INTEGRAL survey of the Cassiopeia region in hard X rays

0.44 MB·English

by  P. R. den Hartog

Astronomy&Astrophysicsmanuscriptno.4711 February5,2008
(DOI:willbeinsertedbyhandlater)
INTEGRAL survey of the Cassiopeia region in hard X rays
P.R.denHartog1,W.Hermsen1,2,L.Kuiper1,J.Vink3,1,J.J.M.in’tZand1,3,andW.Collmar4
1 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,TheNetherlands
2 SterrenkundigInstituutAntonPannekoek,UniversityofAmsterdam,Kruislaan403,1098SJAmsterdam,TheNetherlands
3 SterrekundigInstituut,UniversiteitUtrecht,P.O.Box80000,3508TAUtrecht,TheNetherlands
6
4 Max-Planck-Institutfu¨rextraterrestrischePhysik,POBox1603,85740Garching,Germany
0
0
2 Received2005December16;accepted2006January19

n
Abstract. We report on the results of a deep 1.6 Ms INTEGRAL observation of the Cassiopeia region performed from
a
J December2003toFebruary2004.ElevensourcesweredetectedwiththeimagerIBIS-ISGRIatenergiesabove20keV,includ-
ingthreenewhardX-raysources.MostremarkableisthediscoveryofhardX-rayemissionfromtheanomalousX-raypulsar
7
2 4U0142+61,whichshowsemissionupto 150keVwithaveryhardpower-lawspectrumwithphotonindexΓ=0.73 0.17.
∼ ±
Wederived flux upper limitsfor energies between 0.75 MeV and 30 MeV using archival data fromthe Compton telescope

1 COMPTEL. In order to reconcile the very hard spectrum of 4U 0142+61 measured by INTEGRAL with the COMPTEL
v upper limits, the spectrum has to bend or break between 75 keV and 750 keV. 1E 2259+586, another anomalous X-ray
∼ ∼
4 pulsarinthisregion,wasnotdetected.INTEGRALandCOMPTELupperlimitsareprovided.ThenewINTEGRALsources
4 areIGRJ00370+6122andIGRJ00234+6144. IGRJ00370+6122isanewsupergiantX-raybinarywithanorbitalperiodof
6
15.665 0.006days, derivedfromRXTEAll-SkyMonitor data.Archival BeppoSAXWide-FieldCameradatayieldedfour
1 ±
moredetections.IGRJ00234+6144stillrequiresaproperidentification.OthersourcesforwhichINTEGRALresultsarepre-
0
sentedarehigh-massX-raybinaries2S0114+650,γCas,RXJ0146.9+6121and4U2206+54,intermediatepolarV709Cas
6
and 1ES 0033+595, an AGN of the BL-Lactype. For each of these sources the hard X-ray spectra are fittedwithdifferent
0
modelsandcomparedwithearlierpublishedresults.
/
h
p Keywords. –Stars:neutron–Stars:supergiants–Stars:whitedwarfs–X-rays:binaries–X-rays:individuals:4U0142+61,
-
o 1E 2259+586, IGR J00370+6122, IGR J00234+6144, 2S 0114+650, γ Cas, RX J0146.9+6121, 4U 2206+54, V709 Cas,
r 1ES0033+595–Gammarays:observations
t
s
a
:
v 1. Introduction The most sensitive part of the FOV of a coded-mask instru-
i mentisthefully-codedFOV,whichis9 9 forIBIS-ISGRI
X The INTErnational Gamma-Ray Astrophysics Laboratory ◦× ◦
(see in’tZand 1992, for detailed information on coded-mask
r INTEGRAL (Winkleretal. 2003) is ESA’s currently opera-
a instruments).
tionalspace-basedhardX-ray/softgamma-raytelescope.Since
itslaunchinOctober2002ithasbeenobservingtheskyatpho-
ton energiesbetween3 keV and 2 MeV. Aboard INTEGRAL The main targets of our INTEGRAL observations are the
there are two main hard X-ray/soft gamma-ray instruments, youngGalactic supernovaremnantsCassiopeia A and Tycho.
the imager IBIS (Ubertinietal. 2003) and the spectrometer In this paper these sources will not be addressed in detail;
SPI (Vedrenneetal. 2003), supplementedby two X-ray mon- we refer to Vink (2005) and Renaudetal. (2004) for more
itors,JEM-X(Lundetal.2003)andanopticalmonitor,OMC detailed studies. This paper presents results on compact hard
(Mas-Hesseetal. 2003). These instruments are all wide-field X-ray sources detected in the deep 1.6-Ms observationof the
instruments with observational coverage over a broad spec- Cassiopeia region. In this direction of the Galactic plane the
tralband.InthisworkonlyINTEGRALresultsfromthelow- line of sight crosses the Perseus arm at a distance of 3 kpc.
∼
energydetectorofIBIS,calledISGRI(Lebrunetal.2003),will One may expect that the majority of the sources are likely
be presented. IBIS-ISGRI is a coded-mask instrument with to be associated with this spiral arm. The detected sources
arcminute-resolution (12′ FWHM; Grosetal. 2003) imaging are: (Anomalous X-ray Pulsar; AXP) 4U 0142+61, (high-
capabilities in the energy range from 20 keV to 300 keV. massX-raybinaries)2S0114+650,γCas,RXJ0146.9+6121,
∼ ∼
Itsfieldofview(FOV)is29◦ 29◦ (fullwidthzeroresponse). 4U2206+54,thenewlydiscoveredIGRJ00370+6122,(inter-
×
mediate polar) V709 Cas, (BL-Lac) 1ES 0033+595, and the
Sendoffprintrequeststo:P.R.denHartog currently unidentified newly discovered INTEGRAL source
Correspondenceto:[email protected] IGRJ00234+6141.INTEGRAL-fluxupperlimitsareprovided
2 P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays
Table 1. Summary of our INTEGRAL observations of the Table 2. List of all detected sources in the Cassiopeia re-
Cassiopeia region. For every observation are given the revo- gion ordered by right ascension. Most sources are High-
lutionnumber,year,beginandendtime(t andt )andthe Mass X-ray Binaries (HMXBs), either a Be X-ray Binary
begin end
exposuretime(t ). (BeXB) or a Supergiant X-ray Binary (SXB), with in addi-
exp
tion one Intermediate Polar (IP), one BL-Lac type AGN, one
AnomalousX-ray Pulsar (AXP) and two supernovaremnants
Rev. year date t date t t (ks)
begin end exp
142 2003 Dec.12 15:46:50 Dec.14 21:11:53 178.2 (SNR).
