Compact radio emission in Ultraluminous X-ray sources
aa r X i v : . [ a s t r o - ph . C O ] M a r Astron. Nachr. / AN , No. 99, 99 – 103 (2011) /
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Compact radio emission in Ultraluminous X-ray sources
M. Mezcua ⋆ and A.P. Lobanov
Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, GermanyThe dates of receipt and acceptance should be inserted later
Key words
Galaxies: general – X-rays: general – ISM: HII regionsWe present results from our studies of radio emission from selected Ultraluminous X-ray (ULX) sources, using archivalGiant Metrewave Radio Telescope (GMRT) data and new European VLBI Network (EVN) observations. The GMRT dataare used to find possible faint radio emission from ULX sources located in late-type galaxies in the Chandra Deep Fields.No detections are found at 235, 325 and 610 MHz, and upper limits on the radio flux densities at these frequencies aregiven. The EVN observations target milliarcsecond-scale structures in three ULXs with known radio counterparts (N4449-X1, N4088-X1, and N4861-X2). We confirm an earlier identification of the ULX N4449-X1 with a supernova remnant andobtain the most accurate estimates of its size and age. We detect compact radio emission for the ULX N4088-X1, whichcould harbour an intermediate mass black hole (IMBH) of 10 M ⊙ accreting at a sub-Eddington rate. We detect a compactradio component in the ULX N4861-X2, with a brightness temperature > K and an indication for possible extendedemission. If the extended structure is confirmed, this ULX could be an HII region with a diameter of 8.6 pc and surfacebrightness temperature ≥ K. The compact radio emission may be coming from a ∼ M ⊙ black hole accreting at0.1 ˙ M Edd . c (cid:13) Several scenarios have been proposed to explain the highluminosities ( L X > erg/s) of Ultraluminous X-raysources (ULXs), but none of them is able to reveal the phys-ical nature of all ULXs. If ULXs are powered by accretionat the Eddington rate, this would imply accreting compactobjects of masses - M ⊙ . Such intermediate compactobjects can only be black holes (Colbert & Mushotzky 1999)and they would be the missing link between stellar massblack holes and supermassive black holes in the nuclei ofgalaxies. These Intermediate Mass Black Holes (IMBHs)could form from the death of very massive and hot starsor from multiple stellar interactions in dense stellar clusters(Portegies Zwart 2003). It has also been suggested that ULXobjects may harbour secondary nuclear black holes in post-merger galaxies (Lobanov 2008), with masses in excess of M ⊙ and accreting at sub-Eddington rates. Alternatively,ULXs could be neutron stars or stellar mass BHs apparentlyradiating at super-Eddington luminosities (Begelman 2002).Radio observations of ULXs bear an excellent poten-tial for uncovering the nature of these objects, by detectingand possibly resolving their compact radio emission, mea-suring its brightness temperature and spectral properties,and assessing the physical mechanism for its production.Few ULXs have been studied in the radio domain (Kaaretet al. 2003; K¨ording et al. 2005) and a small sample of ⋆ e-mail: [email protected] of the International Max Planck Research School (IMPRS) forAstronomy and Astrophysics at the Universities of Bonn and Cologne. ULXs has been cross-identified in the existing radio cata-logs (S´anchez-Sutil et al. 2006).An increase of the number of radio detections and sub-sequent Very Long Baseline Interferometry (VLBI) studiesof detected radio counterparts could potentially help to clar-ify the nature of ULX sources. With this aim, we 1) ana-lyze archive images of the Chandra Deep Fields taken withthe Giant Meterwave Radio Telescope (GMRT) looking forfaint radio counterparts of the ULX sources located in thisfields; and 2) initiate an European VLBI Network (EVN)program to detect and study milliarcsecond-scale emissionin ULX objects with known radio counterparts. In Section 2,we present the two samples of ULX objects