An observational argument against accretion in magnetars
Victor Doroshenko, Andrea Santangelo, Valery Suleimanov, Sergey Tsygankov
AAstronomy & Astrophysics manuscript no. start c (cid:13)
ESO 2020September 30, 2020
An observational argument against accretion in magnetars
V. Doroshenko , , A. Santangelo , V. F. Suleimanov , , , and S. S. Tsygankov , Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Str. 84 /
32, Moscow 117997, Russia Astronomy Department, Kazan (Volga region) Federal University, Kremlyovskaya str. 18, 420008 Kazan, Russia Department of Physics and Astronomy, FI-20014 University of Turku, FinlandSeptember 30, 2020
ABSTRACT
The phenomenology of anomalous X-ray pulsars is usually interpreted within the paradigm of very highly magnetized neutron stars,also known as magnetars. According to this paradigm, the persistent emission of anomalous X-ray pulsars (AXPs) is powered bythe decay of the magnetic field. However, an alternative scenario in which the persistent emission is explained through accretionis also discussed in literature. In particular, AXP 4U 0142 +
61 has been suggested to be either an accreting neutron star or a whitedwarf. Here, we rule out this scenario based on the the observed X-ray variability properties of the source. We directly compare theobserved power spectra of 4U 0142 +
61 and of two other magnetars, 1RXS J170849.0 − −
045 with that of the X-raypulsar 1A 0535 + Key words. pulsars: individual: (4U 0142 +
61) – stars: neutron – stars: binaries
1. Introduction
Magnetars, including soft gamma ray repeaters (SGRs) andanomalous X-ray pulsars (AXPs), are defined as isolated neutronstars (NSs) emitting from radio to X-rays, and they are presum-ably powered by the dissipation of their very strong magneticfields ( ∼ G, Thompson et al. 2002; Mereghetti 2008). Suchfields are inferred from their rapid spin-down and extreme flar-ing activity (Katz 1982; Duncan & Thompson 1992; Thompsonet al. 2002). While fields of a similar magnitude have also beensuggested for some accreting pulsars (Doroshenko et al. 2010;Reig et al. 2012; Tsygankov et al. 2016), the emission in this caseis clearly powered by accretion rather than field decay, so theseobjects are generally not considered magnetars.On the other hand, accretion from a fall-back fossil disk,surviving the supernova explosion onto a magnetized NS (vanParadijs et al. 1995; Alpar 2001), or an isolated white dwarf(WD) (Coelho & Malheiro 2014; Borges et al. 2020) has alsobeen invoked to explain the persistent X-ray emission and spinevolution of AXPs. A serious flaw in this scenario is that it cannotexplain the giant flares observed from SGRs, which must still bepowered by the decay or reconnection of ultra-strong magneticfields close to the surface of the NS and triggered by crustalshifts. Accretion is thus currently not considered as a mainstreamexplanation for the AXP phenomenon.Nevertheless, significant observational e ff orts aimed at thedetection of optical or infrared emission from cool fossil disks,potentially powering accretion, have been undertaken and indeedwere successful in revealing the presence of a disk (Hullemanet al. 2000; Kaplan et al. 2001; Ertan & Çalıs , kan 2006; Wanget al. 2006; Mereghetti 2008). We note, however, that evidence for the presence of a disk does not necessarily imply that accretionpowers the observed emission of AXPs.To further investigate the accretion scenario, we have takenan alternative and purely phenomenological approach, basedon the comparison of the aperiodic variability properties ofaccreting objects with that of several magnetars including theprototypical AXP 4U 0142 +
61, often suggested in literatureas an accreting system, and of two other bright magnetars,1RXS J170849.0 − − −
58 for compar-ison.If accretion is the mechanism at the base of the observed emis-sion, one shall expect to observe a similar aperiodic variabilityfor accreting sources and magnetars. Accretion is known to bean intrinsically noisy process (Lyubarskii 1997) and all accretingsystems, from young stellar objects to active galactic nuclei, doexhibit strong red-noise type aperiodic variability (Rappaportet al. 1971; Oda et al. 1974; Revnivtsev et al. 2009; Scaringi et al.2015). There is no reason for magnetars to be an exception. Inthis work, we show, however, that the observed variability prop-erties of AXPs are drastically di ff erent from those of accretingsystems, and thus conclude that the observed emission is likelynot powered by accretion.