143 2003 Dec.15 07:19:13 Dec.17 21:16:51 206.8
144 2003 Dec.18 07:15:25 Dec.20 15:00:42 187.0 Source Exp. Flux(mCrab) Flux(mCrab) Class
145 2003 Dec.21 16:36:36 Dec.23 20:59:51 173.8 (ks) 20–50keV 50-100keV
146 2003 Dec.24 18:50:50 Dec.26 20:23:19 165.0 IGRJ00234+6141 1606 0.72 0.12 1.4 0.3 HMXB?
147 2003 Dec.27 06:58:07 Dec.29 20:43:13 206.8 Tycho 1606 0.43±0.12 1.1±0.3b SNR
148 2003 Dec.30 06:51:06 Jan. 01 15:10:42 189.2 V709Cas 1606 3.84±0.12 2.8±0.4 IP
161 2004 Feb. 07 18:13:47 Feb. 09 18:17:17 163.2 1ES0033+595 1606 0.69±0.13 1.2±0.4b BL-Lac
162 2004 Feb. 10 17:37:54 Feb. 12 18:06:20 163.2 IGRJ00370+6122 1606 0.78±0.13 1.1±0.4b SXB
± ±
IGRJ00370+6122a 489 2.7 0.2 3.6 0.7
± ±
GammaCas 1602 3.8 0.2 1.2 0.4b BeXB
± ±
2S0114+650 1579 7.8 0.2 4.1 0.5 SXB
forsome transientsourcesanda secondAXP(1E 2259+586) ± ±
2S0114+650a 936 12.1 0.2 6.1 0.6
inthisfield. 4U0142+61 1152 1.4 ±0.3 7.0±0.7 AXP
± ±
ForthetwoAXPsources,0.75–30MeVarchivaldatafrom RXJ0146.9+6121 1133 1.6 0.3 1.5 0.8b BeXB
± ±
COMPTEL (Scho¨nfelderetal. 1993) are reanalyzed, deliver- 4U2206+54 731 2.8 0.5 1.8 1.2b BeXB
± ±
ing constraining upper limits for 4U 0142+61. In an attempt CassiopeiaA 1609 3.54 0.13 1.8 0.4 SNR
± ±
to reveal the nature of IGR J00370+6122 archival data from 1mCrab(20–50keV)=6.66 10−6phcm−2s−1keV−1,1mCrab(50–
×
the RXTE All-Sky Monitor and the BeppoSAX Wide-Field 100keV)=1.22 10−6phcm−2s−1keV−1;aoptimizeddataselection;
×
b possibly contaminated by artefact in map. Values to be used with
Camerashavebeenstudied,aswellasaTargetofOpportunity
careorusedasupperlimits.
(ToO)observationwithRXTEPCA.
2. INTEGRALobservationsandanalysis
Our observations of the Cassiopeia region were made dur- For each sourceand energybandthe time-averagedcount
ing nine orbital revolutions between December 12, 2003 and rate was determined by averaging the count rates from each
February 12, 2004 (see Table 1 for details). INTEGRAL per- deconvolvedScW,weightedbythevariance.
formed the observations in a standard rectangular (5 5)
× For converting subsequently count rates into flux values,
ditheringmode(Jensenetal. 2003).The data providedbythe
the source spectra were normalized to the known Crab total
INTEGRAL Science Data Centre (ISDC; Courvoisieretal.
spectrum(nebulaandpulsar),usinga5 5rectangulardither-
2003) were screened for Solar-flare events and erratic count- ×
ing observation of the Crab performed during revolution 102
rate fluctuations due to passages through the Earth’s radi-
as reference. The source fluxes could, thus, be expressed in
ation belts. After screening, the net exposure of the fully-
Crab units and converted in photon fluxes using the average
coded FOV was 1.6 Ms, from 677 pointings called Science
Crab power-law spectrum (F(E) = αE Γ) with photon in-
Windows (ScWs). The data were reduced using the Off-line −
dex Γ = 2.108 and normalization α = 9.59 ph cm 2s 1 at
− −
Scientific Analysis (OSA) software package version 4.2 (see
1 keV (Willingaleetal. 2001). This relation is valid through-
Goldwurmetal. 2003, for IBIS-ISGRI scientific data analy-
outtheIBIS-ISGRIwavebandupto 180keV,aswasverified
sis). For each ScW deconvolved IBIS-ISGRI sky maps were ∼
byKuiperetal.(2006).ThismethodforderivingIBIS-ISGRI
createdindifferentenergyranges.Thesedeconvolvedskymaps
sourcespectraenablesustodeterminethespectrawithoutde-
werecombinedintomosaicmaps.
tailedknowledgeoftheinstrumentresponse,whichcontained
The mosaics were examined for both known and poten-
significantuncertaintiesatthetimeofouranalyses.
tialnewsources.Consistencychecksarenecessarytoeliminate
Spectral-model fitting was performed with XSPEC ver-
spuriousartefacts.Inadditiontodeterminingthedetectionsig-
sion11.3.1(Arnaud1996).Forpower-lawfittingthePEGPWR
nificance,thecorrectsourceshape(Gaussianinapproximation)
modelwasused,whichisanormalpowerlaw,buttheprogram
and the presence or absence of systematic background struc-
returnsthepower-lawindexandtheunabsorbedpower-lawflux
turesinthemapswereverified.Inordertoacceptasourcede-
(F )inachosenenergybandinsteadofanormalization
tection in this work, a significance above 3σ in at least two powerlaw
atagivenenergytodescribethespectrum.Thismodelhasbeen
(consecutive) energy intervals was required. Source detection
choseninordertominimizethecovariancebetweentheparam-
significances( F/dFforabackground-dominatedphotondis-
≃ eters,resultinginthebesterrorestimates.
tribution)weredeterminedinthemosaicsforknownsourcesat
theircatalogpositions,andforcandidatenewsourcesatthepo- Allerrorsquotedinthispaperare1σerrors,unlessstated
sitionsintheskymapswiththehighestdetectionsignificances. otherwise.
P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays 3
5.000
IGR J01363+6610
2S 0114+650
Tycho
4U 0115+63
0.000
4U 0142+614 IGR J00234+6141 1E 2259+586
RX J0146.9+6121
Gam Cas IGR J00370+6122 Cas A
1ES 0033+595
IGR J00291+5934
V709 Cas
-5.000
130.000 125.000 120.000 115.000 110.000
Fig.1.Cassiopeia-regionsignificancemapinGalacticcoordinates(l,b)asseenwithIBIS-ISGRIinthe20–50keVenergyrange.
