Aims: We intend to establish the X-ray properties of Swift J0732.5-1331 and therefore confirm its status as an intermediate polar.



Methods: We analysed 36,240 s of X-ray data from RXTE. Frequency analysis was used to constrain temporal variations and spectral analysis used to characterise the emission and absorption properties. 



Results: The X-ray spin period is confirmed to be 512.4(3) s with a strong first harmonic. No modulation is detected at the candidate orbital period of 5.6 h, but a coherent modulation is present at the candidate 11.3 h period. The spectrum is consistent with a 37 keV bremsstrahlung continuum with an iron line at 6.4 keV absorbed by an equivalent hydrogen column density of around 1022 atoms cm-2. 



Conclusions: Swift J0732-1331 is confirmed to be an intermediate polar

A. J. Norton

Ajello

Barlow

Bradt

E. J. Barlow

Hellier

K. Mukai

Marsh

Masetti

Norton

O. W. Butters

Patterson

Thorstensen

Torres

Wheatley

English

arXiv

Butters, O. W.

Barlow, E. J.

Norton, A. J.

Mukai, K.

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsRXTE confirmation of the intermediate polar status ofSwift J0732.5-1331Journal ItemHow to cite:Butters, O. W.; Barlow, E. J.; Norton, A. J. and Mukai, K. (2007). RXTE confirmation of the intermediatepolar status of Swift J0732.5-1331. Astronomy & Astrophysics, 475(2) L29-L32.For guidance on citations see FAQs.c© 2007 ESOVersion: Version of RecordLink(s) to article on publisher’s website:http://dx.doi.org/doi:10.1051/0004-6361:20078700Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukA&A 475, L29–L32 (2007)DOI: 10.1051/0004-6361:20078700c© ESO 2007Astronomy&AstrophysicsLetter to the EditorRXTE confirmation of the intermediate polar statusof Swift J0732.5–1331O. W. Butters1, E. J. Barlow1, A. J. Norton1, and K. Mukai21 Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UKe-mail: o.w.butters@open.ac.uk2 CRESST and X-ray Astrophysics Laboratory NASA/GSFC, Greenbelt, MD 20771, USA; Department of Physics,University of Maryland, Baltimore county, 1000 Hilltop Circle, Baltimore, MD 21250, USAAccepted 18 September 2007 / Received 11 October 2007ABSTRACTAims. We intend to establish the X-ray properties of Swift J0732.5–1331 and therefore confirm its status as an intermediate polar.Methods. We analysed 36 240 s of X-ray data from RXTE. Frequency analysis was used to constrain temporal variations and spectralanalysis used to characterise the emission and absorption properties.Results. The X-ray spin period is confirmed to be 512.4(3) s with a strong first harmonic. No modulation is detected at the candidateorbital period of 5.6 h, but a coherent modulation is present at the candidate 11.3 h period. The spectrum is consistent with a 37 keVbremsstrahlung continuum with an iron line at 6.4 keV absorbed by an equivalent hydrogen column density of around 1022 atoms cm−2.Conclusions. Swift J0732–1331 is confirmed to be an intermediate polar.Key words. stars: binaries: general – stars: novae, cataclysmic variables – stars: individual: Swift J0732.5–1331 – X-rays: binaries1. Introduction to magnetic cataclysmic variablesIntermediate polars (IPs) are a sub-class of cataclysmic vari-ables (CVs). They fill the phase space, in terms of magnetic fieldstrength, and spin and orbital periods, between non-magneticCVs and the strongly magnetic synchronously rotating polars.The magnetic field strength is believed to be in the range ofa few MG to tens of MG at the white dwarf surface. This islarge enough to dramatically alter the accretion flow, yet notlarge enough to synchronize the spin and orbital periods. Thismagnetic field gives rise to the defining characteristic of the sub-class, that of X-ray variation pulsed at the spin period of thewhite dwarf. For an exhaustive review of CVs see e.g. Warner(1995).There are between twenty six1 and fifty IPs currently known(depending on the selection criteria used). The hard X-ray se-lected object, Swift J0732.5–1331 (hereafter J0732), is a sus-pected IP in need of confirmation. The circumstance of its dis-covery makes J0732 similar to the host of candidate IPs thathave been discovered to be powerful emitters of hard X-rays/softgamma-rays in the 20−100 keV range in the INTEGRAL/IBISsurvey (Barlow et al. 2006). We have embarked on a campaignof pointed RXTE observations of these hard X-ray discoveredcandidate IPs. Here we present the first results of our campaignon J0732.2. Previous observations of Swift J0732.5-1331There is to date no peer-reviewed