astro-ph/9501098 27 Jan 95 B thanks to thea blue SAS starlike opticalacconi counterpart et V al BV and the U source was subsequently asso ciated with source close to the galactic plane Discovered with gon 2 1 by a p owerU law The mo del keV spEXOSAT ectrum of the source isAbstract b est tted Received date accepted date 4 3 D Ricci U EXOSAT U Introduction hole termediate b etween those ofcuto ultrasoft energy sources containing an black approximation of a thermal Comptonisation sp ectrum The Stars neutron Key words and s was found sition a centroid energythe of presence of a eV EW iron Kline emission feature with ing  of high stateto BHCs However theand source the ME sp ectrathe provide source evidence ux for was a high energy tail up highest black soidal photometry of U revealedoptical a counterpart of Sco X and U Optical CCD No evidence corresp onds only a lowamplitude Send oprint requests to Your thesaurus co des are will b e inserted byAA hand manuscript later no International School for AdvancedDipartimento Studies di SISSAISAS Fisica Universitadi Roma La Sapienza Ple Aldo Moro I Roma Italy donavxrmgicrait Aliated to ICRA Osservatorio Astronomico di Brera Via E Bianchi I Merate Italy stellaastmimmiastroit  During an  et keV which strengthens the similarity with the sp ectra candidates b o dy sp ectral comp onent During of mo dulation old al measured by 36 U 1 observations of the low mass Xray binary LMXRB ergs s 4 accreting the GL Israel We is to the orbital p erio d for a p erio dic Xrays general Xrays binaries Black holes  a + present BHCs and relatively keV as well as the Xray colours are in 1 This with  neutron observation higher than in the observation EXOSAT and observations variability with exp onential is a p erio d results LStella  2 lo cated has 4 p o orly mo dulation and L Stella those keV this value is among the star We p ositioning colours The ME light from on timescales of a few hours of hr the within of the system Thorstensen studied of soft LMXRBs the GSPC  similar nd no mag roughly sinu with p erio ds the observation cuto that provides arcsec luminosity analysis capabilities which 3 sp ectra 4 to curves showed evidence those of Uhuru most likely of of Jy was only revealed contain b etween the the Xray of the Mar the for a Gi two p o Via Beirut I Trieste Italy israelvxrmgicrai t the The photon b o dy emitting disk and a hard comp onent with a p ower law colour LMXRBs containing a neutronand star U are similar sp ectral and timing data Details of the The results are discussed in Section sum on containing p er that U might host a BHC However in a recent pa as ysis are summarised in Section Section and describ e our anal OSAT clude U conclusion sources colour energy horsky few tween HEAO ton ultrasoft mass These ndings clearly indicate that U is a low sp ectra with a resolutioncounter of array op erated overThe the Argon chambers of keV the band keV and band pro duced and provided broad band lter sp ectroscopy Cyg X a In this pap er we present a detailed analysis of the The Xray  disk Singh with k T of the derived of  Xray binary that d in was op erational diagram diagram imaging index 36 a EXOSAT hr most Europ ean are ' the ergs s satellites soft Terrel et data an accreting weakly magnetic neutron star Based is source the and lies r values LMC X al obtained keV resp ectively suggesting a similarity in channel of of Ginga comp onent cos source telescopes Schultz yr were 1 assembled  close which observations of U sp ectra for the estimated distance of LMXRB Space i No of was observation 1 Xray also multiplier detected based from May to April Its to the sp ectrum 2 to those of high rep ort signicant are In A et ' found and LE Agencys the medium energy obtained inner which the by km al BHCs on E E with light region that White HEAO up in and the array ASTROPHYSICS radius is they Yaqoob ASTRONOMY i  and longterm the in to an Xray curves the well Xray is binary EXOSAT with LE their o ccupied EXOSAT energies These the and Vela AND GX mo del luminosity CMA EXOSAT and represented A E ME prop ortional et the inclinati on observatory used resp ectively high Marshall B keV temp erature luminosity al results p erio dicity Xray with  Xray Vela of data by observations kp c covered in state  sp ectra A LMXRBs EXOSAT ultrasoft conjunc a 1 B keV suggest colour Pried colour by similar 2 angle black and with such con EX of a Our and low the the b e of of