All excesses with positive significances are drawn. Indicated with white circles and annotation are the sources significantly
detectedduringthese observations.Indicatedwithyellowcirclesandannotationaretransientsourceswhichhavebeenweakly
detectedinthisworkand/ordetectedwithINTEGRALatotherepochs.AXP1E2259+586,notyetdetected,isagoodcandidate
forfuturestudy(seesection4.2).Theweakest(marginally)detectedsourceisTychowith3.5σ,whilethebrightestsourceinthis
field,2S0114+650,isdetectedwith43.1σ
3. Skymapsandsourcelist characteristic age should be interpreted with care, especially
foryoungneutronstars.However,thefactsthatalltheGalactic
Figures1and2showskymapsofalargepartoftheCassiopeia
AXPs (one is located in the SMC) are found within 1 of
◦
region in the energy bands 20–50 keV and 50–100 keV, re- ∼
the Galactic plane and that three of them are associated with
spectively.Forpresentationpurposestheskyregionsathigher
supernovaremnants(Gaensleretal.2001)supporttheiryoung
Galactic latitudes and below Galactic longitude l 108
∼ ◦ nature. These pulsars are called anomalous, because their
were croppedoff. As a result 4U 2206+54,locatedat (l,b) =
X-ray luminosities below 10 keV are orders of magnitude
(100.6, 1.1),isnotshowninFigures1and2.Thesignificantly
◦ − ◦ higher than the rotational energy losses. The magnetic field
detected sources that passed our consistency checks as men-
strengthsof AXPs, B √PP˙, are of the order1014 G, much
tionedinSect.2areindicatedbywhitecirclesandannotation. ∝
stronger than estimated for normal (radio) pulsars (B 1012
The yellow circles and annotation indicate weakly detected ∼
G).
sources, transient sources, that were detected by INTEGRAL
atotherepochsandthesecondAXPinthefield,1E2259+586, In order to explain the energy budget of the AXPs, the
for which there is so far no INTEGRAL detection. The weak magnetar model (Thompson&Duncan 1993, 1996) is cur-
(one energy interval) detections and upper limits of the lat- rently the most accepted model. This model was originally
ter sources are given and discussed in Sects. 4.2 and 6. All aimed at explaining phenomena observed in Soft Gamma-
(weakly)detectedsourcesarelistedinTable2. ray Repeaters (SGRs), but due to recent observationsof sim-
ilar behaviour of SGRs and AXPs, like bursting activity
(Gavriiletal. 2002; Kaspietal. 2003), they are now believed
4. AnomalousX-raypulsars to originate from the same class of objects. This model also
explains the glitching phenomena detected in the spin down
Anomalous X-ray pulsars, originally discussed as a
of two AXPs (Kaspi&Gavriil 2003; Dall’Ossoetal. 2003;
subset of low-mass X-ray binaries (e.g. Hellier 1994;
Woodsetal.2004).ThemagnetarmodelpostulatesthatAXPs
Mereghetti&Stella 1995), are members of a special subset
andSGRsarepoweredbymagnetic-fielddecay.
ofisolatedneutronstars(seeWoods&Thompson2005, fora
recent review). So far, there are seven established AXPs and In the pre-INTEGRAL era AXPs were known as soft X-
oneAXPcandidate.Theirspinperiods(P)arefoundinanar- ray sources visible up to 10 keV. All AXP spectra, below 10
rowrangebetween5.5sand11.8sandtheirperiodderivatives keV, could be described by a black-body component and a
(P˙) are of the order of 10 11 s s 1. Their characteristic ages soft power law. However, this view has changed completely
− −
(τ P/2P˙) indicate th∼at they are young objects, though the sinceINTEGRALdiscoveredhardX-raysabove20keVfrom
∼
4 P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays
5.000
2S 0114+650
0.000
4U 0142+614 IGR J00234+6141
IGR J00370+6122 Cas A
V709 Cas
-5.000
130.000 125.000 120.000 115.000 110.000
Fig.2. Cassiopeia region presented in the same format as in Fig. 1 in the 50–100keV energy range. All detected sources are
indicatedwithwhitecirclesandannotation.Severalsourcesarenolongerapparentinthisenergyrange.IGRJ00370+6122isnot
visibleinthistotal-exposuremap,butisdetectedduringtwosinglerevolutions(seeSect.5.4).Someresidualartefacts/systematics
canbeseeninthecenterofthismap.
the direction of AXPs 1E 1841-045 and 1RXS J170849-
400910 (Molkovetal. 2004; Revnivtsevetal. 2004a). In ad-
dition,denHartogetal. (2004a) reportedthe INTEGRAL de-
tection of 4U 0142+61, in a preliminary analysis of the data
used in this work. Kuiperetal. (2004) provided definite ev-
idence that the hard X-rays from the direction of 1E 1841-
045indeedoriginatedfromtheAXPratherthantheassociated
supernova remnant G27.4+0.0 (Kes 73). They showed that a
significant fraction of the hard X-ray emission is pulsed, us-
ing RXTE PCA and HEXTE coveringthe same energyrange
as INTEGRAL. Also for AXPs 1RXS J170849-400910 and
4U 0142+61 Kuiperetal. (2006) subsequently found pulsed
emissioninthehardX-rayregimeabove20keVusingarchival
RXTEdata.
Fig.3. The broad-band X-ray to gamma-ray spectrum of
4.1.4U0142+61
4U0142+61inanE2Frepresentation.Shownarethe1–8keV
Chandra fit (Pateletal. 2003), the 20–150 keV IBIS-ISGRI
4U 0142+61 was detected by the Uhuru satellite in the early
flux values (1σ error bars) with 2 fits (see Table 3) and the
seventies (Giacconietal. 1972; Formanetal. 1978). The na-
0.75–30 MeV COMPTEL 2σ upper limits. The dotted lines
ture of 4U 0142+61 was unclear for more than two decades,
are extrapolationsof the modelsfitted to the IBIS-ISGRI flux
causedbysourceconfusionwiththenearbylocatedhigh-mass
values. The extrapolation of the single power law shows that
X-ray binary RX J0146.9+6121 (24 from the AXP, see sec-
′ the spectrum has to bend/break in order not to be in conflict
tion5.3).With thediscoveryofRX J0146.9+6121in ROSAT
withtheCOMPTELupperlimits.
0.1–2.4keVdata(Motchetal.1991),Mereghettietal.(1993)
realizedthatthe25-minuteperiodicity,uptothenattributedto
4U0142+61(Whiteetal.1987),wasunlikelytooriginatefrom
4U0142+61,becauseofthecharacteristicsofthemodulations.
