4 Identification of the X-ray sources

Table 6 lists the optical identification of the sources together with their optical magnitudes. The classification we have chosen includes several source classes: Broad-Line Active Galactic Nuclei (BLAGN), Narrow Emission Line Galaxies (NELG, which invariably turned out to be Narrow-Line AGN), non-emission line galaxies (Gal), Active Coronal stars (AC), stars without signs of activity (STAR) and clusters of galaxies (Clus).

 

 

Table 6: Optical identifications of the X-ray sources. Under the last column we quote the main spectral features detected in the spectra.

Source name	RA $_{\rm O}^a$ (J2000)	Dec $_{\rm O}^a$ (J2000)	u	g'	r'	i'	Z	Class	z	L44b	Comments
XMMU J010316.7-065137	01:03:16.43	-06:51:35.83		19.85	19.23	18.74	18.57	BLAGN	1.914	4.221	CIII], CIV, MgII
XMMU J010327.3-064643	01:03:27.41	-06:46:43.31		22.26	21.86	21.77	21.06	NELG?	1.010?	2.036	CIII]?,[OII]?, [NeV]?
XMMU J010328.7-064633	01:03:28.67	-06:46:32.02		20.26	19.81	19.27	19.15	BLAGN	1.820	4.467	SiIV, CIV, CIII], MgII
XMMU J010333.8-064016	01:03:33.88	-06:40:16.21		22.70	21.12	19.76	19.22	NELG	0.692	0.597	[OII], H$\epsilon$?, H$\gamma$, [OIII]
XMMU J010339.8-065224	01:03:39.89	-06:52:25.99		21.39	20.65	20.23	19.99	BLAGN	1.128	2.297	[NIII], CIII], MgII, [OII]
XMMU J010355.6-063710	01:03:55.67	-06:37:10.88		21.82	21.00	20.48	20.11	NELG?	0.314	0.114	H$\beta$, [OIII], H$\delta$, [NeV]?
XMMU J010400.9-064949	01:04:01.09	-06:49:50.89		17.08	15.52	15.24	14.16	AC/dMe			Ca H&K, H$\beta$, H$\alpha$
XMMU J010410.5-063926	01:04:10.54	-06:39:26.69		19.88	19.47	19.08	19.03	BLAGN	0.630	0.774	MgII, H$\gamma$, H$\beta$, [OIII]
XMMU J010420.9-064701	01:04:20.97	-06:47:01.97		19.71	18.98	18.41	18.34	BLAGN	1.536	2.773	CIV, CIII], MgII
XMMU J010430.1-064456	01:04:30.07	-06:44:56.60		18.77	18.27	18.06	17.96	BLAGN	0.910	1.937	CIII], MgII, [OII], H$\gamma$, [NeV]
XMMU J010437.5-064739	01:04:37.55	-06:47:37.23		21.19	20.71	20.24	19.76	BLAGN	2.511	7.248	Ly$\alpha$, SiIV, CIV?, CIII]
XMMU J010444.6-064833	01:04:44.71	-06:48:33.31		20.52	20.32	19.96	19.58	BLAGN	2.256	10.080	Ly$\alpha$, CIV, CIII]
XMMU J121819.4+751919	12:18:19.06	+75:19:22.02	19.53	19.79	19.21	18.96	18.55	BLAGN	2.649	11.209	Ly$\alpha$, CIV, CIII]
XMMU J122017.9+752212	12:20:17.70	+75:22:18.00		12.27c	11.70c			Gal	0.0058	0.0013	NGC 4291d
XMMU J122048.4+751804	12:20:48.25	+75:18:07.29	18.41	18.97	18.47	17.96	17.83	BLAGN	1.687	4.181	CIV, CIII], MgII
XMMU J122051.7+752821	12:20:51.73	+75:28:20.67	20.90	22.05	21.33	20.80		BLAGN	0.181	0.030	H$\beta$, H$\gamma$, H$\delta$? OII?
XMMU J122052.0+750529	12:20:51.27	+75:05:31.62	18.24	18.62	18.20	17.92	17.85	BLAGN	0.646	7.036	[OII], MgII
XMMU J122111.2+751117	12:21:10.62	+75:11:19.39	18.78	19.54	18.79	18.47	18.32	BLAGN	1.259	3.179	EMSS sourcee
XMMU J122120.5+751616	12:21:19.90	+75:16:18.09	20.21	20.90	19.98	19.45	18.85	NELG	0.340	0.156	[OII], [OIII], [NeV]
XMMU J122135.5+750914	12:21:34.92	+75:09:15.99	19.84	20.38	19.11	18.32	17.77	BLAGN	0.330	0.246	H$\beta$, [OIII]
XMMU J122143.8+752235
XMMU J122206.4+752613	12:22:06.66	+75:26:15.36	20.23	20.03	18.68	17.83	17.41	NELG	0.238	0.975	[OII], [OIII]
XMMU J122242.6+751434	12:22:42.69	+75:14:34.68	20.97	21.82	21.13	20.71	19.98	BLAGN	1.065	1.174	MgII, CIII]?
XMMU J122258.1+751934	12:22:58.00	+75:19:34.66		22.66	21.66	21.10		NELG?	0.257	0.161	[OII], [OIII]
XMMU J122318.5+751504	12:23:18.11	+75:15:04.64	20.27	21.15	20.62	20.24		BLAGN	1.509	3.752	CIII], MgII
XMMU J122344.7+751922	12:23:45.65	+75:19:23.01	20.11	20.69	20.26	19.88	19.53	BLAGN	0.757	0.904	MgII, [NeV]?, [OII]
XMMU J122351.0+752227	12:23:50.87	+75:22:28.57	19.51	19.93	19.48	18.76	18.50	BLAGN	0.565	1.718	MgII, [NeV], [OII], H$\gamma$, [OIII]
XMMU J122435.7+750812	12:24:35.31	+75:08:10.10
XMMU J122445.5+752224	12:24:45.42	+75:22:24.92	19.27	20.16	19.82	19.33	19.36	BLAGN	1.852	7.088	CIV, CIII]

