Optical pulsations from the anomalous X-ray pulsar 1E 1048.1-5937

0.46 MB·English

by  V. S. Dhillon

Mon.Not.R.Astron.Soc.000,1–5(2008) Printed24January2009 (MNLATEXstylefilev2.2)
Optical pulsations from the anomalous X-ray pulsar
1E 1048.1–5937
9 V. S. Dhillon,1⋆ T. R. Marsh,2 S. P. Littlefair,1 C. M. Copperwheat,2 P. Kerry,1
0
0 R. Dib,3 M. Durant,4 V. M. Kaspi,3 R. P. Mignani,5 A. Shearer6
2

n 1Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
a 2Department of Physics, University of Warwick, Coventry CV4 7AL, UK
J 3Department of Physics, McGill University,Montreal, Quebec H3A 2T8, Canada

2 4Instituto de Astrof´ısica de Canarias, 38200 La Laguna, Tenerife,Spain
1 5Mullard Space Science Laboratory, UniversityCollege London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
6Centre for Astronomy, National Universityof Ireland, Galway, Newcastle Rd., Galway, Ireland

]
E
H
. Submittedon2008November12.
h
p
-
o ABSTRACT
r We present high-speed optical photometry of the anomalous X-ray pulsar 1E1048.1–
t
s 5937 obtained with ULTRACAM on the 8.2-m Very Large Telescope in June 2007.
a Wedetect1E1048.1–5937atamagnitudeofi′ =25.3±0.2,consistentwiththevalues
[
foundbyWang et al.(2008)andhenceconfirmingtheirconclusionthatthesourcewas

1 approximately 1 mag brighter than in 2003–2006due to an on-going X-ray flare that
v started in March 2007. The increased source brightness enabled us to detect optical
9 pulsations with an identical period (6.458s) to the X-ray pulsations. The rms pulsed
5 fraction in our data is 21±7%, approximately the same as the 2–10 keV X-ray rms
5 pulsed fraction. The optical and X-ray pulse profiles show similar morphologies and
1
appear to be approximately in phase with each other, the latter lagging the former
1. by only 0.06±0.02 cycles. The optical pulsations in 1E1048.1–5937 are very similar
0 in nature to those observed in 4U0142+61. The implications of our observations for
9 models of anomalous X-ray pulsars are discussed.
0
: Key words: pulsars: individual: 1E1048.1–5937– stars: neutron
v
i
X
r
a
1 INTRODUCTION fallback disc models is via optical observations. The mag-
netar model predicts any optical emission must be non-
The anomalous X-ray pulsars (AXPs) are a small group1
thermalandmagnetosphericinorigin.Fourplausiblemecha-
of isolated neutron stars in which the X-ray luminosity far
nismshavebeenconsidered–coherentplasmaemission,syn-
exceedstheenergyavailablefromthespin-down.TheAXPs
chrotron emission from electrons with high Lorentzfactors,
are generally believed to be magnetars, in which the ex-
cyclotron emission from ions in the outer magnetosphere
cess luminosity is powered by the decay of an ultra-strong
andcurvatureemissionfrombunchedelectron-positronpairs
magneticfield,inexcessof1014G(seeWoods & Thompson
in the inner magnetosphere (see Beloborodov & Thompson
2006). An alternative explanation is the fallback disc sce-
(2007) and references therein). The fallback disc model, on
nario, in which some of thesupernova ejecta fails to escape
theotherhand,predictsanyopticalemissionisproducedby
and forms an accretion disc around the neutron star, pro-
reprocessing of the X-ray light in the disc and/or thermal
viding an extra source of energy to power the X-ray emis-
emission from the disc (Pernaet al. 2000).
sion (van Paradijs et al. 1995; Chatterjee et al. 2000;Alpar
2001). Thefirst AXPtobedetected in theoptical part of the
One way of discriminating between the magnetar and spectrum was 4U0142+61 (Hulleman et al. 2000). Optical
pulsationswerediscoveredin4U0142+61byKern & Martin
(2002),andthefactthatthesepulsationshavethesamepe-
⋆ E-mail:vik.dhillon@sheffield.ac.uk riod, morphology and phase as the X-rays, but with 5–7
1 Seehttp://www.physics.mcgill.ca/∼pulsar/magnetar/main.html timesgreaterpulsedfraction,wasreportedbyDhillon et al.
for an up-to-date catalogue of all known AXPs, including the (2005).Theseresultsprovidedstrongsupportforthemagne-
variouswavelengths atwhicheachhasbeendetected. tar model – pulsed optical emission is indicative of a mag-
2 V. S. Dhillon et al.
Table 1. X-rayephemerisfor1E1048.1–5937 (Dibetal.2008).