Hellier (1994) showed with ROSAT PSPC observations that Israeletal. (1994). They foundthat the rotationalenergyloss
thisperiodicityindeedoriginatedfromRXJ0146.9+6121.The of4U0142+61isnotsufficienttoexplainitsX-rayluminosity
spinperiodof8.7secondsof4U 0142+61wasdiscoveredby below10keV.
P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays 5
4.1.2. 4U0142+61:COMPTELarchival-dataanalysis
The Compton telescope COMPTEL (Scho¨nfelderetal. 1993)
aboardtheComptonGamma-RayObservatory(CGRO,1991–
2000) is sensitive to gamma-ray photons between 0.75 MeV
and30MeV,therebycoveringthehardergamma-raybandad-
jacent to the INTEGRAL one. The very-hard spectrum mea-
suredwithIBIS-ISGRIfor4U0142+61promptedustorean-
alyze the COMPTEL data archiveto search for a signal from
4U0142+61.COMPTELhasawidecircularfieldofviewcov-
ering 1steradian.Therefore,duringthelongmissionlifetime
∼
ofCGRO,mostofthesky,particularlytheGalacticplane,has
been viewed with long exposures. Sky maps and source pa-
rameterscanbeobtainedwiththemaximum-likelihoodmethod
(deBoeretal. 1992), which is implemented in the standard
COMPTELdata-analysispackage.Fortheanalysisanddatase-
Fig.4. Fitresidualsforthe4U0142+61spectrum:a)forasin-
lectionstheapproachdescribedbyZhangetal.(2004)wasfol-
glepower-lawfit;b)forapower-lawwithahigh-energycutoff
lowed.Theeffectiveon-axisexposuretimeonthisAXP,accu-
fit.
mulatedoverthetotalmission,amountsto 4.2Ms.Standard
∼
energy intervals were selected for the analysis. In the lower-
energyintervals, 0.75–3MeV and 3–10 MeV, weak, low sig-
4U0142+61wasthefirstAXPforwhichopticalemission nificance (<1.5σ) excesses were found. In the 10–30 MeV
wasdiscovered(Hullemanetal. 2000),whichlaterturnedout ∼
energy band no positive-flux measurement was derived. The
tobepulsed(Kern&Martin2002).Hullemanetal.(2004)also 2σ upper limits for these energy intervals are 3.1 10 8
−
discoverednear-IR emission from this source, further studied phcm 2s 1keV 1,2.9 10 9phcm 2s 1keV 1and2.0×10 10
− − − − − − − −
byMoriietal.(2005)insimultaneousX-ray,near-IRandopti- ph cm 2s 1keV 1 , res×pectively. These are shown in×Fig. 3,
− − −
calobservations.TheydiscoveredinSubaruobservations,that
and can be directly compared with the hard X-ray spectrum.
alsothenear-IRemissionispulsed(Moriietal.2006, private
If the power-law spectrum fitted to the IBIS-ISGRI data ex-
communication).So far, no radio emission has been detected
tends to the MeV energy range, it would result in a photon
(Gaensleretal.2001). flux of 6.7 10 7 ph cm 2s 1keV 1 (0.75–3 MeV), i.e. a
− − − −
The soft X-ray spectrum observed with Chandra by brightC∼OMPT×ELsource,inconsistentwiththeCOMPTELup-
Pateletal. (2003) shows a typical AXP spectrum. The spec- perlimits.Itshowsthatthespectrumhastobendorbreakbe-
trumcanbefittedwithathermalblack-bodymodelwithatem- low 0.75MeV.Tosearchforpossibleindicationsofthisbreak
perature kT = 0.470 0.008 keV and a very soft power-law inth∼eIBIS-ISGRIdata,apower-lawmodelwithahigh-energy
±
componentwithaphotonindexΓ=3.40 0.06. cutoffwasfittedtothedata.Thisresultedinafitimprovement
±
InthisworknoINTEGRAL-timinganalysishasbeenper- of 2.34σ with a ∆χ2 of 7.9 for 2 extra free parameters (see
formedfor4U0142+61.Thiswillbeaddressedinfuturework, Table 3 and Fig. 3). This fit gives an extremely hard power
exploitingalsoadditionalINTEGRALobservations. lawinthelowerenergypartwithaphotonindexΓ=–0.11
±
0.43andahigh-energycutoffat73 15keVwithane-folding
±
energyof 37 18 keV. With this modelno significantflux is
4.1.1. 4U0142+61:INTEGRALresults ±
expectedintheCOMPTELenergyrange.Figure4bshowsthe
4U 0142+61 was significantly detected by INTEGRAL as a residuals from the fits. We merely consider this 2.3σ fit im-
hardX-raysource(see Figs. 1and 2; fora preliminaryreport provementas a hint for a break. With recently performedad-
seedenHartogetal.2004a),eventhoughthissourceislocated ditional INTEGRAL observationswe expectto obtain tighter
17.7 from Cassiopeia A and 9.5 from Tycho, therefore being constraints.
◦ ◦
alwaysoutsidethefully-codedFOVofIBIS-ISGRI.Inthesky-
imagemosaicwithatotalexposureof1.15Ms,4U0142+61is
4.2.1E2259+586
detectedwith5.0σ,9.5σand4.6σsignificancesintheenergy
bands20–50keV,50–100keVand100–150keV,respectively. 1E2259+586hasbeenveryimportantforAXPresearch.Not
The IBIS-ISGRI spectrum for energies above 20 keV, onlywasitthefirstofthecurrentlyknownAXPsrecognizedas
showninFig.3,canbedescribedbyapower-lawmodelwitha apeculiarsource(Fahlman&Gregory1981),butitalsoplayed
photonindexΓ=0.73 0.17.Thisfithasaχ2=2.01(dof=7) akeyroleinunifyingAXPsandSGRsasmagnetarcandidates
± r
andthusa null-hypothesisprobabilityof5%(see Table3 and whenthissourceshowedmorethan80SGR-likeburstsin2002
Fig.4aforthefitresiduals).Strikingistheremarkableharden- (Kaspietal.2003).ThesoftX-rayspectrum,whichcanbede-
ingofthespectrumabove10keVwhencomparedtothevery- scribed with a black bodywith a temperaturekT = 0.41keV
soft Chandra spectrum, also shown in Fig. 3. The transition and a very soft power-law with photon index Γ = 3.6, is a
occursbetweentheChandraandIBIS-ISGRIenergywindows typicalAXP spectrum(see Pateletal. 2001, forrecentX-ray
whereitcanbestudiedwithRXTE(Kuiperetal.2006). observations).Interestingly,Woodsetal. (2004)observedsig-
6 P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays
Table 3. Spectral fit results for AXP 4U 0142+61. The data
can be fitted with a single power-law model. Using a power-
law with a high-energycutoff model, the fit improves 2.34σ,
indicatingahintfortheexpectedbreakinthespectrum.