2. Object selection, observations, and analysis
Being the brightest and arguably the best studied AXPs,4U 0142 +
61 is also the archetypal source discussed in the contextof the accretion scenario, and, in fact, one of the two magnetarsfor which substantial observational arguments exist to supportthis interpretation (the other being 1E 161348 − Article number, page 1 of 4 a r X i v : . [ a s t r o - ph . H E ] S e p & A proofs: manuscript no. start emission from a cool disk around the source (Wang et al. 2006)largely motivated the development of the fall-back accretion sce-nario.The peculiar two-component broadband X-ray spectrum, sim-ilar to that of some accretion-powered pulsars (see i.e., Fig 1 inDoroshenko et al. 2012), has also been interpreted in favor ofaccretion for this source (see also Ertan et al. 2007; Trümper et al.2013; Olausen & Kaspi 2014; Zezas et al. 2015; Borges et al.2020, and references therein). In addition, its high observed fluxand comparatively low spin period ( ∼ . + − − NuSTAR ), which is also used in this work for 4U 0142 +
61 andthe reference accreting sources. Finally, for completeness, wealso include a bright radio pulsar PSR B1509 −
58, which is alsodetected in the hard X-ray band and was observed with
NuSTAR (Chen et al. 2016). On the other hand, we have not included1E 161348 − ∼ . − +
262 ob-served with
NuSTAR in quiescence in Tsygankov et al. (2019a).In this observation, the source was found at a luminosity compa-rable with that of AXPs and it exhibited a two-component energyspectrum similar to that of X Persei and 4U 0142 +
61 (Tsygankovet al. 2019a). This is an important point as the hypothesis is suchthat the persistent emission of AXPs is at least, in part, based onthe similarity of their spectra to that of accretion-powered pulsars(Trümper et al. 2013).Accretion onto a white dwarf has also been suggested topower the persistent emission of AXPs (Coelho & Malheiro2014; Borges et al. 2020). As a reference white dwarf accretor, we have chosen the intermediate polar (IP) GK Persei, whichwas observed by
NuSTAR in outburst and thus has one of thehighest luminosities among all IPs (Suleimanov et al. 2019). Weemphasize, however, that power spectra of all IPs appear to bequalitatively similar (Suleimanov et al. 2019). We stress again thathigh quality data obtained with the same instrument (
NuSTAR ) atcomparable flux levels are available for all considered objects.
As already mentioned, all sources in the sample have been ob-served by
NuSTAR . The summary of the observations used inthe analysis is presented in Table 1. To investigate the observedvariability properties in all objects, we reduced the data and ex-tracted source light curves in the 3-80 keV energy range, with atime resolution of 0.0625 s using the
HEADAS 6.27.1 softwareand current set of calibration files (version 20200526). In eachcase, the source photons were extracted from a region centered onthe source with a radius of 80 (cid:48)(cid:48) . The source signal dominated thecount-rate ( ≥
95% of all counts in all cases) so the backgroundwas not subtracted for a timing analysis. Light curves that wereextracted from the two
NuSTAR units were corrected to the solarsystem barycenter and co-added to improve the counting statistics.PSDs were constructed using the powspec task and convertedto a format that is readable by
XSPEC , as described in Ingram &Done (2012). They were also rebinned by a constant factor toensure that at least 20 points contribute to each frequency bin toreduce the statistical bias (Dahlhaus 1988) associated with theWhittle statistics (Whittle 1953) used to fit the resulting PSDs.To model the PSDs, we chose a broken power law with a breakfixed at the spin frequency of a given source (see i.e., Table 1)because all three sources are expected to be close to co-rotation.A zero-width Lorentzian curve with the same frequency and anadditional constant were also included in the model to accountfor pulsations and white noise. The white noise amplitude wasfixed to the expected level of two, but it was not subtracted fora clearer presentation of the power spectrum of the AXPs. Wealso rescaled the frequency axis for plotting so that the break inthe PSDs appears at the same location to ease the comparisonbetween individual objects, as was done by Revnivtsev et al.(2009). The results are shown in Fig. 1.As it is clearly seen from the figure, low-frequency noisedominates the power spectra of the two, well-established accret-ing objects, 1A 0535 +
262 and GK Per. On the contrary, it iscompletely absent in the PSD of magnetars. To estimate an upperlimit on the noise amplitude, we included a broken power lawcomponent, fixing the indices Γ , below and above the break fre-quency f break to values similar to that obtained for the other tworeference sources, and we calculated the 1 σ confidence boundsfor the amplitudes A noise / pulse of the noise and pulsed signal us-ing the error command in XSPEC . The results are presented inTable 1.The noise amplitude in 4U 0142 +
61 is consistent with zeroand, if there is any, it has to be at least by factor of 180 lower thanthat in the other two objects. In Table 1, we also report the ratio ofthe pulsed signal amplitude to noise amplitude. Also, in this case,the relative noise power in 4U 0142 +
61 is substantially lower (bya factor of at least 75) than that of the reference accreting sources.A similar conclusion also holds for the other two consideredmagnetars. We note that, to our best knowledge, there are alsono reports of aperiodic variability for the persistent emission inthe other magnetars not studied here. Therefore, we concludethat the sources studied in this work do not constitute a specialsample, and the absence of any observed aperiodic variability is
Article number, page 2 of 4. Doroshenko et al.: An observational argument against accretion in magnetars
Source 4U 0142 +
61 1A 0535 +
262 GK Per 1RXS J170849.0 1E 1841 −
045 PSR B1509-58obs.id 30001023003 90401370001 30101021002 30401023002 30001025012 40024001002 L x , erg s − a b c d d ∼ e exposure, ks 143 55 72 93 100 34count-rate, s − f break , Hz 0.115 9 . × − . × − × − × − Γ / Γ / / / / / / A noise ≤ × − + . − . + . − . + . − . × − ≤ × − ≤ × − A noise / A pulse ≤ .
