EVN and MERLIN observations of microquasar candidates at low galactic latitudes

0.31 MB·English

by  M. Ribo

Astronomy&Astrophysicsmanuscriptno.art February5,2008
(DOI:willbeinsertedbyhandlater)
EVN and MERLIN observations of microquasar candidates
at low galactic latitudes
M.Ribo´1,E.Ros2,J.M.Paredes1,M.Massi2,andJ.Mart´ı3
1 Departamentd’AstronomiaiMeteorologia,UniversitatdeBarcelona,Av.Diagonal647,08028Barcelona,Spain
4 2 Max-Planck-Institutfu¨rRadioastronomie,AufdemHu¨gel69,53121Bonn,Germany
0
3 DepartamentodeF´ısica,EscuelaPolite´cnicaSuperior,UniversidaddeJae´n,VirgendelaCabeza2,23071Jae´n,Spain
0
2
Received2August2002/Accepted27August2002

n
a
Abstract. In an attempt to increase the number of known microquasars, Paredes et al. (2002) have presented a long-term
J
project focused on the search for new objects of this type. They performed a cross-identification between X-ray and radio
3
catalogsunder veryrestrictiveselectioncriteriaforsourceswith|b| < 5◦,andobtained asampleof13radio-emittingX-ray
2
sources.Follow-upobservationsof6ofthesesourceswiththeVLAprovidedaccuratecoordinates,whichwereusedtodiscover

1 opticalcounterpartsforallofthem.WehaveobservedthesesixsourceswiththeEVNandMERLINat5GHz.Fiveofthesix
v objectshavebeendetectedandimaged,presentingdifferentmorphologies:onesourcehasatwo-sidedjet,threesourceshave
6 one-sidedjets,andonesourceiscompact.Withallthepresentlyavailableinformation,weconcludethattwoofthesourcesare
1 promisingmicroquasarcandidatesinourGalaxy.
5
1 Keywords. X-rays:binaries–radiocontinuum:stars
0
4
0
1. Introduction Nevertheless,thelackofmeaningfulstatisticalstudiesbecause
/
h
ofthesmallpopulationofmicroquasarswithknownjetveloci-
p Microquasarsarestellar-massblackholesorneutronstarsthat
ties,preventsanydefinitivestatementsofthiskindbeingmade.
- mimic,onsmallerscales,manyofthephenomenaseeninAGN
o Therefore,itisworthsearchingfornewmicroquasarsinorder
r and quasars. Microquasars have been found in X-ray binary toincreasetheknownpopulation.
t
s systems,wherea compactobjectaccretesmatterfroma com-
a panionstar.RadioEmittingX-rayBinaries(REXBs)withrela- In this context, the sources to search for are REXBs,
v: tivisticradiojets,likeSS433,GRS1915+105,GROJ1655−40 which are in fact microquasar candidates. To this end, a
i orCygX-3,aregoodexamplesofmicroquasars(seeMirabel& cross-identification between the X-ray ROSAT all sky Bright
X
Rodr´ıguez1999foradetailedreview),whileotherwellknown Source Catalog (RBSC) (Voges et al. 1999) and the NRAO
r
a sources like LS I +61 303 could turn out to be new relativis- VLA SkySurvey(NVSS) (Condonetal.1998) wasmadefor
tic jetsources(Massietal.2002). Withthe recentadditionof sources with |b| < 5◦ under very restrictive selection crite-
LS 5039 (Paredes et al. 2000), Cyg X-1 (Stirling et al. 2001) ria, and the obtained results have been presented in Paper I.
and XTE J1550−564 (Hannikainenet al. 2001) to the micro- A sample containing 13 sources was obtained,and 6 of them
quasargroup,thecurrentknownnumberofthiskindofsources were observed at radio wavelengths in order to obtain radio
is14,among∼ 50REXBswithinthe∼ 280knownX-raybi- spectra and their variability, as well as accurate radio posi-
naries(Liuetal.2000;Liuetal.2001).Recentstudiesofmi- tions, which allowed the authors to discover the correspond-
croquasarscanbefoundinCastro-Tiradoetal.(2001). ing optical counterparts for all of them. At the end of this
study, two of the sources, namely 1RXS J001442.2+580201
As pointed out in Sect. 1 of Paredes et al. (2002, here-
and 1RXS J013106.4+612035 were classified as promising
after Paper I), the number of known microquasars remains
microquasar candidates. The remaining four sources were
still small, especially when trying to study them from a sta-
classified as weaker candidates for a number of reasons.
tisticalpointofview.Aninterestingaspectofmicroquasarsis
1RXSJ042201.0+485610showsahighlyinvertedspectrumat
theirpossibilityofbeingrelatedtounidentifiedhigh-energyγ-
high radio frequenciesand an optical counterpartslightly ex-
ray sources, as suggested by Paredes et al. (2000). Moreover,
tended.1RXSJ062148.1+174736isanobjectextendedinthe
some parameters like the jet velocity, seem to be related to
optical. 1RXS J072259.5−073131presents a one-sided radio
the mass of the compact object (i.e., with its potential well).
jetatarcsecondscales, supportingthepossibilityofbeingex-
Sendoffprintrequeststo:M.Ribo´, tragalactic.Andfinally,1RXSJ072418.3−071508is a known
e-mail:[email protected] quasar.