The epoch of the frequency and frequency derivative measure-
mentsgivenbelowfallsonthesamenightasourVLTobservations
(09/06/2007 = MJD 54260). BMJD refers to the Barycentric-
corrected Modified Julian Date on the Barycentric Dynamical
Timescale(TDB).Theerrorsonthelasttwodigitsofeachparam-
eter are given in parentheses. This ephemeris is valid for BMJD
54229.0−54280.0.
ν (Hz)....................... 0.1548479469(42)
ν˙ (10−13 Hzs−1)............ –5.413(53)
Epoch(BMJD)............... 54260.0
Figure 1. Left: Summed i′-band image of the field around
1E1048.1–5937, withatotal exposure timeof 10684s. Forclar-
ity, only a portion of one of the two ULTRACAM windows is ters.Fortheobservationspresentedhere,onebeamwasded-
shown.Thepositionsof1E1048.1–5937andthecomparisonstar icated to theSDSSu′ (λ =3543˚A) filter, another to the
eff
areindicatedbycirclesnearthecentreandbottomoftheimage, SDSSg′(4770˚A)filterandthethirdtotheSDSS(7625˚A)i′
respectively. The central box shows the portion of the field that
filter. Because ULTRACAM employs frame-transfer chips,
isplottedatahighercontrastontheright.Theorientationofthe
thedead-timebetween exposuresis negligible: weused UL-
imageismarkedontheupperright-handside.Thepixelscaleis
0.156 arcseconds/pixel, hence the field of view in this image is TRACAMinitstwo-windowedmode,eachof250×200pix-
36×30 arcseconds. The vertical banding is due to residual bias els, resulting in an exposure time of 0.963 s and a dead-
structure.Right:Highercontrastplotofa7.5×7.5arcsecondfield time of 0.024 s. A total of 11095 frames of 1E1048.1–5937
around 1E1048.1–5937, highlighting the detection of the pulsar wereobtained onthenight of2007 June9, witheach frame
inthei′-band time-stampedtoarelative(i.e.frame-to-frame) accuracyof
∼50µs and to an absolute accuracy of ∼1 ms using a ded-
icated GPS system (see Dhillon et al. 2007). Observations
netospheric origin and disc reprocessing is unlikely in this
oftheSDSSstandardG163-51(Smith et al.2002)werealso
case, as theoptical pulsedfraction is higherthan theX-ray
obtainedtofluxcalibratethedata.Thenightwasphotomet-
pulsed fraction and thereisno timedelay between thetwo.
ric,withnomoonandi′-bandseeingof0.65arcseconds.The
Althoughitispossibletocontrivewaysinwhichthefallback
sumofthe11095framesinthei′-bandisshowninfigure1,
discmodelisconsistentwiththeopticalpulsationsobserved
which can be compared to the finding charts presented by
in 4U0142+61, e.g. by assuming that the X-ray pulse pro-
Durant& van Kerkwijk (2005).
file that we observe is different to the X-ray radiation seen
ThedatawerereducedusingtheULTRACAMpipeline
by the disc due to orientation or beaming effects, or by in-
software(Dhillon et al.2007).Allframeswerefirstdebiased
vokingahybriddisc-magnetospheremodel(seeErtan et al.
andthenflat-fielded,thelatterusingthemedianoftwilight
(2007) and references therein), the weight of evidence from
skyframestakenwiththetelescopespiralling.Adoptingthe
the optical and other wavelengths lies heavily on the side
samesuccessfulapproachthatweusedinourstudyofAXP
of the magnetar model (see Mereghetti (2008) for a recent
4U0142+61 (Dhillon et al. 2005), we extracted light curves
review).