Notes to Table: ^a Position of the optical source; ^b Luminosity in the 0.5-4.5 keV rest frame in units of $10^{44}\, {\rm erg}\, {\rm s}^{-1}$; ^c B and R magnitudes from de Vaucouleurs et al. (1991) and Sandage & Visvanathan (1978) quoted under g' and r' respectively; ^d Identification from Humasson et al. (1956); ^e Identification from Stocke et al. (1991).

Finding charts and spectra of these sources can be found under the AXIS programme web pages (http://www.ifca.unican.es/~xray/AXIS). Here we add some notes on individual objects

XMMU J010327.3-064643 is the faintest optical source (in r', i' and Z) with hints of narrow emission lines. We have binned the spectrum into bins of 600 Å each, and applied a photometric redshift technique using a set of galaxy templates as in Fernández-Soto et al. (1999). The minimum $\chi^2$ happens at z=1.01, which allows us to interpret an apparently narrow emission line as CIII], and a dubious [OII]. We therefore tentatively classify this source as a NELG, although its high luminosity ( $1.8 \times 10^{44}\, {\rm erg}\, {\rm s}^{-1}$) would promote it to QSO2. The optical colours of this source do indeed correspond to a QSO or to a starforming galaxy (see Fig. 6, where this source is the empty circle at the bottom of the graph). The classification and redshift of this source are at present very uncertain.

XMMU J122258.1+751934 is a faint source, with a detected but weak emission line that we believe to be [OII] and dubious [OIII] doublet at z=0.257. Since the spectrum is very noisy, we applied the photometric redshift technique to the binned spectrum. The minimum $\chi^2$ is indeed found at $z\sim 0.3$.

XMMU J122120.5+751616 appears to have a very blue spectrum, but the H$\beta$line, which is weak, does not appear to be broad. On the other hand it has strong [NeV], [OII] and [OIII] doublet. Therefore we have classified it as a NELG.

XMMU J010328.7-064633 and XMMU J010316.7-065137 appear to be BAL QSOs. They will be discussed in Sect. 5.4

Figure 5 shows the g'-i' colour of the identified sources as a function of their i'-magnitude. As expected, BLAGNs are usually bluer than the NELGs where the optical light is dominated by the host galaxy rather than by the active nucleus.

Figure 5: Optical colour g'-i' as a function of i'-magnitude. Symbols as in 3.

