Detection of an X-ray periodicity in the Seyfert galaxy IRAS18325-5926

5 Pages·0.43 MB·English

by  K. Iwasawa

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed1February2008 (MNLATEXstylefilev1.4)
Detection of an X-ray periodicity in the Seyfert galaxy
IRAS 18325–5926
K. Iwasawa1, A.C. Fabian1, W.N. Brandt2, H. Kunieda3, K. Misaki3,
C.S. Reynolds4 and Y. Terashima3
1 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA
2 Department of Astronomy and Astrophysics, The Pennsylvania State University, UniversityPark PA 16802, USA
8 3 Department of Astrophysics, Nagoya University,Chikusa-ku, Nagoya 464-01, Japan
9 4 JILA, Universityof Colorado, Campus Box 440, Boulder CO 80309-0440, USA
9
1

n
a
J ABSTRACT
3 Wereportthedetectionofa5.8×104speriodicityinthe0.5–10keVX-raylightcurveof
2 theSeyfertgalaxyIRAS18325–5926,obtainedfroma5-dayASCAobservation.Nearly
9 cycles of the periodic variation are seen; it shows no strong energy dependence and

1 hasanamplitudeofabout15percent.Unlikemostotherwell-studiedSeyfertgalaxies,
v there is no evidence for strong power-law red noise in the X-ray power spectrum of
6
IRAS18325–5926. Scaling from the QPOs found in Galactic black hole candidates
2 suggests that the mass of the black hole in IRAS18325–5926is ∼6×106–4×107M .
2 ⊙
1 Key words: galaxies: individual: IRAS 18325–5926 – galaxies: Seyfert – X-rays:
0
galaxies
8
9
/
h
p
- 1 INTRODUCTION dates; a67 HzQPO in GRS1915+105 (Morgan, Remillard
o &Greiner1997) andQPOatlowerfrequencies,1–10Hz,in
r ASCA X-ray spectra of many Seyfert galaxies show that GX339-4andGS1124–68 (Dotani1992;Bellonietal1997).
t
s the iron emission line is broad and skewed to low ener-
a gies (Tanakaet al1995; Nandraet al1997), indicating that Here we report the discovery of a periodic signal from
v: the X-rays originate from above the surface of an accretion IRAS18325–5926,whichisaSeyfertgalaxywithabroadiron
i disk close to a massive black hole (Fabian et al 1995). The line(Iwasawa etal1995, 1996a). TheX-raysourceisoneof
X
line profiles demonstrate that much of the X-ray emission thecompleteX-raysampleselectedbytheHEAO-1A2sur-
r orginates from about 10–20 gravitational radii, but do not vey (Piccinotti et al 1982) and it was identified with the
a
constrain the mass of the black hole. For that the emission IRASgalaxy(=Fairall 49) byWardet al(1988). Although
radius is required in physical units. itsnarrow-emission-line-dominated opticalspectrumissim-
ilar to that of Seyfert 2 galaxies (DeGrijp et al 1985), the
X-ray variability is an important characteristic of such
presenceofaweakbroadwingtotheBalmerlines(Carteret
activegalacticnuclei(AGN)andmayholdthekeytoobtain-
al1984;Iwasawaetal1995)suggestsadust-obscuredbroad-
ingthatsize.ShorttimescaleX-rayvariationsareprobably
lineregioninthisgalaxy.TheX-rayspectrumismoderately
due to individual flares above the accretion disk and so do
absorbed by a column density of ∼1×1022cm−2, which is
notdeterminetheorbitalradiusunlesstheflaremechanism
consistentwiththepictureofanobscuredSeyfert1nucleus.
is understood much more than is currently the case. A pe-
Apartfromtheslightlysteepcontinuumslope(Γ∼2.1),the
riodicsignalwouldsufficeiftheorigin oftheperiod,orbital
absorption-free hard X-ray emission (> 2 keV) has similar
or otherwise, was known.