of 1E1048.1–5937 usingtwo different techniques:
ThedetectionofopticalpulsationsinotherAXPswould
provide valuable confirmation, or otherwise, of the results
for 4U0142+61 discussed above. Only one other AXP has
2.1 Technique (i)
been unambiguously identified in the optical: 1E1048.1–
5937 (Durant & van Kerkwijk 2005)1. In this paper we re- As part of a long-term monitoring project, 1E1048.1–
port on the first detection of optical pulsations from AXP 5937 has been observed regularly (up to three times per
1E1048.1–5937, obtainedonly∼3monthsafterabrightX- week) since 1997 with the Proportional Counter Array
ray flare in March 2007. (PCA) on board theRossi X-ray Timing Explorer (RXTE)
[Kaspi et al. 2001; Gavriil & Kaspi 2004; Dib et al. 2008].
The X-ray spin frequency and frequency derivative of
1E1048.1–5937 forthenightofourVLTobservations(MJD
2 OBSERVATIONS AND DATA REDUCTION
54260) are given in table 1. For the first light-curve ex-
The observations of 1E1048.1–5937 presented in this pa- traction technique,we shifted and added each of the11095
per were obtained with ULTRACAM (Dhillon et al. 2007) ULTRACAM frames into 10 evenly-spaced phase bins us-
at the Nasmyth focus of Melipal, the 8.2-m Unit 3 of the ing the epoch and spin frequency given in table 1, result-
Very Large Telescope (VLT) in Chile. ULTRACAM is a ing in 10 high signal-to-noise data frames. Anoptimal pho-
CCD camera designed to provide imaging photometry at tometry algorithm (Naylor 1998) was then used to extract
high temporal resolution in three different colours simulta- the counts from 1E1048.1–5937 and an i′∼17 comparison
neously. The instrument provides a 2.66 arcminute field on star ∼12 arcseconds to the south-east of the AXP (see
its three 1024×1024 E2V 47-20 CCDs (i.e. 0.156 arcsec- figure 1), the latter acting as the reference for the profile
onds/pixel). Incident light is first collimated and then split fits and transparency-variation correction. The position of
intothreedifferentbeamsusingapairofdichroicbeamsplit- 1E1048.1–5937 relative to the comparison star was deter-
Optical pulsations from AXP 1E1048.1–5937 3
mined from a sum of all the images, and this offset was
thenheldfixedduringthereductionsoastoavoidaperture
centroiding problems. The sky level was determined from a
clipped mean of the counts in an annulus surrounding the
target stars and subtracted from theobject counts.
2.2 Technique (ii)
The second approach we took to light curve extraction was
identical to that described above, except we omitted the
phase-binning step and simply performed optimal photom-
etry on the11095 individualULTRACAMdataframes fol-
lowedbyaperiodogramanalysisoftheresultingtimeseries.
In other words, we made no assumption about the spin pe-
riod of 1E1048.1–5937.
Figure2.Top:Thesolidanddottedlinesshowopticalpulsepro-
files of 1E1048.1–5937 in the i′-band obtained using techniques
3 RESULTS (i) and (ii), respectively (see sections 2.1 and 2.2). Each pulse
profile was first corrected for transparency variations using the
3.1 Magnitudes
comparisonstarshowninfigure1,althoughthecorrectionmade
We were unable to detect 1E1048.1–5937 in u′ and g′, onlyanegligibledifferencetothelightcurves.Thepulseprofiles
at a 3σ detection limit of u′ > 25.7 and g′ > 27.6, re- werethennormalisedbydividingbythemeannumberofcounts.
Forclarity,twocycles areshown.Bottom:AveragedX-raypulse
spectively. This is unsurprising given the high visual ex-
profileof 1E1048.1–5937 inthe 2–10 keV energy band spanning
tinction to the object (AV = 4.9; Durant & van Kerkwijk theepochoftheopticalobservations(Dibetal.2008).Notethat
2006). We did, however, clearly detect 1E1048.1–5937 in
itisnotpossibletoestimate theX-raypulsedfractionfromthis
i′ at a magnitude of i′ = 25.3 ± 0.2, as shown in fig- profileasthePCAonRXTEhasa1◦ fieldofviewandnoimag-
ure1.Durant & van Kerkwijk(2005)foundthat1E1048.1– ingcapability,renderingthebackgroundleveluncertain.Forthis
5937 was at a magnitude of I = 26.2±0.4 (i′∼26.1)2 on reason,noscaleisgivenontheordinate.