Powerlaw(20–150keV)
Γ 0.73 0.17
±
F (9.7 0.9) 10 11ergcm 2s 1
powerlaw ± × − − −
χ2(dof) 2.01(7)
r
PowerlawwithHigh-Energycutoff(20–150keV)
Γ 0.11 0.43
− ±
F (18 5) 10 11ergcm 2s 1
Powerlaw ± × − − −
E 73 15keV
cutoff ±
E 37 18keV
fold ±
χ2(dof) 1.23(5)
r
F 9.5+0.5 10 11ergcm 2s 1
unabs 3.2× − − −
−
Fig.6. IBIS-ISGRI (20–50 keV) light curve of 2S 0114+650
with 0.1 day binning. It shows periods of flaring as well
as quiescent phases. The epochs of maximum flux, using an
ephemeris based on RXTE-ASM data, are indicated with ar-
rows. The four time intervalsthat are omitted for the spectral
analysisareindicatedasthickhorizontalbars.
AXPabove20keV.Onlyupperlimitscouldbeextractedfrom
the IBIS-ISGRIdata.Intheenergybands20–50keV, 50–100
keV and 100–150 keV the 2σ upper limits are 3.2 10 6
−
×
phcm 2s 1keV 1,1.4 10 6phcm 2s 1keV 1and0.9 10 6
− − − − − − − −
× ×
phcm 2s 1keV 1 ,respectively.Giventhereportedvariability
− − −
below10keV,thedatawerealsoanalyzedinseparaterevolu-
Fig.5. Soft X-ray spectral fits and hard X-ray to gamma-ray
tions,but1E2259+586wasnotdetected.
upperlimits of1E2259+586in a E2F representation.Shown
arefitsmeasuredbyChandra(0.5–7keV;Pateletal.2001)and
by XMM-Newton(2–10keV;Obs. 1, Woodsetal. 2004);the
4.2.2. 1E2259+586:COMPTELarchival-dataanalysis
IBIS-ISGRI20–150keVandCOMPTEL0.75–30MeV2σup-
per limits. The solid staircase line is the IBIS-ISGRI 3σ sen-
Also for 1E 2259+586, analysis of COMPTEL archival data
sitivity for 5-Ms exposure time. A hard high-energy spectral
isperformedasdescribedfor4U0142+61inSect.4.1.2. The
componentasfoundfor4U0142+61isnotinconflictwiththe
effectiveon-axisexposuretimeon1E2259+586,accumulated
INTEGRALupperlimits.
overthetotalmission,amountsto 4.9Ms.Asfor4U0142+61
∼
COMPTELwasnotsensitiveenoughtosignificantlydetectthis
AXP.The2σupperlimitsforthe0.75–3MeV,3–10MeVand
nificant spectral changes (for energies below 10 keV) before
10–30MeVbandsare2.3 10 8phcm 2s 1keV 1,1.7 10 9
andafterthe2002outburst. − − − − −
× ×
ph cm 2s 1keV 1 and 2.0 10 10 ph cm 2s 1keV 1 , respec-
Contrary to 4U 0142+61, 1E 2259+586 is associated − − − − − − −
×
tively.The2σupperlimitsare shownin Fig.5, togetherwith
with a young supernova remnant G109.1-1.0 (CTB 109),
the IBIS-ISGRI upper limits and spectral fits to two different
whichwasdiscoveredbyGregory&Fahlman(1980)(seealso
spectrameasuredbelow10keVbyChandra(Pateletal.2001)
Rho&Petre 1997; Pateletal. 2001; Sasakietal. 2004). The
and by XMM-Newton (Woodsetal. 2004). A spectral shape
supernovaremnantdoesnotinfluence the INTEGRAL analy-
including a hard spectral tail like measured for 4U 0142+61
sisasitisonlydetectedupto 4keV(Sasakietal.2004).
∼ (Fig.3),isallowedbyallupperlimitsabove10keV.
Recently Kuiperetal. (2006) detectedpulsed emission up
4.2.1. 1E2259+586:INTEGRALresults
to 24 keV with the RXTE-PCA, indicating the likely onset
∼
1E 2259+586 is located only 2.9 from Cassiopeia A (see of a hard tail, which is still consistent with the upper limits
◦
Fig. 1). Therefore it was optimally exposed for 1.6 Ms in derived in this work. This detection is promising for a future
the fully-coded FOV. Even with the current high exposure, INTEGRAL detection. The total exposure on 1E 2259+586
the sensitivity of INTEGRAL is not sufficient to detect this willbemorethan5Msbytheendof2006.
P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays 7
5. Binaries
Since launch, INTEGRAL has discovered 107 new hard X-
ray sources1, among which a large number of binaries (see
e.g. Revnivtsevetal. (2004a,b); Molkovetal. (2004, 2005);
Birdetal.(2004,2006)andreferencestherein).Oneoftheim-
portant discoveries of INTEGRAL is the discovery of a pre-
viouslymissedclassofobjects,namelyhighlyabsorbedHigh-
MassX-rayBinaries(HMXBs)(N &1023cm 1;seeKuulkers
H −
2005, forarecentreview).
HMXBs can be divided in two classes (Corbet 1986): the
Be X-ray binaries (BeXBs) and the supergiant X-ray (SXBs)
binaries, which have an OB supergiant or a bright giant as
donor star. Currently there are about 150 HMXBs known,
out of which 65 are located in our own Galaxy. Of these
∼
150 systems, about 20 are of the supergiant class. From the
new INTEGRAL sources, for which follow-up observations
Fig.7. Time averaged IBIS-ISGRI 20–150 keV spectrum of
are performed, already 10 are identified as supergiant X-ray 2S 0114+650in an E2F representation.This spectrumshows
binaries with a high intrinsic absorption. Including the new
ahigh-energytailwhichwasnotobservedbeforeINTEGRAL.
INTEGRAL SXBs, the sample of SXBs has grown to about
The best fit to the data is a bremsstrahlungwith a power-law
30. See e.g. Negueruela (2005); Negueruelaetal. (2006) and
model(seeTable4).Thebest-fitmodelandtheseparatecom-
Lutovinovetal. (2006) forthe latestdevelopmentsand impli-
ponentsareplotted.
cationsofthenewlyfoundHMXBsbyINTEGRAL.
