The Timing Noise of Magnetars

0.44 MB·

by  D. Cerri-Serim

The Timing Noise of Magnetars
D. Çerri-Serim∗
MiddleEastTechnicalUniversity
E-mail: [email protected]
M. M. Serim
7
1 MiddleEastTechnicalUniversity
0 E-mail: [email protected]
2
n D. Yücalan
a MiddleEastTechnicalUniversity
J
E-mail: [email protected]

6
1 S¸. S¸ahiner

] MiddleEastTechnicalUniversity
E
E-mail: [email protected]
H
. S. Ç.˙Inam
h
p Bas¸kentUniversity
- E-mail: [email protected]
o
r
t A. Baykal
s
a MiddleEastTechnicalUniversity
[
E-mail: [email protected]

1
v
We represent noise strength analysis of Anomalous X-Ray Pulsars (AXPs) 4U 0142+61, 1RXS
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3 J170849.9-400910,1E1841-045,1E2259+586andSoftGammaRepeaters(SGRs)SGRJ1833-
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0832,SWIFTJ1822.3-1606andSWIFTJ1834.9-0846togetherwiththeX-RaybinariesGX1+4
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0 and4U1907+09forcomparisonwithaccretingsources.Usingourtimingsolutions,weextracted
.
1 residuals of pulse arrival times after removal of spin down trends and we calculated assoicated
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noisestrengthofeachsource. Ourpreliminaryresultsindicatethatthenoisestrengthisscaling
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1 upwithspin-downrate. Thisindicatesthat,increaseinspin-downrateleadstomoretorquenoise
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v onthemagnetars. Inaddition,wepresentouranalysiswithBayesianstatisticsonthepreviously
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X reportedtransientQPOfeatureof4U1907+09.
r
a
11thINTEGRALConferenceGamma-RayAstrophysicsinMulti-WavelengthPerspective,
10-14October2016
Amsterdam,TheNetherlands
∗Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/
TheTimingNoiseofMagnetars D.Çerri-Serim
1. Introduction
Magnetarsareisolatedneutronstarswithextrememagneticfields(typicallyhigherthan1014G)
(Kouveliotouetal. 1998;Vasisht&Gotthelf1997)andtheiremissionisthoughttobeoriginating
fromdecayofthisfield(Duncan&Thompson1992;Beloborodov2009). TheyexhibitshortX-ray
bursts arising from crustal movements and magnetic activity (Thompson et al. 2002). Magnetars
occasionally show glitches in their spin frequency followed by flux enchancements and structural
changesinpulseprofiles(Dib&Kaspi2014;Rea&Esposito2011). Timingstudiesonmagnetars
revealthattheyposseshightimingnoiselevelcomparedwithstandardpulsars(Woodsetal. 2002;
Gavriil & Kaspi 2002). In this study, we present our timing noise strength measurements on 7
magnetars.
Pulsars in X-ray binary systems are powered by accretion of matter from their companion
stars(Pringle&Rees1972). Angularmomentumcontentofaccretedmatterisasourceofexternal
torque for the pulsar and causes intrinsic variations in spin period. Investigating fluctuations in
torque provides information about accreting matter and the internal structure of the pulsar (Lamb
etal. 1978).
2. AnalysisandNoiseStrengths
We measured pulse frequencies via phase coherent timing solutions for each source. The
constructed pulse frequency histories have approximately same time span. In order to investigate
the torque fluctuations of these sources, we calculated associated noise strengths for the pulse
frequencyvariations(seeCordes1980;Deeter&Boynton1982forapproachandalsoBaykal1997
Figure1: Noisestrengthsof7magnetarsand2X-raybinariesasafunctionoftheirpulsefrequencyderiva-
tives. Square,triangleandasteriksmarksindicateSGRs,AXPsandX-RayBinaries,respectively.