analysis of J0732 publishedin the literature. There are however several mentions of it inAstronomical Telegrams, which we summarise below. These1 http://asd.gsfc.nasa.gov/Koji.Mukai/iphome/iphome.html as of 23/8/7.were all published over the course of a couple of months in early2006.J0732 was first detected by Ajello et al. (2006) with the SwiftBurst Alert Telescope and Swift X-ray telescope (XRT). With theXRT 600 counts in 3400 s were recorded, coincident with theROSAT source 1RXS J073237.6−133113. This is also coinci-dent with the 2MASS source J073237.64−133109.4, a proposedK main-sequence star 400 pc away. Based on its X-ray luminos-ity and colours, Ajello et al. (2006) suggested a CV identificationfor the object.Masetti et al. (2006) used the BFOSC instrument on the G.D.Cassini 1.5 m telescope to obtain the optical spectrum of thecounterpart to J0732. Two objects were found close to the re-ported position, a normal G/K type Galactic star (the 2MASSsource) and a fainter one deemed to be the true optical counter-part. The spectral signature of the system was concluded to bethat of an accretion disc in a low mass X-ray binary.Patterson et al. (2006) also obtained low resolution spectra,this time on the MDM 2.4 m telescope, of the 2MASS opti-cal counterpart proposed by Ajello et al. (2006) (i.e. the fieldstar), concluding that it was indeed a normal G star. In the sametelegram Patterson et al. (2006) also reported optical photome-try (obtained by the small telescope network of the Center forBackyard Astrophysics2) which revealed a stable pulsation pe-riod of 512.42(3) s with most of the power in the first harmonic.This was deemed to be the spin period of a rotating white dwarfand Patterson et al. (2006) consequently suggested an IP classi-fication for the object. A possible 11.3 h orbital signal was alsosuggested, but owing to its low amplitude, this required the bi-nary to be close to face on, as any variation in brightness due tothe projected Roche lobe filling secondary was small.2 http://cba.phys.columbia.edu/Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20078700L30 O. W. Butters et al.: RXTE confirmation of the intermediate polar status of Swift J0732.5–1331Fig. 1. 2−10 keV background subtracted light curve of J0732.5−1331.The zero time corresponds to the start of the observations atJD 2 454 295.30035. The data is binned into bins of 128 s width. Thetypical error on each point is ±0.2.Marsh et al. (2006) subsequently used ULTRACAM mountedon the William Herschel Telescope to observe the optical coun-terpart of J0732. The spin pulsation detected by Patterson et al.(2006) was seen. The counterpart and the non-associated fieldstar were found to be approximately 1.8′′ apart.Torres et al. (2006) performed spectral analysis of the opticalcounterpart at the Mt. Hopkins 1.5 m telescope. Balmer emis-sion lines from Hα to at least Hγ were found. This reinforced itsclassification as a probable IP. However, despite seeing a vari-ation in the radial velocities of the various emission lines, theywere unable to determine an orbital period.Wheatley et al. (2006) later reanalysed the original Swift datareported in Ajello et al. (2006) and found an X-ray pulsation atthe proposed spin period. The modulation was found to be singlepeaked and only present below 2 keV. Reanalysis of the spec-tral data suggested a temperature typical of intermediate polars(kT ∼ 20 keV). This data set is, however, short and suﬀers fromsevere aliasing eﬀects.Thorstensen et al. (2006) carried out time series spec-troscopy at the MDM Observatory on the optical counterpart.The radial velocities of the Hα emission lines were found to varyperiodically with a period of 0.2335(8) days (5.60(2) h), whichwas interpreted as the orbital period of the system. We note thisis close to half the photometric period suggested by Pattersonet al. (2006).Given all this information, J0732 is strongly suspected to bean intermediate polar, but the only way to confirm this is theunambiguous detection of pulsed hard X-ray emission at the spinperiod.3. Observations and data reductionData were obtained from the RXTE satellite (Bradt et al. 1993)with the PCA instrument over two consecutive days, from13th−15th July 2007. The total time on target was 36 240 s,comprising fourteen approximately equal segments of one satel-lite orbit each. Initial data reduction was done with the standardftools, and the flux was normalised according to the number ofcorrectly functioning PCUs. For the light curve analysis PCUs 2,3, and 4 were used; whilst for the spectral analysis only PCU 2was used as the other PCUs were only turned on infrequently.Only the top layer of each PCU was included in the measure-ments and the time resolution of the data was 16 s. Backgroundsubtracted light curves were constructed in four energyFig. 