Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 18 November 2019 (MN LATEX style file v2.2) Accurate positions for the ULXs NGC 7319-X4 and NGC 5474-X1 and limiting magnitudes for their optical counterparts M. Heida,1,2 P. G. Jonker,1,2,3 M. A. P. Torres,1,3 S. Mineo,3 1SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands 2Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands 3Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 18 November 2019 ABSTRACT In this paper we report accurate Chandra positions for two ultraluminous X-ray sources: NGC 7319-X4 at Right Ascension (RA) = 339.02917(2)◦, Declination (Dec) = 33.97476(2)◦ and NGC 5474-X1 at RA = 211.24859(3)◦, Dec = 53.63584(3)◦. We perform bore-sight corrections on the Chandra X-ray Satellite observations of these sources to get to these accurate positions of the X-ray sources and match these posi- tions with archival optical data from the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. We do not find the optical counterparts: the limiting ab- solute magnitudes of the observations in the WFPC2 standard magnitude system are B= −7.9,V= −8.7andI= −9.3 for NGC 7319-X4 and U = −6.4 for NGC 5474-X1. We report on the X-ray spectral properties and we find evidence for X-ray variability in NGC 5474-X1. Finally, we briefly discuss several options for the nature of these ULXs. Key words: galaxies: individual: NGC 5474 – galaxies: individual: NGC 7319 – X- rays: binaries – X-rays: individual: NGC 5474-X1 – X-rays: individual: NGC 7319-X4 1 INTRODUCTION tion just prior to the merger. As a result the black hole receives a kick in the opposite direction. The size of the Ultraluminous X-ray sources (ULXs) are pointlike, off- kick depends on the magnitude and orientation of the spins nuclear X-ray sources that have a luminosity that is larger of the black holes before the merger and on their mass ra- than the Eddington luminosity of a 10 M black hole tio (Damour & Gopakumar 2006, Ferrarese & Merritt 2002, 39 −1 ⊙ arXiv:1206.0597v1 [astro-ph.HE] 4 Jun 2012 (LEdd(10 M ) 10 erg s ; Colbert & Miller 2005). Sev- Merritt et al. 2009). The maximum kick velocity is of the ≈ eral scenarios⊙ have been proposed to explain the high lu- order of 1000 km/s (Baker et al. 2008), which, with time, minosities of these sources. The sources with L . 1040 would allow one to observe these recoiled SMBHs as bright erg s−1 can be explained by stellar mass black holes that off-nuclear X-ray sources (Jonker et al. 2010). Another pos- emit radiation beamed instead of isotropically (King et al. sibility is that some of these ULXs are supernovae. SN 2001). Alternatively, the black hole can be more massive type IIn are the X-ray-brightest supernovae and therefore than the typical 10 M if formed by a low-metallicity the best candidates (Immler & Lewin 2003). Finally, some progenitor, which would⊙ lead to a higher Eddington lu- ULXs could potentially be explained as the remnant BH minosity (Belczynski et al. 2010). ULXs with luminosities from the smaller galaxy that underwent a merger event with above 1041 erg s−1, sometimes called hyperluminous X-ray a larger galaxy. sources, may however need black holes with M & 1000M Finding the optical counterparts of ULXs can help us to explain (Colbert & Miller 2005). Such black holes are re-⊙ distinguish between the different scenarios for their nature. ferred to as intermediate mass black holes (IMBHs). An- If the ULX is a stellar or intermediate mass black hole we other possibility is that some of these sources could be re- expect to see an accretion disc and/or a companion star. A coiling supermassive black holes (SMBHs). In the standard recoiling SMBH would be surrounded by a cluster of stars ΛCDM cosmology galaxies grow through hierarchical merg- (Merritt et al. 2009), whereas a supernova could be distin- ers. The central massive black holes of merging galaxies guished by its decaying light curve. For several ULXs the op- may eventually merge as well. Simulations show that when tical counterpart has been reported (e.g. Motch et al. 2011, two black holes coalesce, gravitational waves are emitted Yang et al. 2011, Wiersema et al. 2010). In this Paper, we that carry away linear momentum in a preferential direc- investigate two ULXs that were observed by the Chandra c 0000 RAS 2 M. Heida et al. X-ray Observatory and the Hubble Space Telescope (HST). called