2 M.Ribo´ etal.:EVNandMERLINobservationsofmicroquasarcandidatesatlowgalacticlatitudes
Table1.MERLINpositionsforthefivedetectedtargetsources,obtainedviaphase-referencing.Theuncertaintiesquotedforthetargetsources
wereprovidedbythejmfittaskinaips(measuringinthephase-referencedimage)anddonotincludesystematicerrors.Thepositionsofall
phase-referencecalibratorsexcept1RXSJ072418.3−071508haveanaccuracybetterthan1mas.
Targetsources Phase-referencecalibrators
1RXSname α(J2000.0) δ(J2000.0) Sourcename α(J2000.0) δ(J2000.0)
J001442.2+580201 00h14m42.s12822+58◦02′01.′′2460 J0007+5706a 00h07m48.s47110+57◦06′10′.′4540
±0s.00013 ±0′.′0022
J013106.4+612035 01h31m07.s23210+61◦20′33.′′3752 J0147+5840a 01h47m46.s54380+58◦40′44′.′9750
±0s.00014 ±0′.′0016
J062148.1+174736 06h21m47.s75264+17◦47′35.′′0818 J0630+1738b 06h30m07.s25870+17◦38′12′.′9300
±0s.00006 ±0′.′0017
J072259.5−073131 07h22m59.s68188−07◦31′34.′′8009 1RXSJ072418.3−071508c 07h24m17.s2912 −07◦15′20′.′339
±0s.00011 ±0′.′0022
J072418.3−071508d 07h24m17.s2912 −07◦15′20.′′339 J0730−116 07h30m19.s1125 −11◦41′12′.′601
±0s.0007 ±0′.′010
a PositionfromPatnaiketal.(1992).
b PositionfromBrowneetal.(1998).
c ThispositionwasobtainedfromVLAobservations(listedinthelinebelow).
d TheentriesinthislinecorrespondtoVLAobservations(seePaperI).
InthispaperwepresentVeryLongBaselineInterferometry Astronomical Image Processing System (aips, developed
(VLBI) observations of these six sources, aimed at revealing and maintained by the US National Radio Astronomy
possiblejet-likefeaturesatmilliarcsecondscales.Wedescribe Observatory).Wedidnotdetect1RXSJ042201.0+485610.All
the observationsandthe data reductionin Sect. 2, presentthe othersourcesweredetected,andaccuratepositionsforthetar-
resultsandadiscussioninSect.3,andwesummarizeourfind- get radio sources were obtained via phase-referencing.These
ingsinSect.4. positions, presented in Table 1, were used later as a priori
information for the VLBI correlation. The position given in
Table1forthequasar1RXSJ072418.3−071508wasobtained
2. Observationsanddatareduction
from the VLA observations presented in Paper I. The posi-
tion of 1RXS J072259.5−073131is deduced from the phase-
We observed the six sources studied in Paper I simultane-
referenceoffsetrelativeto1RXSJ072418.3−071508provided
ously with the Multi-Element Radio-Linked Interferometer
byMERLIN.
Network(MERLIN)andtheEuropeanVLBINetwork(EVN)
onFebruary29th/March1st2000(23:30–23:05UT)at5GHz. To image the sources, we averaged the data in frequency
Since some of the target radio sources were faint, we sched- and exported them to be processed into the difference map-
uled the observations introducing phase-reference calibra- ping software difmap (Shepherd et al. 1994), where we time-
tors with cycle times of around 7 min (compatible with averagedthedatain32sbinsaftercarefulediting.
the expected coherence times). Apart from the calibrators
presented in Table 1, 4C 61.02 was used as calibrator for
2.2.EVN
1RXSJ013106.4+612035,andTXS0422+496andNRAO150
for 1RXS J042201.0+485610. We also observed the fringe- TheEVNobservationswereperformedwiththefollowingar-
finderDA193andtheMERLINfluxdensitycalibrator3C286. ray(name,code,location,diameter):Effelsberg,EB,Germany,
Single dish flux density measurements were carried out with 100 m; JodrellBank, JB, U.K., 25 m; Cambridge,CM, U.K.,
theMPIfR100mantennainEffelsberg,Germany. 32m;Westerbork,WB,TheNetherlands,14×25m;Medicina,
MC,Italy,32m;Noto,NT,Italy,32m;Shanghai,SH,China,
25 m; Torun´,TR, Poland, 32 m; and Onsala85,ON, Sweden,
2.1.MERLIN
25m.DatawererecordedinMkIVmodewith2-bitsamplingat
MERLIN is a connectedradiointerferometeracrossEngland, 256Mbpswithlefthandcircularpolarization.Abandwidthof
with baselines reaching up to 217 km length. This array ob- 64 MHz was used, dividedinto8 intermediatefrequency(IF)
servedwith2-bitsamplingatdualpolarisationwithtwoblocks bands.