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TheTimingNoiseofMagnetars D.Çerri-Serim
forapplications). Theexpectedmeansquareresidual,afterremovalofmth degreepolynomialover
atimespanT,isgivenby
(cid:104)σ2(m,T)(cid:105)=S T2r−1(cid:104)σ2(m,1)(cid:105) (2.1)
R r R e
where (cid:104)σ2(m,1)(cid:105) indicates the expectation value for unit-strength noise process. The numerical
R e
evaluation of these values are presented in Deeter (1984). We removed spin down trends from
thefrequencymeasurementsandcalculatedcorrespondingnoisestrengthS fromthemeansquare
r
residuals. Inthiswork,werepresentnoisestrengthestimationsoffourAXPs(1E2259+586,AXPs
4U0412+61,1RXSJ170849.9-400910and1E1841-045)andtwoX-raybinaries(GX1+4:Serim
et al. (2017); Bildsten et al. (1997) and 4U 1907+09: S¸ahiner et al. 2012) in addition to three
SGRs (SWIFT J1822.3-1606, SGR J1833-0832 and SWIFT J1834-0846; Serim et al. 2012) .
NoisestrengthsversusabsolutevalueofpulsefrequencyderivativesarepresentedinFigure1.
3. QPOof4U1907+09
4U1907+09isanX-raypulsarwithaspinperiodof441.8s. Thesecularspindowntrendof
the pulsar was interrupted by a torque reversal event in 2004 May (Fritz et al. 2006). After 2007
June,thepulsarreturnedtospindownwitharateclosetoitshistoricalsteadyspindownrate(˙Inam
et al. 2009, S¸ahiner et al. 2012). 4U 1907+09 is known to show abrupt variations in X-ray flux
such as flares and dips on time scales of minutes to hours (In’t Zand et al. 1997). Quasi-periodic
oscillations(QPO)ofabout18shadbeenreportedduringa1hrflareon1996February23,which
wasinterpretedasatransientaccretiondiscformation(In’tZandetal. 1998).
We re-analysed the RXTE-PCA observation taken on February 23, 1996. We extracted the
lightcurve of the observation and searched for QPO with the Bayesian search algorithm written
in Python by Huppenkothen1. 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 the
powerlawfitresultfortheBayesiansearchanalysis(right).
s. The probability of detecting a false signal is p(χ2 > 11.277) = 3.6×10−3 , which corresponds
to a 2.91σ level of detection. We also constructed power density spectrum (PDS) of the same
lightcurve, then fit the continuum with power law model and calculated the significance of qpo
signal. Inthismethod,detectionlevelofthesignalyielded3.54σ.
1TheassociatedPythoncodescanbefoundinhttps://github.com/dhuppenkothen/BayesPSD
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TheTimingNoiseofMagnetars D.Çerri-Serim
4. Conclusion
Inconclusion,weextractthetimingresidualsof7magnetars(3SGRsand4AXPs)and2X-
raybinariesusingRXTEandSwiftobservationsandcalculatedassociatedtimingnoisestrengths.
• Ourpulsetimingnoiseanalysison7magnetarsindicatethatthespin-downratesofmagnetars
seem to show a correlation with their noise strengths. These noise strengths could be the
result of micro-scale cracking of the crust due to the magnetic field stress on the crust. We
will extend this work to other magnetars listed in McGill Magnetar Catalog (Olausen &
Kaspi2014).
• We re-analysed the transient QPO feature of 4U 1907+09. The Bayesian method does find
a candidate for a QPO around (cid:39)18s, however, lowers the level of detection to 2.91σ. The
levelofdetectionislessthanthatofthecomparisonanalysis,whichfindsacandidatearound
thesameperiod,with3.54.
WeacknowledgesupportfromTÜB˙ITAK,theScientificandTechnologicalResearchCouncilof
TurkeythroughtheresearchprojectMFAG114F345andScientificResearchProjects
CoordinationUnit(BAP)withtheprojectBAP-01-05-2016-003.
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