studied. Theobservations and data reduction are explained in Section 3.The results obtained are shown in Section 4, leading to adiscussion and final summary presented in Section 5.Throughout this paper we assume a Λ cold dark matter(CDM) cosmology with parameters H = 73 km s − M pc − , Ω Λ = 0 . and Ω m = 0 . . We use archival GMRT data to search for radio counter-parts of 24 ULX objects identified in the Chandra DeepField North (CDFN), Chandra Deep Field South (CDFS),and Extended CDFS (Lehmer et al. 2006). All these ULXshave luminosities L X ≥ erg/s in the 0.5-2.0 keV band,and are located in optically bright irregular and late-spiralgalaxies. Ten of the 24 X-ray sources appear to be coinci-dent with optical knots of emission, with optical propertiesthat are consistent with those of giant HII regions in the c (cid:13)
00 M. Mezcua & A.P. Lobanov: Instructions for authors local universe, suggesting that these ULX sources trace dis-tant star formation (Lehmer et al. 2006).The objects targeted in our EVN observations are se-lected from a sample of 11 ULX objects with radio coun-terparts (S´anchez-Sutil et al. 2006) in the VLA FIRST cat-alog (Becker et al. 1995). We select three ULX which arebrighter than 1 mJy and clearly away from the nuclear re-gion in their respective host galaxies. The first target, N4449-X4, is identified as the most luminous and distant memberof the class of oxygen-rich Supernova Remnants (SNRs)(Blair et al. 1983) and classified as an ULX source by Liu & Bregman 2005. A detailed study of this source is de-scribed in Mezcua & Lobanov (in prep.). The second target,N4088-X1, is an ULX located at a distance of 13.0 Mpc inthe asymmetric spiral galaxy NGC 4088. This ULX is lo-cated within the extended emission of a spiral arm and iscoincident with a conspicous maximum of radio emissionof 1.87 mJy (at 1.4 GHz) with an offset of 3.62” to the X-ray peak (S´anchez-Sutil et al. 2006). The ULX has a X-rayluminosity in the 0.3-8.0 keV band of 5.86 x erg/s (Liu & Bregman 2005, who don’t rule out the possibility of itbeing an HII region). The third target, N4861-X2, is locatedin the spiral galaxy NGC 4861 and has an X-ray luminosityof 8.4 x erg/s (Liu & Bregman 2005). Its radio coun-terpart is offset from the X-ray position by 1.97”. This ULXhas been suggested to coincide with an HII region poweredby massive early OB type stars (Pakull & Mirioni 2002).
We use archival GMRT data of the Hubble Deep Field North(overlapping with our CDFN region of interest) at 235 MHz(experiments 01NIK04 & &
610 MHz (experiments 03JAA01 & & EM072B)have been made on June 1st & Table 1
ULX in the CDFN at 235.5MHz.
Name S [mJy/beam]CXOHDFN J123631.66+620907.3 < . CXOHDFN J123632.55+621039.5 < . CXOHDFN J123637.18+621135.0 < . CXOHDFN J123641.81+621132.1 < . CXOHDFN J123701.47+621845.9 < . CXOHDFN J123701.99+621122.1 < . CXOHDFN J123706.12+621711.9 < . CXOHDFN J123715.94+621158.3 < . CXOHDFN J123721.60+621246.8 < . CXOHDFN J123723.45+621047.9 < . CXOHDFN J123727.71+621034.3 < . CXOHDFN J123730.60+620943.1 < . ing, and tapering of the longest baselines was applied forN4088-X1 and N4861-X2 to improve the detection of ex-tended emission. We show the final GMRT images obtained at 235 MHz and325 MHz in Fig. 1. The primary beam sizes are 114 ar-cmin at 235 MHz, 81 arcmin at 325 MHz, and 43 arcminat 610 MHz. The respective rms noise in the maps at eachfrequency are 1.4, 0.6, and 0.8 mJy/beam. No radio counter-parts of the ULX sources located in the CDFs are detectedin a circle radius for each ULX position of 28 arcsec, whichis more than 10 times the Chandra positional error circle.Upper limits on their flux densities at each frequency aregiven in Table 1 (for ULX located in the CDFN) and Table2 (ULXs in the CDFS), obtained by estimating the local rmsat the ULX locations. These upper limits range between 2-4.6 mJy at 235MHz, 1-2.5 mJy at 332 MHz, and 0.5-2 mJyat 610MHz. The position of only three ULX sources fall inthe images at 610MHz, thus only 3 upper limits are given atthis frequency.