properties to those of Seyfert 1 galaxies. The X-ray source
Previous studies of X-ray variability of AGNs have
has been found to be highly variable, as observed in other
found that power spectra are featureless and power-law in
Seyfert 1 galaxies such as MCG–6-30-15.
shape (e.g., Lawrence et al 1987; McHardy 1989). No char-
acteristic periodicity has been found except for possible A five-daylong ASCAobservation with a netexposure
quasi-periodic oscillations (QPOs) in NGC5548 (∼ 500s, timeof∼200kshasbeencarriedoutwiththeaimofinves-
Papadakis & Lawrence 1993; Tagliaferri et al 1996 argue tigating X-ray variability in the source, including the iron
against the presence of QPO in those data), NGC4051 line and underlying continuum. The general spectral prop-
(∼1hr,Papadakis&Lawrence1995)andRXJ0437.4–4711 erties of the X-ray source are similar to those seen before,
(0.906 ± 0.018 dy, Halpern & Marshall 1996). Very clear and will be discussed in another paper. We report in this
QPO have been seen in several Galactic Black Hole Candi- Letteron a timing analysis of theASCA data.
(cid:13)c 0000RAS
2 K. Iwasawa et al
Figure 1. The ASCA S0 light curve in the 0.5–10 keV band. Each data bin is of exposure time 1000 s for clarity while the timing
analysis used 360 s bins. The epoch of the start is 1997 March 27, 13:21:31 (UT). The nine stars plotted at the top are spaced at the
detected frequency.
2 THE ASCA DATA
Table 1. Mean count rates observed with each of the four de-
IRAS18325–5926 was observed with ASCA for 5 days be- tectors duringtheobservation. Nocorrections forvignetting has
tween 1997 March 27 and 1997 March 31. The X-ray tele- beenmade.
scopes pointed at the source at the 1CCD mode nominal
Detector Countrate
position. Averaged count rates detected with the four de- cts−1
tectors over the total observing run are shown in Table 1.
S0 0.372
TheSolidstateImagingSpectrometers(SIS;S0andS1)are
S1 0.307
operatedwiththeFaintmodeusingonestandardCCDchip
G2 0.280
in each detector. The standard settings of PH mode were
G3 0.332
usedfortheGasImagingSpectrometers(GIS;G2andG3).
Thedata reductionwas performed using FTOOLS(version
4.0) and standard calibration. After data selection, the net observation with many flares on short timescales (Fig. 1).
exposuretimesofusefuldataare204ksfortheSISand180 The light curve appears to have a characteristic time scale
ksfor theGIS. of ∼ 6×104 s as well as random shorter timescales. We
The sourceeventswere collected from acircular region have searched for a possible periodicity of the X-ray mod-
centred on the source with a radius of 4 arcmin for theSIS ulation and examined its significance using a periodogram
and6arcminfor theGIS.Backgroundineach detectorwas technique.
taken from a source-free region on the same detector. The
backgrounds were basically stable with an intensity about
7–8 per cent of the mean counts collected from the source
3 TIMING ANALYSIS
regions.
The mean X-ray flux of the source is 1.7 × TheLombalgorithm(Lomb1976)isappropriateforsearch-
10−11erg cm−2 s−1 in the 2–10 keV band during the ob- ing for a periodic modulation from unevenly sampled data
servation, a factor of ∼2 brighter than the previous ASCA suchasinherentinobservationsfromloworbitsatelliteslike
observation in 1993 but a factor of ∼ 1.5 fainter than the ASCA (Tanaka, Inoue & Holt 1994). In Fig. 2, the power-
Ginga observation in 1989. The absorption-corrected 2–10 spectrum computed using the Lomb periodogram (Press et
keV luminosity is L2−10keV ∼ 3×1043erg s−1 (H0 = 50 al 1992) is shown.