of 16 channels, each channel of 1 MHz bandwidth. We anal- The data were processed at the EVN MkIV correlator at
ysed the left hand circular polarisation data excluding one the Joint Institute for VLBI in Europe (JIVE), in Dwingeloo,
channelat both edgesof the band,yielding a final bandwidth The Netherlands. The correlator integration time was of 4 s.
of14MHz.Thecorrelatorintegrationtimewasof4s. A first post-processing analysis was also carried out at JIVE.
The MERLIN data reduction was carried out at Jodrell The data were processed using aips. A first a priori vis-
Bank Observatory, using standard procedures within the ibility amplitude calibration was performed using antenna
M.Ribo´ etal.:EVNandMERLINobservationsofmicroquasarcandidatesatlowgalacticlatitudes 3
gainsandsystemtemperaturesmeasuredateachantenna.The weaveragedthedifferentGaussians.Welinkedthefluxdensity
fringe fitting (fring) of the residual delays and fringe rates scalebyobservingprimarycalibratorssuchas3C286,3C48,
was performed for all the radio sources. No fringes were orNGC7027(seee.g.Kraus1997;Pengetal.2000).We list
foundfor1RXSJ042201.0+485610and4C61.02.Fringesfor the single dish flux density measurements in the second col-
many baselines were missing for 1RXS J001442.2+580201, umn of Table 2. The flux density values for the main VLBI
1RXSJ062148.1+174736andTXS0422+496. calibratorswereof7.5±0.1Jyfor3C286,and5.9±0.2Jyfor
To improve the fringe detection on all baselines for DA193.
1RXS J001442.2+580201 and 1RXS J062148.1+174736 we
used the delay, rate, and phase solutions from their corre-
3. Resultsanddiscussion
spondingphase-referencecalibrators(J0007+5706,1◦18′ sep-
aration, and J0630+1738, 1◦59′ separation) and interpolated WepresentalltheimagingresultsinFigs.1–5,andtheimage
themtothetargetsourcesusingtheaipstaskclcal.Wefringe- parametersinTable2.Thetotalfluxdensityvaluesforthedif-
fitted the targetsourcesagainusing narrowersearch windows ferentimagesdivergefromeachotherandfromthesingledish
and obtained solutions for all baselines. A similar attempt on measurements,duetotheamplitudeself-calibrationprocessin
1RXSJ042201.0+485610(withrespecttoTXS0422+486and all cases. Therefore, those values should be considered with
NRAO 150) was unfruitful. Effelsberg was used as reference care. Theminimumcontoursin the imagesare thoselisted as
antennathroughouttheaipsdatareductionprocess. Smin in Table 2, while consecutivehighercontoursscale with
31/2.Herefollowsadetaileddiscussiononeachsource.
Wethenaveragedthedatainfrequencyandexportedthem
to be imaged and self-calibrated in difmap. The a priori visi-
bilityamplitudecalibrationwasnotsufficienttoreliablyimage 3.1.1RXSJ001442.2+580201anditstwo-sidedjet
the weakest radio sources. We improvedthat by first imaging
indifmapthecalibratorsourcesJ0007+5706,J0147+5840,and Ascanbeseeninourimages,showninFig.1,thissourceap-
J0630+1738, with appropriate amplitude self-calibration. We pearspoint-likeatMERLINresolution,partiallyresolvedinthe
tapered EVN+MERLIN image and clearly resolved at EVN
deducedcorrectionfactorsfor each antenna, these being con-
scales. In this last case, it shows a two-sided jet-like struc-
sistentforthethreeradiosourceswithin2%.Wecorrectedthe
amplitudecalibrationbackinaipsandexportedthedataagain ture roughly in the north-south direction, with brighter com-
intodifmap,wherethefinalimagingwasperformedafteredit- ponentstowardsthesouth.Thetrendsarevisibleintheclosure
phases,givingusconfidencethatthestructureobservedisnot
ingandaveragingofthevisibilitiesin32sblocks.
aconsequenceofsidelobesorimagingartifacts.Inthetapered
EVN+MERLINimages,thestructureextendsupto20–30mas
2.3.CombiningEVNandMERLIN outside of the core, and more clearly towards the north. This
discrepancycouldbeduetocalibrationproblems.