W-statistics1. The best-fit values are listed in Table NGC 7319-X4 (Liu 2011) is located in the late-type galaxy 2. These values give a 0.5-10.0 keV luminosity of 2 1040 erg −1 × NGC 7319, one of the galaxies in Stephan’s Quintet. NGC s at the distance of the galaxy of 90.75 Mpc (Crook et al. 5474-X1 (Swartz et al. 2011) is the brightest X-ray source 2007). in the highly asymmetrical late-type galaxy NGC 5474. We perform the same analysis on the shorter observa- tion (with obsid 789). The observed background-subtracted number of photons is 20. We fit the same model to this dataset and find that the spectrum is consistent with the same model and flux. 2 OBSERVATIONS, ANALYSIS AND RESULTS To derive the visual extinction from the hydrogen col- 2.1 Chandra X-ray observations umn density we use the relation of Predehl & Schmitt (1995) between NH and optical extinction. We find a visual extinc- We retrieve the observations in Table 1 from the Chandra tion AV = 0.9 0.4 magnitudes. This value is higher than the ± archive and reprocess the events making use of the calibra- Galactic foreground extinction in the direction of NGC 7319 caldb CIAO 2 tion files using version 4.4.7, employing ver- provided by NED , AV = 0.26 mag (Schlegel et al. 1998). sion 4.4. We only use datasets 7924 and 9546 for the bore- However, the dust maps of Schlegel et al. (1998) have the sight correction of the Chandra images, since the datasets same resolution as for IRAS, leaving room for local fluctua- with the shorter exposure times do not contain other sources tions. with enough counts for accurate position measurements. 2.1.2 NGC 5474 2.1.1 NGC 7319 We follow the same procedure for NGC 5474. For the bore- sight correction we use the star USNO B1.0 1436-0232144. For the bore-sight correction of the NGC 7319 field we ◦ ◦ can use only one source, coinciding with the center of the Its position is RA = 211.26471(3) , Dec = 53.63914(3) . It galaxy (see Figure 1). This is a bright (K-band magnitude has no measured proper motion. The position of the cor- responding X-ray source is RA = 211.26499(2)◦ , Dec = = 10) point source in the 2 Micron All Sky Survey (2MASS) ◦ and its position is accurately known: Right Ascension (RA) 53.63916(2) . After correcting the astrometry the offset be- = 339.014833◦ and Declination (Dec) = 33.975750◦ . The tween the reference star and its USNO position is 0.15”. We position of the X-ray source from wavdetect is RA = adopt this as the uncertainty of the X-ray positions with ◦ ◦ respect to the USNO B1.0 frame. The corrected position of 339.014934(4) , Dec = 33.975872(3) . We apply shifts of ◦ ◦ dRA = 0.299” and dDec = 0.440” with wcsupdate. The the ULX is RA = 211.24859(3) , Dec = 53.63584(3) in the − drawback of having only one point with which to do the USNO B1.0 frame. correction is that we can not correct for potential effects of The net number of counts from the source is 5219 in the 0.3–7.0 keV range, with a count rate of 0.175 0.002 rotation. After applying the bore-sight correction the po- ± sition of the core of NGC 7319 differs from the 2MASS counts per second. The background level is less than 0.05 %, Ftools position by 0.03”. This is consistent with the uncertainty corresponding to less than 2 photons. We use the grppha in the determination of the X-ray source on the CCD by task to rebin the spectrum with at least 30 counts wabs pegp- wavdetect. We adopt this as the uncertainty on the bore- in each bin. We fit an absorbed power law ( wrlw 2 × sight corrected Chandra positions. The corrected position ) to the data using χ statistics. Because of the high for the ULX is RA = 339.02917(2)◦ , Dec = 33.97476(2)◦ count rate we also add the pileup model (Davis 2001) to the in the 2MASS frame. These uncertainties stem only from fit function. The best-fit parameters are listed in Table 2 determining the positions of the ULX on the CCD and ty- and the X-ray spectrum with the fitted model is plotted in ing the Chandra frame to the 2MASS frame (and thus do Figure 2. The ULX has an 0.5–10.0 keV X-ray luminosity of 1.9 1040 erg s−1 at the distance of the galaxy of 6.8 Mpc not take into account any uncertainty associated with tying × the 2MASS sources to the International Celestial Reference (Drozdovsky & Karachentsev 2000). System [ICRS]). All quoted uncertainties correspond to 1-σ We use the same extraction regions on the shorter ex- posure with ID 7086.