06/06/2003, just over one year after its first X-ray flare
was observed in April 2002. Wang et al. (2008) observed
1E1048.1–5937 atamagnitudeofI =24.9±0.2(i′∼24.8)2
latter is the average of the X-ray light curves in the pe-
on 07/05/2007, just over one month after its second X-ray
riod23/05/2007–21/06/2007 obtainedaspartoftheRXTE
flarebeganon2007March21.Theseauthorsalsomeasured
a limit of i′ >24.5 on 15/07/2007. Our i′-band magnitude, monitoringcampaign describedbyDib et al. (2008),with a
totaleffectiveintegrationtimeof59.5ks.Bothprofilesshow
whichisslightlyfainterandwasobtainedslightlylaterthan
thesamebroad,single-humpedmorphology.Moreover,since
thefirstI-bandmeasurementofWanget al.(2008),implies
theX-raylightcurveshowninfigure2hasalsobeenphased
thatweobserved1E1048.1–5937whilstitwasstillrelatively
usingtheephemerisgivenintable1,itcanbeseenthatthe
bright, but declining, from the most recent X-ray flare (see
optical and X-ray pulse profiles are approximately in phase
Tam et al. 2008).
with each other. To quantify this, the optical pulse profile
wascross-correlatedwiththeX-raypulseprofile.Theresult-
3.2 Pulse profiles ing peak in the cross-correlation function was fitted with a
Gaussian to derive a phase shift of −0.06±0.02 cycles (i.e.
The two data reduction techniques described in section 2 −0.39±0.13s), where a negative phase shift implies that
result in two different pulseprofiles for 1E1048.1–5937.
the X-ray pulse profile lags the optical pulse profile. This
phaseshiftisonlysignificantatthe3σ level,duetothelow
signal-to-noise and time resolution of the optical data, and
3.2.1 Technique (i)
additionaldatawillberequiredinordertoconfirmthatthe
The first technique produced the pulse profile shown by phase shift is different from zero (discounting the unlikely
the solid line in the top panel of figure 2. The pulse pro- situation in which thetime delay is approximately equal to
file exhibits a broad, single-humped structure with a peak some multiple of the spin period).
around phase 0.5 and a minimum around phase 0. There It should be noted that the morphology of the X-ray
is a great deal of similarity in the morphologies of the op- pulse profile in 1E1048.1–5937 does not appear to be en-
tical pulse profile shown in the top panel of figure 2 and ergysensitive;theshapesofthe2–4keVand6–10keVpulse
the2–10 keVX-ray pulse profile shown below it, where the profiles are virtually identical to the 2–10 keV pulse profile
shown in figure 2, even though the 6–10 keV band is com-
posedprimarilyofnon-thermalphotonswhereasthe2–4keV
2 Noting that the visual extinctions to 1E1048.1–
bandiscomposedofboththermalandnon-thermalphotons
5937 and 4U0142+61 are approximately the same
(F. Gavriil, privatecommunication).
(Durant&vanKerkwijk2006),wehaveassumedthatthecolours
of 1E1048.1–5937 are the same as 4U0142+61 (Hullemanetal. The modulation amplitude of the pulses presented in
2004; Dhillonetal. 2005) and then used the equations of figure2canbemeasuredusingapeak-to-troughpulsedfrac-
Smithetal.(2002)toconvertfromI toi′. tion, hpt, defined as follows:
4 V. S. Dhillon et al.
profilederivedusingtechnique(i).Ratherthanadoptingthe
X-rayephemeris given in table 1, we can instead determine
thepulseperioddirectlyfrom ouropticaldatausingaperi-
odogram.Measuring thesameperiod intheopticalandthe
X-rays would prove without doubt that we have detected
optical pulsations from 1E1048.1–5937.