TheEVNandMERLINarrayshaveonecommonbaseline,be-
Model fitting of the EVN visibilities with circular
tweenJBandCM,whichallowstocombinebothdatasetsand
Gaussians provides a parametrization of the inner structure.
map them together. We processed the data within aips in or-
Fivecomponentsreproducethevisibilities.Thecentralonehas
dertocombinebotharrays.TheB1950.0(u,v)coordinatesof
7.8 mJy, with a FWHM of 0.4 mas. Towards the north, one
theMERLINdatahadtobecorrectedtotheonesoftheEVN
component (N2) of 0.6 mJy at 3.6 mas (P.A. −10◦, FWHM
forthesamereferencesystem(J2000.0)withuvfix.Then,the
0.7mas)andanotherone(N1)of0.5mJyat8.6mas(P.A.2◦,
MERLINdatawereself-calibratedwith theEVNimages(see
extended over 3 mas) are needed. Brighter components are
below), and the phase solutions were limited to the longest
presentsouthwards,one(S2)of1.1mJyat5.0mas(P.A.177◦,
MERLIN baselines. The MERLIN data were imaged and the
FWHM of 0.8 mas) and the other one (S1) of 0.4 mJy at
peak-of-brightnessof bothdata sets were checkedto be simi-
13.7mas(P.A.177◦,FWHMbelow0.3mas).
lar. As a nextstep, the EVN data were averagedin frequency
Ifwe assume thatcomponentsS1 andN1 correspondto a
to correspond to the MERLIN data. The aips headers of both
pairofplasmacloudsejectedatthesameepochnearthecom-
data sets were modified conveniently to match together, and
pact object and perpendicularly to the accretion disk, we can
the weightingof bothdata sets was also modifiedto be equal
estimate some parameters of the jets by using the following
with wtmod. Finally, both data sets were concatenated (using
equation:
dbcon)andexportedtodifmaptobeimagedwithdifferentdata
weighting in (u,v) distance (tapering) after time averaging in βcosθ= µa−µr = da−dr , (1)
32sbins. µ +µ d +d
a r a r
βbeingthevelocityofthecloudsinunitsofthespeedoflight,
2.4.Fluxdensitymeasurementsatthe100mantenna θ the angle between the direction of motion of the ejecta and
the line of sight and µ and µ the proper motions of the ap-
inEffelsberg a r
proaching and receding components, respectively (Mirabel &
Weinterleavedcross-scans(inazimuthandelevation)withthe Rodr´ıguez1999).Althoughwedonotknowtheepochofejec-
100mEffelsbergantennatomeasuretheradiosourcefluxden- tionoftheclouds,wecancancelthetimevariablebyusingthe
sities(A.Kraus,privatecommunication).WefittedaGaussian relativedistancesto the cored and d, as expressedin Eq. 1.
a r
functiontotheflux-densityresponseforeverycross-scan,and Sincebothvariables,βandcosθ,takevaluesbetween0and1,it
4 M.Ribo´ etal.:EVNandMERLINobservationsofmicroquasarcandidatesatlowgalacticlatitudes
Table2.FluxdensitiesandparametersusedtoproducetheimagesinFigs.1–5.
Singledish Arraya
1RXSname S (taperFWHM) beamsize P.A. S S S
EB tot peak min
[mJy] [Mλ] [mas]×[mas] [◦] [mJy] [mJybeam−1] [mJybeam−1]
J001442.2+580201 6.5±0.5b M 57×51 37 6.2 5.8 0.1
E+M(10) 7.7×7.2 −4 10.1 9.9 0.18
E 1.77×0.86 −20 11.5 7.0 0.15
J013106.4+612035 20.1±0.6 M 71×39 −63 17.5 17.9 0.5
E+M(15) 7.8×6.2 −75 19.2 18.7 0.8
E 1.04×0.99 −1 17.6 11.6 0.4
J042201.0+485610 <5 — — — — — —
J062148.1+174736 7.1±0.7b M 88×39 24 5.8 5.6 0.2
E+M(8) 13.1×11.1 −24 6.0 6.8 0.3
E 8.8×4.1 47 7.0 6.4 0.3
J072259.5−073131 67.9±1.1 M 115×66 4 66.0 62.9 0.7
E+M(15) 9.5×7.3 −61 52.1 41.9 0.9
E 5.74×1.12 11 46.9 36.2 0.9
J072418.3−071508 282.2±4.1 M 120×64 8 301.0 285.7 0.9
E+M(10) 11.7×9.8 −61 287.0 263.4 2.0
E 5.73×1.02 11 248.0 184.8 0.7
a M:MERLIN.E+M:EVN+MERLIN,FWHMofthetaperingfunction(weightingofvisibilities)inparenthesis.E:EVN.
b ValueswithlowSNRintheGaussianfits.