In this section the binaries that were detected in the
Cassiopeiafield(seeTable2)areintroducedandtheresultsare
presented. It concerns four HMXBs, one Intermediate-Polar
(IP)andthenewsourceIGRJ00234+6141whichismostlikely
anHMXB.
5.1.2S0114+650
2S0114+650isasupergiantX-raybinary.Thesupergiantcom-
panionLSI +65 010isoftheB1Iatype(Reigetal.1996).This
◦
system exhibitsX-raymodulationswith a periodof2.7hours
(Finleyetal. 1992; Corbetetal. 1999; Bonning&Falanga
2005) which are interpreted as originating from the neutron
star’s spin - the slowest known to date. The orbital period
of 11.59 days (Cramptonetal. 1985) is quite normal for a
SXB.Recently,Farrelletal.(2004)foundevidenceforasuper-
Fig.8. Residuals for IBIS-ISGRI 20–150 keV spectral fits
orbitalperiodof 30.7days,which they interpretas an indica- of 2S 0114+650. Panel a) single power-law model; b) sin-
tionforthepresenceofaprecessingwarpedaccretiondisk.
gle bremsstrahlung model; c) bremsstrahlung plus power-law
2S0114+650hasbeenobservedinthesoftX-raybandsev- model.
eraltimes(<10keV;e.g.Halletal.2000;Apparaoetal.1991).
However, before INTEGRAL little information was available
for the high-energypart of the spectrum. Koenigsbergeretal. presented in Fig. 6, shows a variable source with periods of
(1983) published an OSO-8 spectrum taken in 1976, which flaring and quiescence, both lasting of the order of less than
couldbefittedwithahardpowerlawwithphotonindexΓ=1.2 a day to days. This source is known to show flaring activ-
and a high-energy cutoff E 14 keV. The source was de- ity adding up to a single broad pulse when folding with the
cut
tectedupto 30keV. ∼ orbital ephemeris over a long time span (Koenigsbergeretal.
∼
1983;Apparaoetal.1991;Corbetetal.1999;Halletal.2000).
Using an ephemeris (maximum flux expected at MJD =
5.1.1. 2S0114+650:INTEGRALresults 51825d.2 0d.3+ N 11d.5995(53))created with the available
± ×
onlineRXTE-ASMdata(MJD50087–53100),wefindthatthe
INTEGRAL observed 2S 0114+650 for 1.58 Ms. It was de-
phasesoftheflaresaredistributedconsistentlyoverthephase
tected at significance levels of 43.1σ and 8.6σ in the 20–
intervalofthebroadtime-averagedpulse.
50 keV and 50–100keV bands, respectively.The light curve,
For the spectral analysis, four time intervals in which the
1 see http://isdc.unige.ch/ rodrigue/html/igrsources.html for the sourcewasinaweak/quietstateareomitted.Theomittedinter-
∼
completeIGR-sourceslist vals in MJD are: 52988–52990, 52992–52996,52997–53000
8 P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays
and53004–53006(theseintervalsareindicatedinFig.6).The Table4.Spectralfitresultsfor2S0114+650.
remainingdataconsistof374ScWsaddinguptoanexposure
of 935.8ks. For this selection of observationtimes the detec- Powerlaw(20–150keV)
tion significances increased to 52.3σ and 10.0σ in the 20–50 Γ 3.01 0.06
±
keVand50–100keVbands,respectively. F (17.2 0.5) 10 11ergcm 2s 1
powerlaw ± × − − −
χ2(dof) 1.21(14)
r
Thetime-averagedhigh-energyspectrum(20–150keV)of Bremsstrahlung(20–150keV)
2S 0114+650 is shown in Fig. 7. A fit with a single power- kT 20.4 0.9keV
±
law model (Γ = 3.01 0.06) gives an acceptable fit up to norm (5.9 0.4) 10−2
± χ2(dof) 2.45±(14) ×
150 keV (see Table 4 and Fig. 8a). A more physicalthermal- r
F (14.9 0.4) 10 11ergcm 2s 1
bremsstrahlung model could not fit the data satisfactorily up unabs ± × − − −
Bremsstrahlung+Powerlaw(20–150keV)
to 150 keV. Above 70 keV it underestimates the high-energy
kT 14.1 2.4keV
flux and the fit is therefore not acceptable (see Table 4 and ±
norm (7.2 1.5) 10 2
Fig.8b).Addingapower-lawcomponenttothebremsstrahlung ± × −
Γ 2.0 0.7
model,a goodfitisachieved.However,statistically itcannot F (8.7± 3.7) 10 11ergcm 2s 1
befavouredover,nordistinguishedfromthesinglepower-law χ2po(wdeorlfa)w 1.12±(12) × − − −
r
model(see Table4 andFig.8a,c).Toexplainthehardestpart F (17.7 1.0) 10 11ergcm 2s 1
unabs ± × − − −
ofthespectrumcomptonizationmodelsareinvoked.Themod-
els by Titarchuk (1994, compTT) and Poutanen&Svensson
(1996, compPS) are able to model the high-energy part of
5.2.γCas
the spectrum better than the bremsstrahlung model, but the
parameters are poorly constrained.Therefore the high-energy
γCasisabrightB0.5IVestarandrepresentstheprototypical
tailwhichhasbeenobservedbyINTEGRALforthefirsttime
Bestar(Secchi1867).ItwasdiscoveredinX-raysbyJernigan
canbedescribedbyeithercomptonizationinathermalornon-
(1976) and Masonetal. (1976). The X-ray characteristics of
thermal plasma. It is not clear whether it originates in a hot
γ Cas made it a peculiar star because its X-ray luminosity is
coronaorina putativeaccretiondisk.Theaccretion-disksce-
severaltimeshigherthanmeasuredforotherBestars,but 20
nario,however,islesslikelyforSXBsystems. ∼
timesweakerthantheweakestBeXB(Murakamietal.1986).
Throughradialvelocityanalysisofspectrallines,itwasfound
Recently Filippovaetal. (2005) and Bonning&Falanga
tobeabinary(Harmanecetal.2000).