km s−1 Mpc−1). Alargepeakwithaperiodjustlongerthanthedataset
The minimum time resolution of the data is 4 s. X-ray and due to the trend seen across the light curve, lies at
light curves from the four detectors have been binned into the lowest frequency shown in Fig. 2. There are two other
360 s for each data point for the timing analysis presented significant peaks.Thehigherfrequencypeakat1.72×10−4
below. The count rate decreased gradually over the whole Hzor 96.9 min is likely to originate in theorbital period of
(cid:13)c 0000RAS,MNRAS000,000–000
X-ray periodicity in IRAS18325–5926 3
Table2.ResultsoftheLombperiodgramanalysis.Probabilities
that the signals could occur randomly from the four detectors
are shown for the peak at 1.725×10−5 Hz. G2 (which had the
lowestcountratefromthesource)hasamoresignificantpeakat
afrequency1.27×10−3 Hz,notseenintheotherdetectors.
Detector Probability
S0 1×10−10
S1 9×10−9
G2 6×10−4
G3 3×10−6
Figure3.TheASCAS0lightcurvefoldedatthedetectedperiod.
Twocycles areshownforclarity. Theoriginaldata (dotted line)
are averaged into 10 bins per cycle (thick solidline). Error bars
tocountratesarecalculatedfromthesquarerootofthenumber
of counts integrated over each bin while the standard deviation
ineachbinistypically∼0.12cts−1.
ilarmean count-rateandamplitudetotherealobservation.
Thusthe error of the period is estimated to be ∼0.4 ks.
Figure 2.ThepowerspectrumderivedfromtheASCAS0light The folded light curve at the detected period is shown
curveofIRAS18325–5926 inthe0.5–10keVband.Apeakatthe inFig.3.Themodulationisarchorsinusoidalinshapewith
rightcorresponds tothe ASCAorbitalperiod.Thehighestpeak an amplitude of ∼15 per cent. There is no strong evidence
at1.725×10−5 Hzisintrinsictothesource.Largepoweratthe for X-ray colour variation associated with the periodicity.
low frequency below 3×10−6 Hz is due to the decreasing trend
We note that the power spectra of the variability of
acrossthe wholeobservation. Notethat afrequencycorrespond-
Seyfertgalaxiesistypicallypower-lawinshapeoverthefre-
ingtothewholeobservationlengthis∼2×10−6 Hz.
quencyintervalsshown inFig. 2(McHardy1989). Thepos-
sibility ofsuchrednoisecomplicates thetrueassignment of
significancetothepeakinFig.2.TheprobabilitiesinTable2
ASCA(Tanakaetal1994).Another,moresignificant,oneis arestrictlyforasinusoidalsignalinwhitenoise.Apartfrom
foundat1.725×10−5 Hz,correspondingtoaperiodof58.0 a large peak at the lowest frequencies due to the trend in
ks. This peak is found strongly in all detectors (Table 2) thelightcurve,wedohoweverseenoevidencefor rednoise.
exceptforG2inwhichtheobservedcountrateisthelowest Overthefrequencyrange3×10−6 Hztoabove10−4 Hz,the
amongthefourdetectorsthusthedataaremost noisy.The peaks in the power spectrum are similar to those expected
probabilities of the peak being random are shown in Table fromwhitenoise,apartfromthespikeat1.73×10−5 Hz.Ifit
2.Ifthereissignificantclumpinginthedatasampling,then isarguedthatthespikeisduetorednoise,thenwemustask
thederivationoftheprobabilitycouldbeaffected(Horne& whytherearenosimilar large peaksat 10−5 Hzandbelow.