N1
N2
S2
S1
Fig.1.Imagesof1RXSJ001442.2+580201usingthedifferentarraysandwiththeparametersgiveninTable2.Theaxesunitsareinmas.A
(u,v)-taperingwithaFWHMat10MλhasbeenusedtoperformthecombinedEVN+MERLINimage.IntheEVNimage,N1,N2,S2andS1
indicatethecomponentsdiscussedinthetext.
isclearthatknowingβcosθallowsustocomputealowerlimit of the components using the following equation (Mirabel &
forthe velocity(β ) and an upperlimit forthe angle (θ ). Rodr´ıguez1999):
min max
The same applies for the S2 and N2 components. In Table 3
1/(k−α)
we list the positions of the components obtained from model (cid:16)Sa/Sr(cid:17) −1
βcosθ= , (2)
fitting,togetherwiththederivedvaluesfromβmin andθmax for 1/(k−α)
eachoneofthepairs.Theslightlydifferentresultsobtainedus- (cid:16)Sa/Sr(cid:17) +1
ing pair 1 or 2, could be due to the fact that the position for
whereS andS arethefluxdensitiesoftheapproachingand
a r
theS1componentobtainedwithmodelfittinghappenstobeat
recedingcomponents,respectively,kequals2foracontinuous
the lower part of this elongated component, hence increasing
jetand3fordiscretecondensations,andαisthespectralindex
βcosθ, or to intrinsic different velocities for each one of the
of the emission (S ∝ ν+α). However, the equation above is
pairs.Hereafterwewilluseβ>0.20±0.02andθ<78±1◦. ν
onlyvalidwhenthecomponentsareatthesamedistancefrom
thecore.Ifthisisnotthecase(i.e.,d <d ),theratioS /S will
r a a r
A similar approach to obtain the jet parameters of the belowerthantheonethatshouldbeusedinEq.2(becausethe
source can be performed thanks to the brightness asymmetry fluxdensitydecreaseswithincreasingdistancefromthecore).
M.Ribo´ etal.:EVNandMERLINobservationsofmicroquasarcandidatesatlowgalacticlatitudes 5
Table 3. Model fitted positions of the components in the wardsthenorthwestintheEVNimage.Modelfittingwithcir-
1RXSJ001442.2+580201radiojetsandobtainedjetparameters. cularGaussiancomponentscanreproducetheobservedvisibil-
ities as follows:a central15.4mJy componentwith 0.64mas
Comp. Distance P.A. β θ FWHM,andanorthwest2.1mJycomponentwithaFWHMof
min max
[mas] [◦] [◦] 0.74mas,locatedat1.8masinP.A.−73◦.Ascanbeseen,this
lastcomponentis locatedata distanceof ∼ 2 timesthebeam
N1 8.6±0.3 2 0.23±0.02 77±1
sizefromthecore.
N2 3.6±0.2 −10 0.16±0.02 81±1
Using the fact that we do not detect a counter-jet,we can
S2 5.0±0.1 177 0.16±0.02 81±1
S1 13.7±0.3 177 0.23±0.02 77±1 useEq.2replacingSrwiththe3σlevelvalue.This,ofcourse,
willonlyprovidealowerlimittoβcosθ,expressedasfollows:
1/(k−α)
(cid:16)Sa/3σ(cid:17) −1
βcosθ> . (3)
Inconsequence,Eq.2onlyallowsustoobtainalowerlimitfor 1/(k−α)
βcosθ. (cid:16)Sa/3σ(cid:17) +1
In order to use this approach we will consider k = 3, Using Sa = 2.1 mJy, 3σ = 0.30 mJy (the 1σ value has
because the components seem to be discrete condensations, been taken as the root mean square noise in the image), α =
and α = −0.20 ± 0.05, according to the overall spectral in- −0.05±0.05(seePaperI),andk = 3tobeconsistentwiththe
dex reported in Paper I, since we do not have spectral index lowestlimit,weobtainβcosθ > 0.31±0.05(β > 0.31±0.05
information of the components. The use of the flux densities andθ < 72±3).Hence,a lowerlimitof β ≥ 0.3is obtained,
obtained after model fitting gives βcosθ > 0.09 ± 0.04 for pointingtowardsrelativisticradiojetsastheoriginoftheelon-
the S2–N2 pair and βcosθ > −0.07 ± 0.08 for the S1–N1 gatedradioemissionpresentintheEVNimage.Althoughthe
pair. This last value is certainly surprising,althoughit can be one-sidedjetmorphologyatmasscalesisfoundmostlyinex-
explained by the fact that the S1 flux density obtained after tragalacticsources,itisalsopresentinsomegalacticREXBs,
modelfittingdoesnotaccountforthetotalfluxofthisplasma like Cyg X-3 (Mioduszewski et al. 2001) or LS I +61 303
cloud.Infact,betterestimatesofthefluxdensitiescanbeob- (Massietal.2001).Hence,wecannotruleoutapossiblegalac-
tained summing together the flux densities of the clean com- ticnatureonthebasisofthedetectedmorphology.