Figure 3 shows the Lomb-Scargle periodograms
(Press & Rybicki 1989) for the 11095 points in the i′-band
light curves. The light curve was first corrected for trans-
parency variations. The highest peak in the resulting pe-
riodogram occurs at a frequency of 0.15484 ±0.00004 Hz
(6.458±0.002 s), where the error is given by thewidth (σ)
of a Gaussian fit to the peak in the periodogram. This fre-
quency is consistent to the fifth decimal place with the X-
raypulsefrequencygivenintable1,therebyconfirmingthat
we have indeed detected the X-ray pulsation of 1E1048.1–
Figure 3. Lomb-Scargle periodograms of 1E1048.1–5937 inthe 5937 in theoptical. Wefurthertestedtherobustnessof our
i′-band, obtained usingthe light curves fromtechnique (ii) [sec- period detection by constructing 10000 randomised light
tion 2.2]. The dotted line shows the X-ray pulse frequency of
curvesfromtheoriginallightcurvesbyrandomlyre-ordering
0.1548479469 Hzgivenintable1.
the time-series. None of the resulting 10000 periodograms
showed a higher peak at 0.15484 Hz.To check for artifacts,
hpt = FFmmaaxx−+FFmmiinn, (1) wskeycaanlcdutlahteedcomthpeaLriosomnb-sStacrar–glneeiptehreirodshogorwaemdsanofybevotidhenthcee
where Fmax and Fmin are the maximum and minimum foraperiodicityat0.15484Hz,oratanyotherfrequency.We
flux in the pulse profile, respectively. We find a value of also searched for evidence of optical bursts and/or longer-
hpt =52±15%.Thepeak-to-troughpulsedfractiondefined timescale periodicities in our 10684s light curve – nothing
in equation 1 effectively adds any noise present in the light significant was found.
curvetothetruepulsedfraction,therebytendingtoincrease Foldingthe11095pointsofthetechnique(ii)lightcurve
theresultingmeasurement.Amorerobustestimateisgiven on the X-ray period given in table 1 results in the pulse
by the root-mean-square (rms) pulsed fraction, hrms, de- profileshownbythedottedlineinthetoppaneloffigure2.
fined as follows: Note that the phasing of both the technique (i) and (ii)
profiles in figure 2 can be directly compared, as all data
1
hrms = y1¯ n1 n (yi−y¯)2−σi2 2 , (2) honaveewboeuelndfeoxldpeedctu,stihneg ttehcehnzeiqrouep(oiiin)tliggihvetncuinrvteabshleow1.nAiss
" #
Xi=1 inexcellentagreementintermsofmorphology,phasingand
where n is the number of phase bins per cycle, yi is the pulsed fraction (hrms = 26±8%) with the technique (i)
number of counts in the ith phase bin, σi is the error on light curve, lending additional confidence to our reduction
yi and y¯ is the mean number of counts in the cycle. As and analysis techniques.
expected, measuring the optical pulsed fraction in this way
gives a lower valueof hrms =21±7%.
For comparison, the 2–10 keV X-ray rms pulsed frac-
4 DISCUSSION AND CONCLUSIONS
tion at the same epoch as the optical observations was
hrms = 28.7±0.5%. Since it isn’t possible to measure the Theresults presented in section 3demonstrate conclusively
rms pulsed fraction directly from the RXTE pulse profile that we have detected optical pulsations from 1E1048.1–
due to the uncertain background (see caption to figure 2), 5937 on theX-rayspin period.
we derived this value as follows. We first averaged the rms It is instructive to compare the optical light curve of
pulsedflux,definedastheproductof thetotal fluxand the 1E1048.1–5937 with that of the only other AXP to have
rms pulsed fraction, measured with RXTE between 2007 beenstudiedinthisway–4U0142+61(Dhillon et al.2005).