(2005) have independently observed 2S 0114+65 with
Determiningwhetherthenatureofthecompactcompanion
INTEGRAL. Filippovaetal. (2005) caught the source in a
is a neutron star or a white dwarf was a challenge for a long
low state and measured a simple power law (Γ = 2.3
± time. Rappaport&vandenHeuvel (1982) argued that the X-
0.4) which is marginally consistent (2σ) with our findings.
ray luminosity of 1033 erg s 1 combined with a wide orbit,
Bonning&Falanga (2005) observed 2S 0114+6 for 180 ks ∼ −
and the hardnessof its spectrum,supporta neutron-starcom-
in December 2004. They find similar results compared to
panion.However,mostauthorsagreethatawhitedwarfcom-
Halletal. (2000) combining JEM-X and IBIS-ISGRI data.
panion is more likely: its spectrum can be fitted with a ther-
Theirbest-fitmodelisapowerlawwitha high-energycutoff:
malopticallythinplasmawithanextremelyhightemperature
photonindexΓ=1.6 0.5andhigh-energycutoffE =9 11
± cut ± of 10–12 keV; a strong Fe emission line at 6.8 keV with an
keV with an e-foldingenergy E = 22.1+12.1 keV (90%er-
fold 6.0 equivalentwidth of 280 eV is present (Murakamietal. 1986;
rors). Fitting our data with these parameter−s, leaving the nor-
Owensetal. 1999); and also rapid fluctuations on timescales
malizationfree,givesanunacceptablefit.Thismodelalsofails
of seconds hinting towards a white-dwarf companion are de-
tofitthehardtailandthereforeunderestimatesthefluxabove
tected(Kuboetal.1998;Smithetal.1998).
70keV.Alreadyintheirspectrumitcanbeseenthatthemodel
Owensetal.(1999)observedγ Caswith BeppoSAX.The
isbelowtheirdatapointsabove70keV,butjustbecauseofthe
spectrumofγ Cas couldbe extractedupto 30keV withthe
larger statistical errors their model was still acceptable to de-
∼
HPGSPCand 40keVwiththePDS.ThebestfittotheirX-ray
scribe their spectrum. Bonning&Falanga (2005) note the in-
∼
spectrumfrom0.2–40keVisachievedwithaMEKALmodel
dicationfora possiblecyclotronabsorptionfeatureat44keV.
(Meweetal.1995)withanadditionalCarbonline(at0.3keV),
Ourdatadonotconfirmthisindication,despitethemuchlonger
or a two component MEKAL model. The temperature of the
exposuretime.
hotplasmainbothmodelsis12.4 0.6keV.
±
Evenmorerecently,Masettietal.(2005)publishedresults
froma BeppoSAXobservationof 63ks with the narrowfield
5.2.1. γCas:INTEGRALresults
instruments,sensitivebetween0.1keVand300keV.Thespec-
trum of 2S 0114+65 is determined in the 1.5–100 keV band. INTEGRALdetectedγCaswithhighsignificanceupto50keV
Their findings agree with the findings of Bonning&Falanga (25.1σ).Between50keVand100keVa3σfluxpointismea-
(2005), except that they also do not find evidence for a cy- sured(seeTable2),however,thisfluxpointislikelycontami-
clotronabsorptionline.Comparedtoourspectrumtheirfitre- natedbysurroundingartefacts.Forthisreasonitwasdiscarded
sultsalsofailtodetectthehardtail. inthespectrum,showninFig.9.
P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays 9
5.3.RXJ0146.9+6121
RX J0146.9+6121 (V* V831 Cas), a BeXB located 24 from
′
AXP 4U 0142+61 (see section 4.1), was discovered
by Motchetal. (1991) with ROSAT. Its companion star
(LS I +61 235) was spectrally classified as a B1Ve by
Reigetal.(1997).Whiteetal.(1987)discovereda25minutes
periodicity which was then attributed to AXP 4U 0142+61.
However, Hellier (1994) proved that this periodicity origi-
natedfromRXJ0146.9+6121,usingtheROSAT-PSPCimager.
RXJ0146.9+612alsoshowslong-term,uptoanorderofmag-
nitude,variability(Haberletal.1998a).
X-rayspectraderivedfromASCA,RXTEandBeppoSAX
data (Haberletal. 1998b,a; Mereghettietal. 2000) could be
modelled up to 25 keV with power laws with or without
Fig.9.IBIS-ISGRIspectrumofγCasinanE2Frepresentation. high-energycuto∼ffsorwith bremsstrahlungmodels.However,
Overplottedisthebest-fitmodel,whichisapower-lawwitha thespectraderivedfromthenon-imaginginstrumentsonboard
high-energycutoff(see Table 5). Thebottompanelshowsthe RXTE and BeppoSAX are contaminated by 4U 0142+61.
fitresiduals. For the soft X-ray spectrum (<10 keV) the contribution of
4U 0142+61 was corrected for by including fixed model pa-
Table5.SpectralfitresultsforγCassiopeia. rameters,butforthehardX-rayspectrum(>10keV)thecon-
tribution of 4U 0142+61 was still unknown. The GIS instru-
ment onboard ASCA was able to resolve RX J0146.9+6121
PowerlawwithHigh-Energycutoff(20–50keV)
and4U0142+61inthe0.5–10keVband.
Γ 3.1 0.3
±
F (4.2 0.3) 10 11ergcm 2s 1
powerlaw ± × − − −
E 34.1 1.9keV
cutoff ± 5.3.1. RXJ0146.9+6121:INTEGRALresults
E 7 2keV
fold ±
χ2r (dof) 2.81(4) LikeAXP4U0142+61,thissourcewasalwaysinthepartially-
F (3.56 0.14) 10 11ergcm 2s 1
unabs ± × − − − coded FOV of IBIS-ISGRI with a total exposure of 1.13 Ms.
Bremsstrahlung(20–40keV)
With the high spatial resolution of INTEGRAL it is possible
kT 13.6 1.5keV
± toresolveRXJ0146.9+6121andtheAXP,andthereforethere
norm 3.6 0.7 10 2
± × − is no source confusion. In the 20–50 keV mosaic the source
χ2(dof) 1.44(5)
Fr 3.4+20 10 11ergcm 2s 1 isdetectedwith adetectionsignificanceof5.3σwith a corre-
unabs −0.8× − − − spondingphotonfluxof(1.1 0.2) 10−5 phcm−2s−1keV−1 .
± ×
In narrow-band (5 keV) mosaics the source is only detected
in the 20–25 keV and 25–30 keV bands with photon fluxes
(3.5 0.7) 10 5phcm 2s 1keV 1(4.8σ)and(1.8 0.6) 10 5
− − − − −
TheINTEGRALspectrumofγ Cascanbestbe described phc±m 2s ×1keV 1 (3.3σ),respectively.Inthe30–4±0keV×band
− − −
byapower-lawmodelwithahigh-energycutoff.However,the RXJ0146.9+6121isstill(just)visiblewith(0.8 0.3) 10 5
residuals are large. With χ2r = 2.81(dof = 4) and thusa null- phcm 2s 1keV 1(2.7σ),butinthe40–50keVba±ndonl×ya2−σ
− − −
hypothesis probability of 2.4%, this fit can hardly be consid- upperlimitof4.2 10 6phcm 2s 1keV 1couldbeextracted.