ponents obtained within each one of the four plasma clouds. Asdonefortheprevioussource,wecancomparetheVLA
This yields to βcosθ > 0.13± 0.05 for the S2–N2 pair and position with the MERLIN one, and find that they differ in
βcosθ > 0.17±0.02 for the S1–N1 pair, in good agreement ∆αcosδ = 39 ± 10 mas and ∆δ = −0.8 ± 10 mas. If the
withthevaluescomputedusingthedistancesfromthecompo- offsets are real, this would imply µαcosδ = 64±16 mas yr−1
nentstothecore,showninTable3. andµδ = −1±16masyr−1. Hence,itseemspossiblethatwe
If we compare the VLA position reported in Paper I with havedetectedapropermotioninrightascensionata4σlevel.
theMERLINpositioninTable1wecanseethattheyarediffer- However,wemustbecautioussince,asinthepreviouscase,the
ent.Infact,takingintoaccounttheerrors,theMERLIN−VLA phase-referencesourcesandobservingfrequencieswerediffer-
position offsets can be expressed as: ∆αcosδ = 16±10 mas entineachobservation.
and ∆δ = 27 ± 10 mas. Assuming that the difference in po- Overall,theseresultsareindicativeofrelativisticradiojets,
sition is due to intrinsic proper motions and considering the andhence,togetherwiththeresultsreportedinPaperI, allow
time span between both observations, 224 days, we obtain: ustoclassifythissourceasapromisingmicroquasarcandidate.
µ =26±16masyr−1andµ =44±16masyr−1.Although
αcosδ δ
apropermotionofthesourcewouldclearlyindicateitsmicro-
3.3.1RXSJ042201.0+485610,anon-detectedsource
quasarnature,becausenopropermotionwouldbedetectedin
an extragalactic source, we must be cautious with this result.
First ofall, the phase-referencesourceswhere differentin the This radio source was marginallyseen in the cross-scans car-
VLAandMERLINobservations,aswellastheobservingfre- ried outin Effelsberg.In fact, this is compatible with the low
quencies from which positions were estimated. On the other flux density of 2.3±0.4 mJy at 1.4 GHz listed in the NVSS.
hand,noneoftheseresultsexceedsthe3σvalue,preventingto ThesourcewasnotdetectedwithMERLINorwiththe EVN,
statethatapropermotionhasbeendetected. mainly due to problems with the phase-reference sources, as
Summarizing, our results indicate that this source ex- pointedoutinSect.2.2.Infact,asdiscussedinPaperI,VLAA
hibits relativistic radio jets with β > 0.20 and, therefore, configuration observations showed a flux density of 0.4 mJy
together with the results reported in Paper I, we consider at5GHzandaninvertedspectrumwithaspectralindexupto
1RXS J001442.2+580201 as a very promising microquasar α=+1.6,suggestingthermalradio-emissionresolvedathigher
candidate. angularresolutions.However,wecannotexcludethepossibil-
ityofhavingahighlyvariablesource.
3.2.1RXSJ013106.4+612035anditsone-sidedjet
3.4.1RXSJ062148.1+174736,acompactsource
We show in Fig. 2 the images obtained after our observa-
tions. Although the source appears compact at MERLIN and Ascanbeseeninourimages,showninFig.3,theradiosource
EVN+MERLINscales,thereisaweakone-sidedradiojetto- iscompactonallscales.Infact,modelfittingoftheEVNvisi-
6 M.Ribo´ etal.:EVNandMERLINobservationsofmicroquasarcandidatesatlowgalacticlatitudes
Fig.2.Imagesof1RXSJ013106.4+612035usingthedifferentarraysandwiththeparametersgiveninTable2.Theaxesunitsareinmas.A
(u,v)-taperingwithaFWHMat15MλhasbeenusedtoperformthecombinedEVN+MERLINimage.