Table EXOSAT observations and count rates

1

Time Instrument Filter or Energy Range Exp osure s Counts s

CMALE A Lexan 

August CMALE AluminiumParyl ene 

CMALE Boron 

CMALE Polypropylene 

ME keV 

GSPC keV 

CMALE A Lexan 

May CMALE AlumimiumParylene 

CMALE Boron 

ME keV 

GSPC keV 

light curves with high throughput The ME was often op er

Fig ME and LE sp ectrum from the observation the cut

ated with one half of the detector array oset from the source

opl mo del has b een used

in order to provide a simultaneous monitor of the particle back

ground the p ointing directions of the two halves were usually

swapped every hours in order to minimize the systematic

uncertainties in the background subtraction The gas scintil

lation proportional counter GSPC provided a factor of 

improved sp ectral resolution in the keV band with a fac

tor of  lower eective area than the ME

EXOSAT observed U on August for

ab out hr and on May for ab out hr The data

were obtained from the EXOSAT database available within

High Energy Astrophysics Database Service at the Astronomi

cal Observatory of Brera Tagliaferri Stella except for

the high time resolution light curves presented in Section

which were accumulated by using ESAs online interactive

analysis system Table gives a summary of the source count

rate and exp osure times in each instrument during the two ob

servations In all cases the instrumental background was stable

Energy Sp ectra

ME and LE data

The keV pulse height source sp ectra from the ME Argon

chambers were analysed together with the source count rates

derived from the LECMA and each of the lters used The

data from the ME Xenon chambers were excluded due to sys

tematic uncertainties in the background subtraction Several

trial sp ectral mo dels were convolved through the instrumental

resp onse and tted to the data Neither a single p ower law a

thermal bremsstrahlung nor a black b o dy mo del pro duced an

We note that the integrated sp ectrum of an accretion disk

acceptable representation of the sp ectrum of U

lo cally emitting like a black b o dy provided also a go o d t to

A p owerlaw mo del with an exp onential highenergy cut

the data However the standard viscosity disk mo del

o cutopl provided instead a go o d t to the data

disk in table and cf Shakura Sunyaev requires

for a p owerlaw photon index of  and a cuto energy

S

39 1

a disk luminosity of  ergs s or  times the Ed

of E  keV Table and Fig The column density

cut

20 2

dington limit for the estimated mass  M of the accreting

was constrained to a value of   H cm mainly

black hole Reconciling this luminosity with the observed L

X

through the LE rates measurements



requires an unrealistic inclination of ab out and the pres

ence of Xray eclipses which are excluded eg by the The thermal Comptonisation mo del of Sunyaev

EXOSAT light curves A similarly go o d t was obtained by Titarchuck CompST of which the p ower law with

using the simple blackbo dy disk mo del adopted by Yaqoob et exp onential cuto mo del provides an approximation gave in

al diskbb in Table and see also Mitsuda stead a substantially worse t In this case an electron tem

in which the disk inner radius and temp erature are allowed p erature of k T  keV and a Thomson optical depth of

e

12

to freely vary The b est t values r cos i ' km and  were estimated For b oth mo dels the keV lumi

in es

36 1

k T ' keV are similar to those obtained with the Ginga nosity of the source was derived to b e L '  ergs s

in X

Table Sp ectral ts of the data Errors in the sp ectral parameters are at condence level

2

Date Mo del dof Best t parameters

cutopl  E  k eV

S cut

Aug compST k T  k eV 

es

a 3

disk M M M M norm 

E dd

b

diskbb T  k eV norm

in

a 2

normcos id where i is the inclination of the disk and d is the distance in units of kp c

10

10

b 2

normR d
cos i where R in Km is the inner disk radius

in 10 in

Fig ME and LE sp ectrum from the observation In panel A the single comp onent cutopl mo del has b een used In panel B a

second p owerlaw comp onent with a slop e of has b een added in order to t the high energy excess



sp ectra In turn a very high inclinati on i would b e range b etween and Therefore we xed its value to

required if the derived inner disk radius were to b e compatible compatible with the measurement by Ginga Yaqoob et al

2

with the neutron star radius or the marginally stable orbit of A dof of was obtained in this case The sta

a few solar masses black hole tistical signicance of the additional p ower law comp onent was

2

evaluated through an Ftest to b e  Fig B Therefore

The source count rate increased by  during the

we conclude that the EXOSAT data conrm the Ginga

observation The values obtained from the sp ectral tting of

result of an additional sp ectral comp onent ab ove  keV

the data are similar to those discussed ab ove Table

The keV source luminosity during the observation

except for a rather noticeable high energyexcess in the ME

36 1

was L '  ergs s with the high energy comp onent

X

rates ab ove keV This excess was not adequately mo delled

contributing up to  of the total

by any of the single comp onent mo dels adopted for the

observation The cuto p ower law provided the b est single com

2

p onent mo del with a of for degrees of freedom dof

see Fig A To mo del the highenergy excess an additional

GSPC observations

sp ectral comp onent consisting of a p ower law was added Ow

ing to p o or statistics the photon index of this high energy The keV GSPC sp ectra from b oth observations were t

p ower law could not b e obtained with an acceptable accuracy ted with the same continuum mo del that provided the b est t