Baliunas 1986). We have verified that this is not the case We find that the power spectrum (and lightcurve) are not
using simulated light curves, one in which random count- at all similar in shape to simulations made with red noise.
rates (with the same mean and variance as the data) are (The light curve of MCG–6-30-15 from the previous ASCA
assigned to the observed times and the other in which the longlook (∼4dy,Yaqoobet al1997) doesshowred-noise.)
count rates were shuffled. Wemade1000 simulated lightcurveswith theobserved
Theperiodicityisconfirmedwithanepoch-foldingtech- time sequence and (phase) randomized f−1 noise, normal-
nique. The 90 per cent statistical error to the 58.0 ks peak izedtogivethesamevarianceastherealdataset.Themin-
fortheobserveddataintheχ2–(trialperiod)diagram,when imum power (in the same units as Fig. 2) in the frequency
itisfittedwithagaussian,is∼0.3ks.Thesystematicerror range from 3×10−6 Hz to 1.5×10−5 Hz was 42, decreas-
in deriving a coherent periodicity is found to be ∼ 0.2 ks, ing to 25 when the lower frequency was 4×10−6 Hz. If the
applyingthesametechniquetosimulatedsinewaveswitha power varies as f−1.5, which is more typical of AGN, then
period of 58.0 ks,holding thesame time locations and sim- these values rise to 80 and 60 respectively. All of these far
(cid:13)c 0000RAS,MNRAS000,000–000
4 K. Iwasawa et al
IIRRAASS1188332255 from the Piccinotti et al (1982) X-ray sample. This implies
about 100 Galactic objects in the Sky above |b| = 20◦ at
the flux level of IRAS18325–5926 and thus probabilities of
0000″″ about3×10−6 and10−7 ofafurtherobjectinthe1arcmin
and 10 arcsec radius error boxes obtained from the ASCA
andROSATimages,respectively.Wenotethatbothimages
2233′′ 3300″″ are consistent with a single point source. As mentioned in
theIntroduction,theobservedX-rayabsorptionisfullycon-
sistent with the optical classification of the Seyfert galaxy,
Declination (2000)Declination (2000) 2244′′ 00330000″″″″ bmwtouhortesstruwenuatlnshikleidekluXeyle-yrftoatoryhaaavtacGrwaiataeablacaicllriytetsiycmredsipceoetuvearcatctirenei.adgbWbltehyee4nc0cooannasc-reilcmumodafiegnNitnfGhrgoaCmdt6e8ittt1eh4cies-,
galaxy(Madejskietal1993).TheIRASgalaxyisbyfarthe
most probable X-raysource.
0000″″
4 DISCUSSION
--5599°° 2255′′ 3300″″
The periodicity found in the light curve from the long ob-
0055ss 3377mm 0000ss 5555ss 1188hh 3366mm 5500ss servation is about 58 ks which is probably intrinsic to the
RRiigghhtt AAsscceennssiioonn ((22000000))
source whilst the 97 min peak is due to the ASCA orbital
period.AsimilarX-rayfluxvariationisapparentinthepre-
Figure 4.TheROSATPSPC0.5–2keVcontour imageoverlaid viousASCAPV(Iwasawaetal1996a) andGinga(Iwasawa
onthedigitizedUKSchmidtsurveyplateoftheIRAS18325–5926 et al1995; Smith& Done1996) observations. Theduration
region. ofeachofthoseobservationswasaboutonedayandsoonly
slightly longer than the period (∼16 hr). The data sets are
too widely spaced to phase together.
exceed the power seen in that frequency range in the real The(Newtonian) orbitalperiod at5RS = 10rg is3.4×
dataset. If the f−1 power is scaled down to match the ob- 104M8R1 s where the mass is in 108M8M⊙ and the radius
servations then the maximum power seen in the simulation is10R1rg.Soitcouldeasilybeoccurringat10rg fora1.7×
beyond1.5×10−5 islessthan10.Weconcludethattheob- 108M⊙ blackholeorat20rg fora2.1×107M⊙ one.Inother
servations are inconsistent with any simple power-law red words, a 16 hr period is reasonable for orbital variations at
noise model. 10–20 gravitational radii in a disk around a black hole of
Thepeakat1.725×10−5 Hzisnearly10timesthefre- mass2×107−108M⊙.TheEddingtonlimitforsuchmasses
quency of the peak due to the ASCA orbital period. We is above 1045erg s−1, well above that observed.