Fig.3.Imagesof1RXSJ062148.1+174736usingthedifferentarraysandwiththeparametersgiveninTable2.Theaxesunitsareinmas.A
(u,v)-taperingwithaFWHMat8MλhasbeenusedtoperformthecombinedEVN+MERLINimage.
bilities convergesto a point-like radio source (the FWHM of EVN+MERLINimagesshowaclearone-sidedjettowardsthe
a circular Gaussian tends to zero). We must note that when east,withaslightbenttowardsthesouthatlargercoresepara-
this source was observed it was below the horizon in SH. tions.Theclosurephasesclearlyshow thatthesourcedeparts
Therefore,theobtainedbeamsizefortheEVNimageislarger fromsymmetry,withpreferredemissiontotheeast,bothinthe
than the ones obtained for the other sources, as can be seen MERLINandtheEVNdatasets.Twodistinctcomponentsare
in Table 2, hence providing lower angular resolution than in presentintheEVNimage,at9 and17masfromthe compact
theothercases.AcomparisonbetweentheVLAandMERLIN core(P.A.of89and113◦,respectively).Thosecomponentscan
positionsrevealsthattheyareperfectlycompatiblewithinthe bemodelfittedwithellipticalGaussians,yieldingfluxdensities
errors,suggestinganextragalacticnatureorasmallpropermo- of3.7and2.8mJy,respectively,foracoreof40.3mJy.
tion if it turns out to be a galactic microquasar. Although the
compactnessofthesourceisnotindicativeofagalacticoran Usingagainthefactthatwedonotdetectacounter-jet,we
extragalacticorigin,theextendedopticalcounterpart,reported canuseEq.3withSa =3.7mJy(theclosestcomponenttothe
in Paper I, suggests an extragalactic origin for this source. In corein theEVNimage),3σ = 0.54mJy (aspreviouslydone,
fact, the detected radio variability from 8 to 11 mJy within a the 1σ value has been taken as the rootmean square noise in
week reportedin PaperI,is compatiblewith thecompactness the image), α = −0.25± 0.2 (see Paper I), and k = 3 to be
in an extragalactic object (see IDV phenomenon, Wagner & consistentwiththediscretenatureofthecomponents,toobtain
Witzel1995). βcosθ>0.29±0.05,andhenceβ>0.29±0.05andθ<73±3◦.
Therefore,these results pointtowardsrelativistic radiojets as
theoriginoftheelongatedradioemissionpresentintheimages.
3.5.1RXSJ072259.5−073131anditsbentone-sided
jet Aspointedoutfor1RXSJ013106.4+612035,theone-sided
jetmorphologydoesnotruleoutamicroquasarnatureforthis
The images obtained at different resolutions are plotted in object. However, the bending of the jet at such small angular
Fig.4.TheMERLINimagepresentsacompactstructurewith scales resembles the ones seen in blazars. In fact, as reported
some elongation eastwards, while the EVN and combined in Paper I, a one-sided arcsecond scale jet is also present in
M.Ribo´ etal.:EVNandMERLINobservationsofmicroquasarcandidatesatlowgalacticlatitudes 7
Fig.4.Imagesof1RXSJ072259.5−073131usingthedifferentarraysandwiththeparametersgiveninTable2.Theaxesunitsareinmas.A
(u,v)-taperingwithaFWHMat15MλhasbeenusedtoperformthecombinedEVN+MERLINimage.
Fig.5.Imagesofthealreadyidentifiedquasar1RXSJ072418.3−071508 usingthedifferentarraysandwiththeparametersgiveninTable2.
Theaxesunitsareinmas.A(u,v)-taperingwithaFWHMat10MλhasbeenusedtoperformthecombinedEVN+MERLINimage.
VLAAconfigurationobservationsat1.4GHz,anunusualfea- the observations. It presents (Fig. 5) a one-sided pc-scale jet
tureinthealreadyknownmicroquasars. orientedtowardsthenortheast,changingfromaP.A.of∼ 50◦
A comparison between the VLA and MERLIN positions at5masfromthecore(EVNimage)to20◦ upto200mas,at
reveals that they agree within the errors, suggesting an extra- MERLINscales.
galacticnatureor a smallpropermotionif itturnsouttobe a Model fitting of the EVN visibilities with circular
galacticmicroquasar. Gaussians reveals a compact core (0.7 mas FWHM) with
245.4 mJy, and two distinct components, one with 11.9 mJy
Overall, these results are indicative of relativistic radio
at6.2mas(P.A.46◦,1.9masFWHM,presentintherightpanel
jets,althoughthissourceshowscharacteristicsmoresimilarto
image ofFig. 5) andanotheronewith the size of the beam,a
blazarsthantomicroquasars.
flux density of 1.7 mJy at a distance of 29.6 mas in P.A. 27◦
(visibleinthemiddlepanelimageofFig.5).