Table Sp ectral ts of the data

2

date mo del dof Best t parameters

cutopl  E  k eV

S cut

May compST k T  k eV 

3

disk M M M M norm 

edd

diskbb T  k eV norm

in

Fig GSPC sp ectrum from the observation Panel A shows the t using the cutopl mo del plus a Gaussian line Panel B shows

the residuals with resp ect to the continuum

to the ME and LE data note that no evidence for a high energy the FWHM We note that by adopting the alternative Xray

excess is found in the ME data b elow keV The photo elec continuum mo dels discussed in Sect the derived line pa

tric absorption was kept constant at the b est t value obtained rameters remain essentially unchanged

2

from the ME and LE data The b est t mo del gave a dof

of and for the and observations

resp ectively Fig

Time variability

Adding a Gaussian line prole with a centroid energy in

Light curves

the keV range did not improve signicantly the t of the

GSPC sp ectrum The condence upp er limit to the

equivalent width of such a feature was  eV On the con During the observations of and the ME provided

2

trary adding a Gaussian line prole decreased the dof of resp ectively and channel pulse height analyser data from

the GSPC sp ectra to corresp onding to an F b oth the source and background detectors Using these data

test probabili ty of  see also Gottwald White lightcurves with and s time resolution in the

The centroid energy and equivalent width of the line were de energy ranges keV keV and keV were extracted

termined to b e  keV and  eV resp ectively from the EXOSAT database The lightcurves display a small

The width of the line could not b e resolved by the GSPC and amplitude ap erio dic variability on long timescales which is

an upp er limit of keV condence was obtained for more pronounced in the observation see Fig

Fig keV ME lightcurves from b oth observations Each bin corresp onds to an integration time of s

Power Spectra condence upp er limits to the mo dulation semiamplitude are

given in Table for selected p erio ds

The lightcurves describ ed ab ove were used to carry out a de

tailed search for coherent pulsations Since the observa

tion is longer and has a higher average count rate than the

Table upp er limits to the semiamplitude of pulsations

it allows for a more sensitive search In order to max

for selected p erio ds

imize the Fourier resolution and therefore the sensitivity a

single p ower sp ectrum was calculated for the

Period Upp er limits

and s resolved lightcurves of each observation and energy

s August May

range Missing data p oints were replaced with the average

count rate In Fig the p ower sp ectra are marked with a

and normalised such that the white noise arising from counting

statistics corresp onds to a p ower of

A red noise comp onent is dominant in the frequency range

5 4

from   Hz to   Hz This comp onent results

a

from the longtimescale variability visible in the lightcurves

a

The presence of this continuum p ower sp ectrum comp onent



a

aects the statistical distribution of the p ower estimates for

a

long p erio ds This means that techniques which assign proba

2

bilities to p ower sp ectrum p eaks assuming the simple dis

a

From the high time resolution ME data keV

tribution of p ower exp ected from counting statistics noise see

eg Leahy et al may signicantly overestimate their sig

nicance Israel Stella developed a technique for re

For short p erio ds up to  s the sensitivity of the

liably searching for p ower sp ectrum p eaks corresp onding to a

search is mainly limited by counting statistics noise and up

p erio dic mo dulation in the presence of coloured continuum

p er limits of  are obtained for any sinusoidal mo dulation

p ower sp ectrum comp onents A search for coherent pulsations

For longer p erio ds the source red noise comp onent plays an

is carried out by lo oking for signicant p eaks ab ove the es

increasingly imp ortant role in reducing the sensitivity of the

timated continuum sp ectrum If no p eaks with a probabili ty

search The corresp onding upp er limits have values of 

of chance o ccurrence lower than a given threshold are found

for p erio ds b etween and s The EXOSAT ob

then an upp er limit to the semiamplitude of a sinusoidal mo d

servations were not long enough to search for a low amplitude

ulation is worked out for each Fourier frequency of the p ower

Xray mo dulation at the hr orbital p erio d

sp ectrum through a generalisation of the metho d outlined by

We analysed also the p ower sp ectra of the and s

Leahy et al By using this technique each of the p ower

resolved and keV ME light curves from the 

sp ectra of U was searched for p eaks with a probabil

hr August observation All the p oints in the p ower

ity of  condence level of exceeding by chance the

sp ectra are well b elow the detection threshold and thus no sig

level of the continuum p ower sp ectrum comp onents

nicant p erio diciti es are found Upp er limits for selected fre

quencies are given in Table and shown in Fig for the For the hr May observation p ower sp ec

keV energy range tra were calculated from the and s time resolved

ME count rates from the entire energy range of the Ar lightcurves in the and keV bands No signif

chambers keV were also available with a resolution of icant p eaks were found in any of the p ower sp ectra The