In Fig. 2 we show the final images of N4088-X1 (left) andN4861-X2 (right). The noise levels achieved are 26 µ Jy/beamfor N4088-X1 and 3 µ Jy/beam for N4861-X2, and the restor-ing beams are 31 x 29 mas and 11 x 5 mas, respectively.For N4088-X1, we identify a compact component offlux density 0.1 mJy at a 5 σ level. The component is cen-tered atRA(J2000) = 12 h m s ± s ,DEC(J2000) = 50 ◦ ± B > K and an upper limit of 34 x 26 mas for thesize. Adopting a distance of 13.0 Mpc yields, for N4088-X1, an integrated 1.6 GHz radio luminosity of 3.8 x 10 erg/s. c (cid:13) stron. Nachr. / AN (2011) 101 GREY: HDF-N IPOL 235.500 MHZ FINAL HDFN.ICL001.1Grey scale flux range= -29.1 347.5 MilliJY/BEAM0 100 200 300 D E C L I NA T I O N ( J2000 ) RIGHT ASCENSION (J2000)12 42 41 40 39 38 37 36 35 3462 403020100061 5040 GREY: CDFS IPOL 332.125 MHZ FINAL CDFS.ICL001.1Grey scale flux range= -28.4 201.9 MilliJY/BEAM0 50 100 150 200 D E C L I NA T I O N ( J2000 ) RIGHT ASCENSION (J2000)03 34 30 00 33 30 00 32 30 00 31 30 00 30 30-27 304050-28 001020
Fig. 1
GMRT images of the HDFN at 235 MHz (left) and the CDFS at 325 MHz (right). The restoring beam sizes are20.1 x 16.3 arcsec at 235 MHz and 13.7 x 11.8 arcsec at 325 MHz. The best rms sensitivities achieved are 1.4, and 0.6mJy/beam, respectively. The ULXs positions are marked with squares.
Table 2
ULX in the CDFS and ECDFS at 332MHz and610MHz.
Name S S [mJy/beam] [mJy/beam]CXOECDFS J033122.00-273620.1 < . ...CXOECDFS J033128.84-275904.8 < . ...CXOECDFS J033139.05-280221.1 < . < . CXOECDFS J033143.46-275527.8 < . ...CXOECDFS J033143.48-275103.0 < . ...CXOCDFS J033219.10-274445.6 < . ...CXOCDFS J033221.91-275427.2 < . ...CXOCDFS J033230.01-274404.0 < . ...CXOCDFS J033234.73-275533.8 < . ...CXOECDFS J033249.26-273610.6 < . ...CXOECDFS J033316.29-275040.7 < . < . CXOECDFS J033322.97-273430.7 < . < . The ULX N4861-X2 (Fig 2, right) has a compact com-ponent A centered atRA(J2000) = 12 h m s ± s ,DEC(J2000) = 34 ◦ ± ∼ µ Jy (for which we derive a radioluminosity L . GHz = 3.3 x 10 erg/s assuming a distanceto the host galaxy of 14.80 Mpc) and a size upper limit of9.8 x 3.8 mas, corresponding to a brightness temperature T B > K. Two additional components (B and C), witha total flux density of ∼ µ Jy are detected, but cannot befirmly localized with the present data. If this extention wereconfirmed, the whole structure (including component A, B & C) would have a total flux density of 0.18 mJy, a luminos-ity of L . GHz = 7.7 x 10 erg/s and diameter D ∼
120 mas.This diameter corresponds to 8.6 pc at the distance of thehost galaxy, and it is in agreement with the typical size ofHII regions found in our Galaxy, like G18.2-0.3 (F ¨ u rst et al.1987), which has a size of 200 mas, a luminosity of L . GHz = 1.1 x 10 erg/s and is formed by several discrete sources. We use the upper limits obtained from the GMRT data onthe radio flux densities of the ULX objects to locate themin the fundamental plane of sub-Eddington accreting blackholes ( cf. , Corbel et al. 2003; Gallo et al. 2003; Merloni et al.2003; Falcke et al. 2004) as defined by a correlation betweenradio core ( L R ) and X-ray ( L X ) luminosity and black holemass, M BH , log L R = 0 . L X + 0 .