June 7 and June 12 (Dib et al. 2008). This value, which Both objects show optical pulsations on the X-ray spin pe-
is background independent, was identical to that measured riod, which is 6.458s in the case of 1E1048.1–5937 and
with Chandra on 2007 April 28 by Tam et al. (2008), who 8.688sin4U0142+61.Bothobjectsshowopticalpulsations
alsofindastronganticorrelationbetweentotalfluxandrms with similar morphologies to their 2–10 keV X-ray light
pulsed fraction. Hence if the pulsed flux was the same for curves,with1E1048.1–5937exhibitingasingle-humpedpul-
the RXTE and Chandra observations, we can be confident sation and 4U0142+61 a double-humped pulsation. Both
thatthermspulsedfractionwasthesameaswell,andhence objects exhibit optical pulsations which are approximately
we haveadopted the Chandra rms pulsed fraction from the in phase with the X-ray pulsations, with 1E1048.1–5937
2007 April 28 observation of Tam et al. (2008). showing only marginal evidence for the optical leading the
X-rays and 4U0142+61 showing only marginal evidence
for the optical lagging the X-rays. Even the optical pulsed
3.2.2 Technique (ii)
fractions of the two objects are similar, with values of
The second data reduction technique (section 2.2) can be hpt = 52±15% and hrms = 21±7% in 1E1048.1–5937,
usedtoprovideacheckonthereliabilityoftheopticalpulse and hpt =58±16% and hrms =29±8% in 4U0142+61.
Optical pulsations from AXP 1E1048.1–5937 5
The only major difference when comparing this study STFC grant PP/E001777/1. TRM and CC are supported
of 1E1048.1–5937 with Dhillon et al.’s (2005) study of under STFC grant ST/F002599/1. VMK holds the Lorne
4U0142+61 is the ratio of theoptical to X-ray pulsed frac- Trottier Chair in Astrophysics and Cosmology, a Canada
tion: in 1E1048.1–5937 it is approximately unity, whereas Research Chair and acknowledges support from an NSERC
in 4U0142+61 the optical pulsations had an rms pulsed Discovery Grant, CIFAR and FQRNT. RPM acknowledges
fraction 5–7 times that of the X-rays. However, whereas STFC for support through its rolling grant programme.
the optical and X-ray pulsed fractions for 1E1048.1–5937 Based on observations collected at ESO,Chile.
were measured contemporaneously, those of 4U0142+61
were not. The X-ray pulsed fractions of 4U0142+61 re-
ported by Dhillon et al. (2005) were quoted from the work REFERENCES
of Patel et al. (2003), who obtained their Chandra data in
Alpar M. A., 2001, ApJ, 554, 1245
2000, over two years prior to the optical observations. We
Beloborodov A. M., Thompson C., 2007, ApJ,657, 967
nowknowfrom theworkofDib et al.(2007),however,that
Chatterjee P., Hernquist L., Narayan R., 2000, ApJ, 534,
4U0142+61 exhibitedan increase in its pulsedlight of 36%
373
in the 2–10 keV band between 2002 and 2004. Hence it is
Dhillon V. S., Marsh T. R., Hulleman F., van Kerkwijk
possible that at least part of the discrepancy between the
M. H., Shearer A., Littlefair S. P., Gavriil F. P., Kaspi
optical/X-ray pulsed fraction ratio in 1E1048.1–5937 and
V.M., 2005, MNRAS,363, 609
4U0142+61isduetovariabilityinthelattersource.Thishy-
Dhillon V. S., Marsh T. R., Stevenson M. J., Atkinson
pothesis is further supported bythefact that Dhillon et al.
(2005) measured a value of hpt = 58±16% in September D. C., Kerry P., Peacocke P. T., Vick A. J. A., Beard
2002whereasKern & Martin(2002)measuredhpt =27±8% S. M., Ives D. J., Lunney D. W., McLay S. A., Tierney
C. J., Kelly J., Littlefair S. P., Nicholson R., Pashley R.,
in November 2001, although there are other possible rea-
Harlaftis E. T., O’Brien K., 2007, MNRAS,378, 825
sons for this discrepancy (see Dhillon et al. 2005 for a dis-
Dib R., Kaspi V. M., Gavriil F. P., 2007, ApJ,666, 1152
cussion). It is also possible that the discrepancy between
Dib et al. R.,2008, in preparation
the optical/X-ray pulsed fraction ratio in 1E1048.1–5937
Durant M., vanKerkwijk M. H.,2005, ApJ, 627, 376
and 4U0142+61 is due to variability in 1E1048.1–5937:
Durant M., vanKerkwijk M. H.,2006, ApJ, 650, 1082
as discussed in section 3.2.1, the X-ray pulsed fraction of
ErtanU¨.,ErkutM.H.,Ek¸siK.Y.,AlparM.A.,2007,ApJ,
1E1048.1–5937 is inversely proportional to the total 2–10
657, 441
keV flux. The optical pulsed fraction might not vary in the
Gavriil F. P., Kaspi V.M., 2004, ApJ,609, L67
same way, implying that the optical/X-ray pulsed fraction
Hulleman F., van Kerkwijk M. H., Kulkarni S. R., 2000,
ratio in 1E1048.1–5937 could be variable and we just hap-
Nat,408, 689
pened to observe it when it was unity.