2. cleaned periodogram. The upper plot shows the raw peri-odogram, with the window function inset; the lower plot shows the de-convolved (cleaned) periodogram.Table 1. Modulation depths of the pulse profile in diﬀerent energybands. Modulation depth is defined here as the semi-amplitude of a fit-ted sinusoid divided by the fitted mean.Energy band Modulation depth Uncertainty Fitted mean(keV) (%) (%) (ct s−1 PCU−1)2–10 8 1 1.312–4 16 3 0.254–6 7 2 0.466–10 7 2 0.5610–20 10 4 0.28bands: 2−4 keV, 4−6 keV, 6−10 keV and 10−20 keV, as wellas a combined 2−10 keV band for maximum signal-to-noise. Amean X-ray spectrum was also extracted.3.1. Light curve analysisThe raw count rate varied between 3.9 and 5.4 count s−1 PCU−1.The background count rate, generated from the calibration files,varied between 2.9 and 3.8 count s−1 PCU−1. The backgroundsubtracted 2−10 keV light curve is shown in Fig. 1. The datawere subsequently analysed with a variable gain implementa-tion of the clean algorithm (Lehto 1997) to discover any peri-odicities and discount any aliasing eﬀects. The results of this areshown in Fig. 2.3.1.1. Spin periodStrong peaks are evident in the cleaned periodogram at 168 cy-cles day−1 (512 s) and at its first harmonic, in the 2−10 keVenergy band. Similar signals are seen in each energy band.Analysis of the peaks yields a pulsation period of 512.4(3) s.The data in each of the energy resolved light curves were thenfolded at the 512.4 s period, and Fig. 3 shows the result in the2−10 keV energy band. The modulation depths of the pulse pro-file were then estimated by fitting a sinusoid to the folded data ineach energy band and dividing the semi-amplitude by the fittedmean. The results of this are shown in Table 1.3.1.2. Orbital periodThe windowing of the data is such that no reliable signal canbe extracted for periods of a few hours from the periodogram,O. W. Butters et al.: RXTE confirmation of the intermediate polar status of Swift J0732.5–1331 L31Fig. 3. 2−10 keV light curve folded at the 512.4 s pulse period with anarbitrary zero point. Two cycles are shown for clarity.therefore no reliable orbital period can be found. Folding theX-ray data at the 5.6 h spectroscopic period (Thorstensen et al.2006) yields no coherent modulation, but folding it at the 11.3 hphotometric period suggested by Patterson et al. (2006) gives aplausible signal (see Fig. 4).3.2. Spectral analysisAnalysis of the X-ray spectrum was carried out with thexspec package. The best fit for a simple photoelectricallyabsorbed bremsstrahlung model had the parameters kT = 37 ±7 keV and nH = (2.0 ± 0.5) × 1022 cm−2 (reduced χ2 = 3.0). Theresiduals of this plot indicate the presence of an excess at ap-proximately 6.5 keV. Keeping the temperature and column den-sity fixed and fitting a Gaussian to this region indicates an ironline at 6.4 ± 0.1 keV with a width of σ = 0.3 ± 0.1 keV (reducedχ2 = 1.0), as shown in Fig. 5.4. DiscussionThe strong X-ray signal at the 512.4(3) s pulse period seen at allenergies is characteristic of IPs and confirms the nature of theobject. Patterson et al. (2006) found a much stronger peak at thefirst harmonic in their frequency analysis of the optical photom-etry data. This is characteristic of a double-peaked pulse profileand indicates that two emission regions can be seen. The X-raydata reported here exhibit the same periodicity, but a somewhatdiﬀerent profile. The first harmonic in the X-ray data is stillpresent, and the pulse profile consequently shows a second min-imum superimposed on the pulse maximum, but the overall pro-file is only marginally double-peaked. The most likely geometryof this system is therefore such that one magnetic pole can al-ways be seen, the other being behind the WD for most of the cy-cle. If the heights of the accretion columns are such that a frac-tion of the hidden pole’s column can be seen at certain phasesthen the X-ray profile may be explained. If the optical emissionarises from reprocessed X-rays (i.e. further up the accretion col-umn) then it may be seen from both poles and this would explainthe optical signal of Patterson et al. (2006).The modulation depth of the X-ray pulse profile is approxi-mately constant above 4 keV, implying that the dominant eﬀectshaping the profile is geometric, probably self-occultation by thewhite dwarf. At the lowest energies (2−4 keV) the modulationdepth is higher, which implies that phase-varying photoelectricabsorption (as well as occultation) is the process which causesthe modulation. The spectral fitting also indicates the presenceFig. 