email protected]
We represent noise strength analysis of Anomalous X-Ray Pulsars (AXPs) 4U 0142+61, 1RXSJ170849.9-400910, 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+4and 4U 1907+09 for comparison with accreting sources. Using our timing solutions, we extractedresiduals of pulse arrival times after removal of spin down trends and we calculated assoicatednoise strength of each source. Our preliminary results indicate that the noise strength is scalingup with spin-down rate. This indicates that, increase in spin-down rate leads to more torque noiseon the magnetars. In addition, we present our analysis with Bayesian statistics on the previouslyreported transient QPO feature of 4U 1907+09. ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] J a n he Timing Noise of Magnetars D. Çerri-Serim
1. Introduction
Magnetars are isolated neutron stars with extreme magnetic fields (typically higher than 10 G )(Kouveliotou et al. 1998; Vasisht & Gotthelf 1997) and their emission is thought to be originatingfrom decay of this field (Duncan &Thompson 1992;Beloborodov 2009). They exhibit short X-raybursts arising from crustal movements and magnetic activity (Thompson et al. 2002). Magnetarsoccasionally show glitches in their spin frequency followed by flux enchancements and structuralchanges in pulse profiles (Dib & Kaspi 2014; Rea & Esposito 2011). Timing studies on magnetarsreveal that they posses high timing noise level compared with standard pulsars (Woods et al. 2002;Gavriil & Kaspi 2002). In this study, we present our timing noise strength measurements on 7magnetars.Pulsars in X-ray binary systems are powered by accretion of matter from their companionstars (Pringle & Rees 1972). Angular momentum content of accreted matter is a source of externaltorque for the pulsar and causes intrinsic variations in spin period. Investigating fluctuations intorque provides information about accreting matter and the internal structure of the pulsar (Lambet al. 1978).
2. Analysis and Noise Strengths
We measured pulse frequencies via phase coherent timing solutions for each source. Theconstructed pulse frequency histories have approximately same time span. In order to investigatethe torque fluctuations of these sources, we calculated associated noise strengths for the pulsefrequency variations (see Cordes 1980;Deeter & Boynton 1982 for approach and also Baykal 1997
Figure 1:
Noise strengths of 7 magnetars and 2 X-ray binaries as a function of their pulse frequency deriva-tives. Square, triangle and asteriks marks indicate SGRs, AXPs and X-Ray Binaries, respectively. he Timing Noise of Magnetars D. Çerri-Serim for applications). The expected mean square residual, after removal of m th degree polynomial overa time span T , is given by (cid:104) σ R ( m , T ) (cid:105) = S r T r − (cid:104) σ R ( m , ) (cid:105) e (2.1)where (cid:104) σ R ( m , ) (cid:105) e indicates the expectation value for unit-strength noise process. The numericalevaluation of these values are presented in Deeter (1984). We removed spin down trends fromthe frequency measurements and calculated corresponding noise strength S r from the mean squareresiduals. In this work, we represent noise strength estimations of four AXPs (1E 2259+586, AXPs4U 0412+61, 1RXS J170849.9-400910 and 1E 1841-045) and two X-ray binaries (GX 1+4:Serimet al. (2017); Bildsten et al. (1997) and 4U 1907+09: ¸Sahiner et al. 2012) in addition to threeSGRs (SWIFT J1822.3-1606, SGR J1833-0832 and SWIFT J1834-0846; Serim et al. 2012) .Noise strengths versus absolute value of pulse frequency derivatives are presented in Figure 1.
3. QPO of 4U 1907+09
4U 1907+09 is an X-ray pulsar with a spin period of 441.8 s. The secular spin down trend ofthe pulsar was interrupted by a torque reversal event in 2004 May (Fritz et al. 2006). After 2007June, the pulsar returned to spin down with a rate close to its historical steady spin down rate (˙Inamet al. 2009, ¸Sahiner et al. 2012). 4U 1907+09 is known to show abrupt variations in X-ray fluxsuch as flares and dips on time scales of minutes to hours (In’t Zand et al. 1997). Quasi-periodicoscillations (QPO) of about 18 s had been reported during a 1 hr flare on 1996 February 23, whichwas interpreted as a transient accretion disc formation (In’t Zand et al. 1998).We re-analysed the RXTE-PCA observation taken on February 23, 1996. We extracted thelightcurve of the observation and searched for QPO with the Bayesian search algorithm writtenin Python by Huppenkothen . The Bayesian search method found a QPO candidate around 17.9 Figure 2:
Lightcurve of 4U 1907+09 February 23, 1996 observation (left) and the periodogram and thepower law fit result for the Bayesian search analysis (right). s. The probability of detecting a false signal is p( χ > 11.277) = 3 . × − , which correspondsto a 2.91 σ level of detection. We also constructed power density spectrum (PDS) of the samelightcurve, then fit the continuum with power law model and calculated the significance of qposignal. In this method, detection level of the signal yielded 3.54 σ . The associated Python codes can be found in https://github.com/dhuppenkothen/BayesPSD he Timing Noise of Magnetars D. Çerri-Serim
4. Conclusion
In conclusion, we extract the timing residuals of 7 magnetars (3 SGRs and 4 AXPs) and 2 X-ray binaries using RXTE and Swift observations and calculated associated timing noise strengths. • Our pulse timing noise analysis on 7 magnetars indicate that the spin-down rates of magnetarsseem to show a correlation with their noise strengths. These noise strengths could be theresult of micro-scale cracking of the crust due to the magnetic field stress on the crust. Wewill extend this work to other magnetars listed in McGill Magnetar Catalog (Olausen &Kaspi 2014). • We re-analysed the transient QPO feature of 4U 1907+09. The Bayesian method does finda candidate for a QPO around (cid:39) σ . Thelevel of detection is less than that of the comparison analysis, which finds a candidate aroundthe same period, with 3.54.We acknowledge support from TÜB˙ITAK, the Scientific and Technological Research Council ofTurkey through the research project MFAG 114F345 and Scientific Research ProjectsCoordination Unit (BAP) with the project BAP-01-05-2016-003. References [1] Baykal, A.,
The torque and X-ray flux changes of OAO 1657-415 , AAP 318, 515 (1997)[2] Beloborodov, A. M.,
Untwisting Magnetospheres of Neutron Stars , ApJ 703, 1044 (2009)[3] Bildsten, L. et al.,
Observations of Accreting Pulsars , ApJS 113, 367 (1997)[4] Cordes, J. M.,
Pulsar timing. II - Analysis of random walk timing noise - Application to the Crabpulsar , ApJ 237, 216 (1980)[5] Deeter, J. E. and Boynton, P. E.,
Techniques for the estimation of red power spectra. I - Context andmethodology , ApJ 261, 337 (1982)[6] Deeter, J. E.,
Techniques for the estimation of red power spectra. II Evaluation of alternative methods ,ApJ 281, 482 (1984)[7] Dib, R. and Kaspi, V. M.,
16 yr of RXTE Monitoring of Five Anomalous X-Ray Pulsars , ApJ 784, 37(2014)[8] Duncan, R. C. and Thompson, C.,
Formation of very strongly magnetized neutron stars - Implicationsfor gamma-ray bursts , ApJL 392, 9 (1992)[9] Fritz, S. et al.,
A torque reversal of 4U 1907+09 , AAP 358, 885 (2006)[10] Gavriil, F. P. and Kaspi, V. M.,
Long-Term Rossi X-Ray Timing Explorer Monitoring of AnomalousX-Ray Pulsars , ApJ 567, 1067 (2002)[11] Inam, S. Ç. and ¸Sahiner, ¸S. and Baykal, A.,
Recent torque reversal of 4U1907+09 , MNRAS 395,1015 (2009)[12] in ’t Zand, J. J. M. and Strohmayer, T. E. and Baykal, A., ApJL 479, 47 (1997)[13] in ’t Zand, J. J. M. and Baykal, A. and Strohmayer, T. E.,
Recent X-Ray Measurements of theAccretion-powered Pulsar 4U 1907+09 , ApJ 496, 386 (1998) he Timing Noise of Magnetars D. Çerri-Serim[14] Lamb, F. K. and Pines, D. and Shaham, J.,
Period variations in pulsating X-ray sources. II - Torquevariations and stellar response , ApJ 225, 582 (1978)[15] Kouveliotou, C. et al.,
An X-ray pulsar with a superstrong magnetic field in the soft γ -ray repeaterSGR1806 - 20 , Nature 393, 237 (1998)[16] Olausen, S. A. and Kaspi, V. M., The McGill Magnetar Catalog , ApJS 212, 6 (2014)[17] Pringle, J. E. and Rees, M. J.,
Accretion Disc Models for Compact X-Ray Sources , AAP 21, 1 (1972)[18] Rea, N. et al.,
The X-ray Quiescence of Swift J195509.6+261406 (GRB 070610): An Optical BurstingX-ray Binary? , ApJL 729, 21 (2011)[19] Serim, M. M. and Inam, S. Ç. and Baykal, A.,
Noise Strength Estimates of Three SGRs: SwiftJ1822.3-1606, SGR J1833-0832 and Swift J1834.9-0846 , ASPC 466, 255 (2012)[20] Serim et al.,
Comprehensive Timing and Spectral Analysis of GX 1+4 to be submitted to MNRAS[21] ¸Sahiner, ¸S. and Inam, S. Ç. and Baykal, A.,
A comprehensive study of RXTE and INTEGRALobservations of the X-ray pulsar 4U 1907+09 , MNRAS 421, 2079 (2012)[22] Thompson, C. and Lyutikov, M. and Kulkarni, S. R.,
Electrodynamics of Magnetars: Implications forthe Persistent X-Ray Emission and Spin-down of the Soft Gamma Repeaters and Anomalous X-RayPulsars , ApJ 574, 332 (2002)[23] Vasisht, G. and Gotthelf, E. V.,
The Discovery of an Anomalous X-Ray Pulsar in the SupernovaRemnant Kes 73 , ApJL 486, 129 (1997)[24] Woods, P. M. et al.,
Large Torque Variations in Two Soft Gamma Repeaters , ApJ 576, 381 (2002), ApJ 576, 381 (2002)