35 40 + − + − ≤ . ≤ . ≤ − Table 1.
Summary of the data used and derived PSD parameters for 4U 0142 +
61, 1A 0535 + a -using fluxes reported in Weng &Gö˘güs , (2015) and distance from Borges et al. (2020), b -Tsygankov et al. (2019a), c -Suleimanov et al. (2019), d -Olausen & Kaspi (2014), e -Chenet al. (2016). All uncertainties are reported at the 1 σ confidence level. Frequency × Period P o w e r GK Per1A 0535+2624U 0142+611E 1841-0451RXS J170849.0-400910PSR B1509-58
Fig. 1.
Power density spectra of the magnetized compact objects dis-cussed in the text and labeled in the legend. The frequency is expressedin units of respective objects spin frequency and power is normalizedsuch that expected white noise level is 2. a strong general argument against the accretion-powered originof emission from magnetars.
3. Conclusions
The recent discovery of the the two-component X-ray spectra typ-ical of AXPs in several accreting X-ray pulsars at low luminosity(see Tsygankov et al. 2019b,a) has suggested that such a spectralshape might be a common feature of accretion-powered pulsarsat low luminosities. The similarity of the spectra of these objectsand AXPs (Doroshenko et al. 2012) can thus be viewed as anargument in favor of the common origin of the observed emissionin X-ray pulsars and AXPs, that is to say accretion (Doroshenkoet al. 2012; Trümper et al. 2013). We examined, therefore, the hy-pothesis that the persistent emission of AXP 4U 0142 +
61 couldindeed be powered by accretion.We find, however, that, despite the similarity of the ob-served X-ray spectra, the aperiodic variability that is univer-sally observed in all accreting systems, including the low-luminous X-ray pulsars and accreting white dwarfs (Revnivt-sev et al. 2009; Scaringi et al. 2015), is completely absentin 4U 0142 + − − +
262 (observed in the low luminosity state) and of the intermediate polar GK Persei, which can be considered as repre-sentative objects of an accreting neutron star and a white dwarf,respectively. All of these objects were observed with the
NuSTAR observatory at a comparable flux and luminosity level, which al-lowed us to obtain power spectra of a similar quality. We find thatdespite similar luminosities, counting statistics, and energy spec-tra, the variability properties of accreting objects and magnetarsare drastically di ff erent and no evidence for the low-frequencyred noise that is typical for accreting sources is detected in themagnetars of the sample. We emphasize that the choice of otherreference objects would not alter our conclusions since aperiodicvariability is an established feature of accreting systems.We conclude, therefore, that the observed persistent emissionfrom 4U 0142 +
61, 1RXS J170849.0 − − −
58 is not due to accretion, as expected. Consider-ing that 4U 0142 +
61 is a prime candidate for accretion-poweredAXPs and the lack of detected (or reported) variability in any ofthe magnetar candidates, we conclude that our finding constitutesa strong independent argument against the accretion-poweredorigin of the persistent X-ray emission in magnetars. This con-clusion can be further verified by extending a similar analysis tomore sources.
Acknowledgements.
VD thanks Joachim Trümper for asking the questionwhether accreting pulsars in low-luminosity state are indeed magnetars. A ques-tion which this paper aims to answer. This work was supported by the RussianScience Foundation (grant 19-12-00423). VFS thanks Deutsche Forschungsge-meinschaft for financial support (grant DFG-GZ WE 1312 / References
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