To further explore the optical colours, we plot in Fig. 6 the r'-i' versus g'-r' optical colour-colour diagram for the identified sources. We overlay median colours for QSOs, as derived from the Sloan Digitized Sky Survey (Richards et al. 2001), in the redshift range z= 0-3 and for E/S0 galaxies, as derived from the Coleman et al. (1980) template, in the redshift range z= 0-1. For clarity we do not show the expected colours for starforming galaxies as they are mostly coincident with those of QSOs, but they can follow the track marked by the E/S0 colours by adding increasing amounts of reddening. The influence of cosmological Lyman-$\alpha$ absorption has not been included, as it is only relevant in the g'-band for redshifts z> 3. Clearly, the X-ray sources classified as BLAGNs have optical colours as expected for QSOs, and the X-ray sources identified as NELGs fall in the region where the galaxies lie (either E/S0 or more likely reddened starforming galaxies). This implies again that in NELGs most of the optical light we see comes from the host galaxy, unlike the X-ray emission which comes from the nucleus.

Figure 6: Optical colour-colour diagram. Symbols as in 3. The continuous line represents the colours of E/S0 galaxies in the redshift range z= 0-1 and the dashed line the median colours of SDSS QSOs in the redshift range z= 0-3.

4.1 Additional identifications

Besides the identifications corresponding to the complete sample of X-ray selected sources down to an X-ray flux of $2\times 10^{-14}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$, we have identified a few sources fainter than these. These 10 additional sources are not part of the sample considered here, but still we list their identifications to help other researchers in possible identification work of fainter sources. These identifications are listed in Table 7.

 

 

Table 7: Additional identifications below $2\times 10^{-14}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$.

Source name	RA $_{\rm X}^a$ (J2000)	Dec $_{\rm X}^a$ (J2000)	Perrb	Fluxc	RA $_{\rm O}^d$ (J2000)	Dec $_{\rm O}^d$ (J2000)	i'	Class	z	L44e
XMMU J010311.7-064038	01:03:11.79	-06:40:38.25	1.93	$0.83\pm 0.24$	01:03:11.83	-06:40:39.14	18.98	NELG	0.187	0.133
XMMU J010359.8-065318	01:03:59.82	-06:53:18.46	1.95	$1.25\pm 0.65$	01:03:59.87	-06:53:18.87	20.92	BLAGN	1.270	0.980
XMMU J010400.9-063027	01:04:00.98	-06:30:27.36	2.34	$1.55\pm 0.30$	01:04:01.15	-06:30:28.35	20.09	BLAGN	1.693	2.199
XMMU J010402.9-063600	01:04:02.91	-06:36:00.14	1.14	$1.51\pm 0.20$	01:04:02.84	-06:35:59.70	21.69	BLAGN	0.932	0.629
XMMU J010405.5-065359	01:04:05.57	-06:53:59.21	2.45	$1.27\pm 0.26$	01:04:05.33	-06:53:59.17	21.13	BLAGN	2.821	5.093
XMMU J010410.0-063012	01:04:10.06	-06:30:12.92	1.77	$1.63\pm 0.72$	01:04:10.13	-06:30:12.78	21.13	BLAGN	1.190	1.579
XMMU J010411.5-065209	01:04:11.53	-06:52:09.60	2.25	$1.56\pm 0.26$	01:04:11.40	-06:52:08.86	20.90	BLAGN	1.224	1.138
XMMU J010439.3-064629	01:04:39.34	-06:46:29.88	1.60	$1.49\pm 0.24$	01:04:39.33	-06:46:27.24	20.08	BLAGN	1.620	1.927
XMMU J121937.5+751042	12:19:37.57	+75:10:42.96	1.46	$1.61\pm 0.28$	12:19:37.34	+75:10:43.79	20.77	STAR
XMMU J122425.4+751818	12:24:25.46	+75:18:18.04	1.34	$1.36\pm 0.22$	12:24:25.50	+75:18:19.74	15.72	AC/dMe

Notes to Table: ^a Position of the X-ray source; ^b Position error (Statistical) of the X-ray source in arcsec; ^c Flux in the 0.5-4.5 keV band, in units of $10^{-14}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$; ^d Position of the optical source; ^e Luminosity in the 0.5-4.5 keV band.