Fig Power sp ectra a of the ME lightcurve obtained from the keV data panel A and from the high resolution data in the

entire range of the ME Ar chambers panel B during the May observation The : condence threshold for the detection

of a sinusoidal signal is shown b The b ottom curves c give the corresp onding upp er limits to the fractional semiamplitude of the

mo dulation

 ms for the May observation We divided the the characteristic of its Comptonised sp ectral comp onent is

lightcurve in intervals in order to prevent the Doppler fre therefore premature

quency variations asso ciated to the orbital motion from smear

On the other hand the EXOSAT sp ectra and esp e

ing the p ower of a sinusoidal signal over more than one Fourier

cially the Ginga sp ectra show the presence of a high energy

frequency These intervals were used to calculate p ower

p owerlaw comp onent ab ove energies of keV This kind of

sp ectra of indip ende nt Fourier frequencies

twocomponent sp ectra has b een observed from several BHCs

The p ower sp ectrum resulting from the average of these

in their high state the luminosity of which however is a factor

p ower sp ectra was then analysed with the same technique dis

of higher than that of U

cussed ab ove No signicant p erio dicities were detected The

Our decomp osition of the EXOSAT sp ectrum of

upp er limits are rep orted in Table and plotted in Fig

U is similar to that of Singh et al who in

their analysis used also the ME Xenon chambers data and ex

Discussion

cluded the LECMA rates However according to these au

The main comp onent of the EXOSAT sp ectra is consistent with thors the EXOSAT sp ectrum of U consists of

a p ower law mo del with exp onential cuto characterised by a the sum of a  keV temp erature black b o dy accounting for

sp ectral index of  and a cuto energy of  keV for b oth ab out of the total luminosity plus a Comptonised sp ec

the and observations This sp ectral form provides trum extending up to  keV In this case U would

a simple analytical approximation to the thermal Comptoni p osses an Xray sp ectrum similar to that of the high luminos

37 1

sation sp ectrum of Sunyaev Titarchuk and b esides ity LMXRBs ergs s containing an old neutron star

semplicity it involves no conceptual dierence Indeed thermal the socalled ZsourcesWhite et al Hasinger van

Comptonisation mo dels and their approximations are found der Klis Such a sp ectral decomp osition of the EX

to provide very go o d ts to the main sp ectral comp onent of OSAT data app ears however unlikely for the following reasons

a number of high state BHCs and LMXRBs containing old a despite the mo derate  increase of source luminosity

weakly magnetic neutron stars White et al the parameters of the Comptonised sp ectral comp onent would

In high state BHCs this sp ectral comp onent is somewhat change drastically from the to the observation a

softer then that of U E  keV in LMC X factor of and  variation in electron temp erature and

cut

and LMC X Ricci In LMXRBs containing an old neu Thomson depth resp ectively when it would b e virtually in

tron star the comp onent is instead harder eg E  keV distinguish abl e from a p ower law b neutron star LMXRBs

cut

in U and E  keV in U Ricci of luminosity comparable to that of U do not show

cut

A classicati on of the compact ob ject in U based on evidence for an additional black b o dy comp onent with upp er

Fig Same as Figure but for the keV ME lightcurve of the August observation

limits usually in range sp ectra and light curves were obtained through the High En

ergy Astrophysics Database Service at the Brera Astronomical

Black b o dy emission disk mo dels can b e t to the data in

Observatory and the online interactive analysis system at the

place of the cuto p ower law mo del or the Comptonisation



Space Science Department of ESA ESTEC This work was

mo del but they require an unrealistic inclinatio n of i

partially supp orted by ASI grants

that cannot b e reconciled with the absence of Xray eclipses

This is also unlike other high state BHCs and a number of high

luminosity LMXRBs White et al

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Acknowledgements We are grateful to Dr S Ilovaisky for help in press

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Note added in proof In the recently distributed preprint EX

OSAT GSPC iron line catalog by Gottwald et al AA

in press no iron line detection is rep orted for the GSPC

sp ectra of U despite the fact that the continuum

mo del is the same as that used in our pap er This is surpising

since we have veried that the detection condition of Gottwald

2

et al a improvement of  is met also for the energy

range used in their pap er We can sp eculate that p erhaps their

2

automatic tting pro cedure failed to reach the minimum in

the case of U

a

This article was pro cessed by the author using SpringerVerlag L T X

E AA style le