4. 2−10 keV light curve folded at the 11.3 h photometric periodsuggested by Patterson et al. (2006) with an arbitrary zero point. Twocycles are shown for clarity.0.10.050.2normalized counts s−1  keV−1Swift J0732.5−1331105−0.0100.01residualsEnergy (keV)Fig. 5. 2.5−15 keV mean spectrum fitted with a photoelectrically ab-sorbed bremsstrahlung plus iron line profile. Fitting parameters arekT = 37 ± 7 keV, nH = (2.0 ± 0.5) × 1022 cm−2, iron line fitted with aGaussian centred on 6.4 ± 0.1 keV with σ = 0.3 ± 0.1 keV. Reducedχ2 = 1.0.of a significant local absorbing column, and has parameters thatare typical of other IPs.There is still some ambiguity about the orbital period of thissystem. The chance of it being the 11.3 h period suggested byPatterson et al. (2006) is now increased, given the X-ray signalseen here, but the possibility of aliasing in this data set meansthat it cannot be definitively said to be so and we cannot ruleout the 5.6 h spectroscopic period found by Thorstensen et al.(2006). The absence of a beat period in the frequency analysisof the optical data does suggest that the true orbital period maybe long, since no reprocessed radiation is seen from the face ofthe secondary star and therefore the bodies are likely to be farapart.The lack of an X-ray beat signal in these RXTE data indicatesthat there is no significant stream-fed component to the flow.This suggests a relatively weak magnetic field strength and isconsistent with the small Pspin/Porb ratio of the system (Nortonet al. 2004), namely 0.025 or 0.013 depending on which is thecorrect orbital period.The high temperature bremsstrahlung continuum and thepresence of an iron line at 6.4 keV reinforce the case for J0732to be an IP since these are both features often seen in other IPs(Hellier & Mukai 2004).L32 O. W. Butters et al.: RXTE confirmation of the intermediate polar status of Swift J0732.5–1331Finally, we note that the average RXTE count rate(1.3 ct s−1 PCU−1 in the 2−10 keV band) is consistent3 with thevalue obtained with the Swift satellite, indicating that the systemhas not changed significantly in brightness since its discovery.5. ConclusionThe unambiguous X-ray spin period detection at 512.4(3) s,along with the spectral fit to an absorbed 37 keV bremsstrahlungmodel with an iron line, confirm the intermediate polar statusof Swift J0732.5−1331. We are unable to determine the orbitalperiod from these RXTE data although there is some indica-tion of modulation at the previously suggested photometric pe-riod of 11.3 h and none at the spectroscopic period of 5.6 h.To conclude we note that this system is similar, in terms of itssmall Pspin/Porb value, to the IPs RX J2133.7+5107 and NY Lup(IGR J15479–4529). Both of these are INTEGRAL hard X-raysources and both also have soft X-ray components. We mighttherefore expect that Swift J0732.5−1331 would also displaysuch characteristics upon further study.3 Using webpimms:http://heasarc.gsfc.nasa.gov/Tools/w3pimms.htmlReferencesAjello, M., Greiner, J., Rau, A., et al. 2006, The Astronomer’s Telegram, 697, 1Barlow, E. J., Knigge, C., Bird, A. J., et al. 2006, MNRAS, 372, 224Bradt, H. V., Rothschild, R. E., & Swank, J. H. 1993, A&AS, 97, 355Hellier, C., & Mukai, K. 2004, MNRAS, 352, 1037Lehto, H. J. 1997, in Applications of time series analysis in astronomy and me-teorology, ed. T. Subba Rao, M. B. Priestley, & O. Lessi (London Chapmanand Hall)Marsh, T., Littlefair, S., & Dhillon, V. 2006, The Astronomer’s Telegram, 760, 1Masetti, N., Bassani, L., Dean, A. J., Ubertini, P., & Walter, R. 2006, TheAstronomer’s Telegram, 735, 1Norton, A. J., Wynn, G. A., & Somerscales, R. V. 2004, ApJ, 614, 349Patterson, J., Halpern, J., Mirabal, N., et al. 2006, The Astronomer’s Telegram,757, 1Thorstensen, J. R., Patterson, J., Halpern, J., & Mirabal, N. 2006, TheAstronomer’s Telegram, 767, 1Torres, M. A. P., Steeghs, D., Garcia, M. R., et al. 2006, The Astronomer’sTelegram, 763, 1Warner, B. 1995, Cataclysmic variable stars, Cambridge Astrophysics Series(Cambridge, New York: Cambridge University Press)Wheatley, P. J., Marsh, T. R., & Clarkson, W. 2006, The Astronomer’s Telegram,765, 1