78 log M BH + 7 . .For our calculations, we assume a radio spectral index α R ≃ . and a X-ray spectral index α X ≃ − . adopted previ-ously by Falcke et al. (2004). The resulting radio and X-rayluminosities of the ULX objects in our sample are comparedin Fig. 3 to the results of Corbel et al. (2003) and Merloni etal. (2003). The resulting high upper limits on the BH massesdo not provide strong constraints on the nature of these ULXobjects.A similar relation is shown in Fig. 4 for the most com-pact components in the EVN images of N4088-X1 and N4861-X2. Using our radio luminosity at 1.6 GHz appropriatelyscaled to 5 GHz, the X-ray luminosity scaled to the 2-10 c (cid:13)
02 M. Mezcua & A.P. Lobanov: Instructions for authors
CONT: N4088-X1 IPOL 1650.561 MHZ N4088-LOW.ICL001.1Cont peak flux = 1.3259E-04 JY/BEAM Levs = 2.599E-05 * (-2, 2, 3, 4, 5) D E C L I NA T I O N ( J2000 ) RIGHT ASCENSION (J2000)12 05 31.718 31.716 31.714 31.712 31.710 31.708 31.706 31.70450 32 46.8246.8046.7846.7646.7446.7246.7046.6846.66
Fig. 2 ◦ for N4088-X1 and 11.36 ◦ for N4861-X2. The contours for the left image are (-2, 2, 3, 4, 5) x 26 µ Jy/beam, therms noise off-source. For the ULX on the right, the contours are (-32, 2, 3, 4, 5, 6, 7, 10, 20, 30, 40, 50) x 3 µ Jy/beam.These radio counterparts are offset from the X-ray peak position by 3.62” for N4088-X1, and by 1.97” for N4861-X2.These offsets lie within the X-ray positional error. The compact component of N4861-X2 is indicated with an A. Possibleextended emission might be detected in regions B and C.keV band, and assuming a sub-Eddington accretion regime,we derive a black hole mass of 10 . M ⊙ and of 10 . M ⊙ for N4088-X1 and N4861-X2 (component A), respectively.These masses are in agreement with the IMBH scenario forboth objects.Higher sensitivity observations are needed, and obser-vational time has already been guaranteed, to try to detectand/or confirm possible extended structure for both N4088-X1 and N4861-X2. Radio observations of ULX sources can help to unveil thenature of these objects. Analysis of archival GMRT data ofthe Chandra Deep Fields at 235, 325 &
610 MHz have notyielded any radio counterparts for these sources but yieldedupper limits on their flux densities. These ULXs are tooweak for deep field radio observations so higher sensitivityis needed in order to detect any faint radio emission.New EVN observations of three ULXs with known ra-dio counterparts yielded first milliarcsecond-scale imagesof all three objects. The EVN observations have confirmedthe earlier identification of the ULX N4449-X1 with a SNRand obtained the most accurate estimates of its size andage (Mezcua & Lobanov in prep.). For the two other ULXsstudied, N4088-X1 and N4861-X2, the EVN measurements have provided improved estimates of the compact radio flux,yielding better localizations of these objects in the L X -L radio diagram. The suggested nature of these objects can be bestverified with more sentitive observations at 5 GHz aimedat both improving the brightness temperature estimates andobtaining spectral index information. The success of the EVNobservations also calls for expanding this study to more ULXobjects. Acknowledgements.
The authors are grateful to M. L´opez-Corredoiraand M. W. Pakull for their valuable comments. M. Mezcua wassupported for this research through a stipend from the InternationalMax Planck Research School (IMPRS) for Radio and Infrared As-tronomy at the Universities of Bonn and Cologne.
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Location of the 24 ULXs (red arrows) in the fun-damental plane of sub-Eddington accreting black holes. Theparallel lines correspond to the labeled black hole mass rel-ative to that of the Sun. We show for comparison the Corbelet al. (2003) data for the X-ray binary GX 339-4 (filled cir-cles), and the Merloni et al. (2003) data for the Low Lumi-nosity AGN (LLAGN) NGC 2787, NGC 3147, NGC 3169,NGC 3226, and NGC 4143 (inverted triangles).
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26 28 30 32 34 36 38 40 log L R (erg/s) l og L x ( e r g / s ) ULXGX 339-4LLAGN N4861-X2N4088-X1
Fig. 4
Old (S´anchez-Sutil et al. 2006, grey squares) andnew (this work, red squares) location of the ULX sourcesN4088-X1 and N4861-X2 in the fundamental plane of sub-Eddington accreting black holes. The parallel lines corre-spond to the labeled black hole mass relative to that ofthe Sun. We show for comparison the Corbel et al. (2003)data for the X-ray binary GX 339-4 (filled circles), andthe Merloni et al. 2003 data for the Low Luminosity AGN(LLAGN) NGC 2787, NGC 3147, NGC 3169, NGC 3226,and NGC 4143 (inverted triangles). c (cid:13)(cid:13)