− − − −
eredstatisticallyacceptable.However,othermodels,likeasin- ×
We fitted a power-lawmodelto these data pointsto obtainan
glepowerlaw(χ2 =7.32,dof=6)orabremsstrahlungmodel
r indication for the photon index and flux. The photon index is
(χ2r =4.77,dof=7), result in significantly worse fits. This is Γ=3.4 0.9withaflux(1.5 0.2) 10 11ergcm 2s 1(20–40
− − −
dueto the verylow fluxin the 40–50keV bin with respectto ± ± ×
keV).
the20–40keVspectrum,requiringacutoff.
Comparingthese resultswith the previouslypublishedre-
A bremsstrahlung fit over the 20–40 keV interval yields sultsfromHaberletal.(1998b,a)andMereghettietal.(2000),
a temperature of (13.6 ± 1.5) keV with χ2r = 1.44 (dof = 5), our data points nicely extend the previously observed spec-
which is (within errors) comparableto the temperature found tra. Leaving the normalization as the only free parameter,
by Owensetal. (1999). However,over the 20–50keV energy acceptable fits can be found for all reported fits, except
range the best-fit temperature is 9.8 0.5 keV. This temper- for one, namely the single power-law fit with photon index
±
ature is in agreement with reports by Kuboetal. (1998) and Γ = 1.46 observed with ASCA (Haberletal. 1998b), does
Smithetal.(1998),butthequalityofthebremsstrahlungfitto not give an acceptable fit to our data points. However, the
ourspectrumisunacceptable. bremsstrahlung model fitted to the same ASCA data returns
ThespectralpropertiesofγCasaremorereminiscentofa the best slope for the INTEGRAL energy band between 20
typicalHMXBspectrumthana WD-binaryspectrum,leaving and 40 keV. The fluxes in the 0.5–10 keV band are also
the question about the nature of the compact companion still comparable with 3.9 10 11 erg cm 2s 1 for ASCA and
− − −
∼ ×
open. (2.9 0.6) 10 11ergcm 2s 1forINTEGRAL(extrapolatedto-
− − −
± ×
10 P.R.denHartogetal.:INTEGRALsurveyoftheCassiopeiaregioninhardXrays
Rev 0142 (Phase 0.97 - 0.12) Rev 0143 (Phase 0.14 - 0.31) Rev 0144 (Phase 0.33 - 0.48)
Rev 0145 (Phase 0.55 - 0.69) Rev 0146 (Phase 0.75 - 0.88) Rev 0147 (Phase 0.91 - 0.07)
Fig.10. Series of 20–50 keV mosaics centered on the coordinates of IGR J00370+6122. The source was discovered in the
revolution-147 mosaic as a 12.3σ source. In each panel the corresponding orbital phases of each revolution are given. Both
detectionsfallwithintheexpectedepochofmaximumflux.
wardslowerenergies).Thepower-lawwithahigh-energycut- with a significance of 5.6σ and a flux of (1.4 0.3) 10 5
−
± ×
offmodel(Γ=2.05, E =17.2, E = 7),fittedtoRXTE phcm 2s 1keV 1(2.1mCrab,20–50keV).
cutoff fold − − −
data Mereghettietal. (2000), gives a slightly too steep spec-
Weredeterminedthesourcepositioninthe20–50keVmo-
tralslopeintheINTEGRALenergyrange.However,theirnor- saicofrevolution147:R.A.=0h37m07s.3,Decl.=+61 2150
◦ ′ ′′
malization of this power law is in full agreementwith the to-
(J2000) with an estimated 2 accuracy (90% confidence),
′
wards lower energiesextrapolatedINTEGRAL normalization
which is 0.37 from the originally distributed coordinates
′
of(1.4 0.2) 10 10ergcm 2s 1.
− − − (denHartogetal.2004b).
± ×
Recently, Filippovaetal. (2005) reported the detection of
A search through SIMBAD yields a possible ROSAT X-
RX J0146.9+6121 with independent INTEGRAL observa- raycounterpart,1RXSJ003709.6+612131(Vogesetal.1999),
tions. They reported a weak detection of 3 mCrab (18–60
located only 0.43 from the updated coordinates. The ROSAT
∼ ′
keV)andaspectrumfittedwithasinglepowerlawwithphoton
source has an optical counterpartwithin its 17 error radius,
′′
indexΓ=2.9+1.1.Theslopesofourspectraarefullyconsistent,
0.8 namelya 10th-magnitude(B andV bands)B1Ib supergiant,
howeverdurin−gtheirobservationthesourcewasabouttwiceas BD +60 7∼3 (Rutledgeetal. 2000). Follow-up optical obser-
bright.
vations by Reigetal. (2005) reassessed the spectral type of
BD+6073tobeBN0.5II-III,whichbelongstothebrightgiant
class.Theestimateddistancetothesourceis 3kpc.Noradio
5.4.IGRJ00370+6122 ∼
emissionisfoundfromthissourceintheNVSSonlinearchive.
AlsoCameronetal.(2004)didnotfindaradiosignalwithnew
5.4.1. IGRJ00370+6122:Discoveryandidentification
VLAobservations.
IGRJ00370+6122isthefirstnewlyfoundINTEGRALsource WeobtainedaRXTEAll-SkyMonitor(RXTE-ASM)light
intheCassiopeiaregion(denHartogetal.2004b).Thesource curve(MJD50087–53124)andfoundaperiodicityof(15.665
didnotshowupintheintegratedmosaicsofrevolutions142– 0.006)days.ThefoldedlightcurveinFig.11showsasingle
±
148 (1.6 Ms), but it did appear in two single revolutions. For peakwithapeakfluxof3.3mCrabandaquiescentlevelbelow
thispaperthedatawerereanalyzedwithOSA4.2,confirming 1 mCrab (after subtracting a 1 mCrab systematic bias). The
ourinitialresults(seeFig.10).Thesourceissignificantlyde- peak flux remains above 2 mCrab for 3 days. The epoch of
∼
tectedinrevolution147withasignificanceof12.3σandaflux maximum flux during the INTEGRAL and contemporaneous
of(3.1 0.3) 10 5phcm 2s 1keV 1(4.6mCrab)inthe20–50 RXTE-ASM observations is MJD53001.7 0.3. A HMXB
− − − −
± × ±
keVenergyband.Intheotherrevolutionsthesourceisnotde- with an eccentric orbit where the compact object is accreting
tectedexceptinrevolution142whereitappearedmoreweakly from wind accretionnear periastron,can exhibita light curve