consider this to be a chance coincidence. To the best of What we do not know from the 9 cycles that we have
our knowledge, there is no corresponding frequency origi- seen is whether the variation is strictly- or quasi-periodic.
nating in the spacecraft. This has been checked using the Further observations are necessary to test whether the ob-
light curves of the calibration source of the two GIS detec- servedperiodicity islong-term orpartof aQPO.Thequal-
tors during the present observation and the light curve of ity factor of the current variation is at least 30 and could
two-day long ASCA observation of a stable X-ray source, be much higher. A strict periodicity will be difficult to ex-
the Centaurus cluster, kindly provided by S. Allen. An ex- plain. The most obvious clock would be an object in orbit
amination of the X-ray image rules out a possibility that about the accreting black hole. The lifetime against spiral-
wobbles of the spacecraft causes the observed X-ray modu- induetogravitationalradiation losseswouldbeonlyweeks
lation. for another black hole of similar mass to the primary (say
Note that the X-ray source is the brightest both in 107M⊙),increasingastheinversemassformuchsmallerob-
the ASCA and ROSAT PSPC images of the region and is jects. A star orbiting within an accretion disk does isolate
located at the optical position of the galaxy (18h36m58s, a particular radius (see discussion by Syer, Clarke & Rees
−59◦24′09′′, J2000). Simultaneous ROSAT and ASCA ob- 1991) andcould last 100,000 yrbutit isnot obvioushowit
servations of the source in 1993 (Iwasawa et al 1996) show could modulate theX-ray emission.
a correlated decrease in flux by a factor of about two, con- Atransientperiodicitycouldbeduetoaverylong-lived
firming that the ROSAT and ASCA sources are the same. flare, or flare patch, on the disk. The appearance of small
WeshowinFig.4,theROSATPSPCimageoverlaidonthe variation originating at a few Schwartzschild radii may be
digitizedUKSTUopticalimage.TheoffsetoftheX-rayand boostedrelativistically byorbitalmotion (seee.g., Bolleret
optical position is ∼ 10 arcsec, which is within the PSPC al 1997).
positional uncertainty. A 67 Hz QPO has recently been discovered with
To eliminate further any residual doubt that the vari- the Rossi X-ray Timing Expolorer (RXTE) in the Galac-
abilityreportedhereisfromIRAS18325–5926, sincethepe- tic black hole candidate GRS 1915+105 (Morgan, Remil-
riod is in the range of that of low-mass X-ray binaries and lard & Greiner 1997). If this and the periodicity seen in
somecataclysmicvariables,wehaveestimatedtheprobabil- IRAS18325–5926 are related to the orbital period at the
ity of a chance alignment with a Galactic object by scaling sameradius(ingravitationalunits)ineachsystem,thenthe
(cid:13)c 0000RAS,MNRAS000,000–000
X-ray periodicity in IRAS18325–5926 5
masses of their black holes should scale as 67/1.73×10−5 FabianA.C.,NandraK.,ReynoldsC.S.,BrandtW.N.,OtaniC.,
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respectively.
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ACKNOWLEDGEMENTS
We thank all the member of the ASCA team. T. Dotani
is thankedfor helpfuldiscussion. The ROSATPSPC image
was retrieved from the ROSAT archive maintained at the
GoddardSpaceFlight Center.TheopticalUKSchmidtim-
age was taken from the Digitized Sky Survey produced by
SpaceTelescopeScienceInstituteandtheoriginaldatawere
takenbyRoyalObservatoryEdinburgh.ACFandKIthank
Royal Society and PPARC, respectively, for support. CSR
thanks the National Science Foundation for support under
grant AST-9529175.
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