3.6.1RXSJ072418.3−071508,aquasarwithabent Usingagainthefactthatwedonotdetectacounter-jet,we
one-sidedjet canuseEq.3withS =11.9mJy(theclosestcomponenttothe
a
core),3σ=0.45mJy,α=0.07±0.02(seePaperI),andk=3
Thisradiosourcehasrecentlybeen(March2002)classifiedas
tobeconsistentwiththediscretenatureofthecomponents,to
aquasarintheSIMBADdatabase,anditisnotanymoreami- obtainβcosθ>0.51±0.04.Hence,anupperlimitofθ<60±3◦
croquasarcandidate.ItiscataloguedintheParkes-MIT-NRAO and a lower limitof β > 0.51±0.04is obtained,pointingto-
(Griffith et al. 1994) and the Texas Survey (Douglas et al.
wardsrelativisticradiojetsastheoriginoftheelongatedradio
1996). It is listed as PMN J0724−0715in the NED database,
emissionpresentintheimagesofthisalreadyidentifiedquasar.
andisthesourceWGAJ0724.3−0715inPerlmanetal.(1998),
who reported a faint and quite broad Hα emission line (rest-
4. Summary
frame W = 30.3 Å, FWHM=4000 km s−1), and classified
λ
it as a Flat Spectrum Radio Quasar (FSRQ) with z = 0.270. We have presented EVN and MERLIN observations of the
Nevertheless, we have reported here our observationalresults six sources studied by Paredes et al. (2002) in their search
for this source, since it was a candidate when we performed for microquasarcandidates at low galactic latitudes. The first
8 M.Ribo´ etal.:EVNandMERLINobservationsofmicroquasarcandidatesatlowgalacticlatitudes
Table4.SummaryoftheobtainedresultsaftertheVLAandopticalobservations(PaperI),andtheEVNandMERLINobservationsreported
here.Anasteriskindicatesanon-expectedbehaviourformicroquasars.
1RXSname VLAobs. Opticalobs. EVN+MERLINobs. Notes
Structure α Structure Imag. Structure β θ[◦]
J001442.2+580201 compact −0.2 point-like 19.9 two-sidedjet >0.20 <78 Promisingcandidate
J013106.4+612035 compact −0.1 point-like 17.9 one-sidedjet >0.31 <72 Promisingcandidate
J042201.0+485610 compact +1.6∗ extended∗ 17.5 notdetected∗ — — Thermalsource?
J062148.1+174736 compact +0.1 extended∗ 17.6 compact — — Galaxy?
J072259.5−073131 one-sidedjet∗ −0.2 point-like 16.8 bentone-sidedjet∗ >0.29 <73 Blazar?
J072418.3−071508 compact +0.1 point-like 17.2 bentone-sidedjet∗ >0.51 <60 Quasar(FSRQ)
one, namely 1RXS J001442.2+580201, displays a two-sided scaleFacilitiesprogrammeundercontractNo.ERBFMGECT950012.
radio jet, which after analysis implies β > 0.20 ± 0.02 and Part of the data reduction was done at JIVE withthe support of the
θ < 78±1◦. 1RXS J013106.4+612035,displays a one-sided European Community - Access to Research Infrastructure action of
radiojet,requiringβ > 0.31±0.05andθ < 72±3◦.Thethird theImprovingHumanPotentialProgrammeundercontractNo.HPRI-
one, namely 1RXS J042201.0+485610,was not detected due CT-1999-00045. M. R.,J. M. P.and J. M. acknowledge partial sup-
port by DGI of the Ministerio de Ciencia y Tecnolog´ıa (Spain) un-
to its low flux density and/or to phase-referencing problems.
dergrantAYA2001-3092, aswellaspartialsupportbytheEuropean
1RXSJ062148.1+174736appearedcompactatallscales.The
Regional Development Fund (ERDF/FEDER). During this work,
fifth one, namely 1RXS J072259.5−073131, displays a bent
M.R.hasbeensupportedbytwofellowshipsfromCIRIT(Generalitat
one-sidedradiojet,implyingβ>0.29±0.05andθ< 73±3◦.
deCatalunya,ref.1998BEAI200293and1999FI00199).J.M.has
Finally,1RXSJ072418.3−071508showsalsoabentone-sided beenaidedinthisworkbyanHenriChre´tienInternationalResearch
jet,requiringβ>0.51±0.04andθ<60±3◦. Grant administered by the American Astronomical Society, and has
After a detailed analysis of our data, we show beenpartiallysupportedbytheJuntadeAndaluc´ıa.Thisresearchhas
in Table 4 a summary of the results obtained after madeuseoftheNASA’sAstrophysicsDataSystemAbstractService,
the VLA and optical observations (Paper I), and the oftheSIMBADdatabase,operatedatCDS,Strasbourg,France,andof
EVN+MERLIN observations reported here. As can be theNASA/IPACExtragalacticDatabase(NED)whichisoperatedby
seen, the first two sources, 1RXS J001442.2+580201 and theJetPropulsionLaboratory,CaliforniaInstituteofTechnology,un-
dercontractwiththeNationalAeronauticsandSpaceAdministration.
1RXS J013106.4+612035, are promising microquasar candi-
dates.1RXSJ042201.0+485610isprobablyofthermalnature
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