Hulleman F., van Kerkwijk M. H., Kulkarni S. R., 2004,
Viewed in isolation, the data on 1E1048.1–5937 pre-
A&A,416, 1037
sentedinthispaperdonotallowustodiscriminatebetween
Kaspi V. M., Gavriil F. P., Chakrabarty D., Lackey J. R.,
themagnetarandfallbackdiscmodels,astheopticalandX-
MunoM. P., 2001, ApJ, 558, 253
ray pulsed fractions are approximately equal, although the
Kern B., Martin C., 2002, Nat, 417, 527
tentative evidence we present for the optical pulses lead-
Mereghetti S., 2008, A&AR,15, 287
ing theX-raysis irreconcilable with reprocessing. Arguably
Naylor T., 1998, MNRAS,296, 339
the most important result of this paper, however, is that
Patel S. K., Kouveliotou C., Woods P. M., Tennant A.F.,
it confirms the existence of pulsed optical light in a sec-
Weisskopf M. C., Finger M. H., Wilson C. A., G¨o˘gu¨¸s E.,
ond AXP, 1E1048.1–5937, thereby demonstrating that the
vander Klis M., Belloni T., 2003, ApJ, 587, 367
properties of the optical emission in 4U0142+61 are not
Perna R., Hernquist L., Narayan R., 2000, ApJ,541, 344
unique. The observed similarities between the optical and
Press W. H.,RybickiG. B., 1989, ApJ, 338, 277
X-ray emission in both 1E1048.1–5937 and 4U0142+61 in-
Smith J. A., Tucker D. L., Kent S., Richmond M. W.,
dicate that closely related populations of particles, located
Fukugita M., Ichikawa T., Ichikawa S., Jorgensen A. M.,
in the same region of the magnetosphere, are probably re-
Uomoto A., Gunn J. E., Hamabe M., Watanabe M.,
sponsible for the emission. This paper has also highlighted
Tolea A., Henden A., AnnisJ., Pier J. R., McKay T. A.,
the way forward for time-resolved optical studies of these
BrinkmannJ.,ChenB.,HoltzmanJ.,ShimasakuK.,York
incredibly faint objects, as thedata presented in thispaper
D.G., 2002, AJ, 123, 2121
would have been unobtainable had 1E1048.1–5937 been in
Tam C. R., Gavriil F. P., Dib R., Kaspi V. M., Woods
afaintstate.BytargettingobservationsduringbrightX-ray
P.M., Bassa C., 2008, ApJ,677, 514
states, it should be possible to study otherAXPs, and pos-
vanParadijsJ.,TaamR.E.,vandenHeuvelE.P.J.,1995,
sibly also soft gamma repeaters, in the optical part of the
A&A,299, L41
spectrum,although thiswill stillrequireaccesstosensitive,
WangZ., Bassa C., KaspiV.M., Bryant J. J., Morrell N.,
high-speedcameraslikeULTRACAMontheworld’slargest
2008, ApJ, 679, 1443
telescopes.
Woods P. M., Thompson C., 2006, in Lewin W. H. G.,
van der Klis M., eds, Compact Stellar X-ray Sources.
CUP,Cambridge, p. 547
ACKNOWLEDGMENTS
ULTRACAM is supported by STFC grant PP/D002370/1.
SPLacknowledges thesupportofan RCUKFellowship and