RXTE confirmation of the intermediate polar status of Swift J0732.5-1331

Crossref

RXTE confirmation of the intermediate polar status   of Swift J0732.5–1331

Aims.We intend to establish the X-ray properties of Swift J0732.5–1331 and therefore
 confirm its status as an intermediate polar.
        Methods.We analysed 36 240 s of X-ray data from RXTE. 
Frequency analysis was used to constrain temporal variations and
spectral analysis used to characterise the emission and
 absorption properties.
        Results.The X-ray spin period is confirmed to be 512.4(3) s with a strong first
harmonic. No modulation is detected at the candidate orbital period of
5.6 h, but a coherent modulation is present at the candidate 11.3 h
period. The spectrum is consistent with a 37 keV
bremsstrahlung continuum with an iron line at 6.4 keV absorbed by an
equivalent hydrogen column density of around 1022 atoms cm-2.
        Conclusions.Swift J0732–1331 is confirmed to be an intermediate polar

EDP Sciences OAI-PMH repository (1.2.0)

Aims. We intend to establish the X-ray properties of Swift J0732.5-1331 and therefore confirm its status as an intermediate polar.Methods. We analysed 36 240 s of X-ray data from RXTE. Frequency analysis was used to constrain temporal variations and spectral analysis used to characterise the emission and absorption properties.Results. The X-ray spin period is confirmed to be 512.4(3) s with a strong first harmonic. No modulation is detected at the candidate orbital period of 5.6 h, but a coherent modulation is present at the candidate 11.3 h period. The spectrum is consistent with a 37 keV bremsstrahlung continuum with an iron line at 6.4 keV absorbed by an equivalent hydrogen column density of around 1022 atoms cm(-2).Conclusions. Swift J0732-1331 is confirmed to be an intermediate polar.</p

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RXTE confirmation of the intermediate polar status of Swift J0732.5-1331

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