1 Introduction

The X-ray emission from Seyfert host galaxies comprises the contribution of a number of discrete sources plus the hot interstellar plasma (Fabbiano 1989). Most of the discrete sources appear to be close accreting binaries, with a compact companion. Einstein observations of the bulge of M 31 revealed a population of about 100 low mass X-ray binaries (Fabbiano et al. 1987). Later, Supper et al. (1997) showed, by using ROSAT data, that the most luminous of these objects in M 31 has $L_{\rm X}=2\times 10^{38}$ erg s^-1, close to the Eddington limit for a $1.4~ M_{\odot}$ neutron star.

Recently, several sources with X-ray luminosities higher than the Eddington limit for a typical neutron star have been detected in nearby galaxies (e.g., Read et al. 1997; Colbert & Mushotzky 1999; Makishima et al. 2000; La Parola et al. 2001; Zezas et al. 2001). Fabbiano et al. (2001) found with Chandra 14 pointlike sources in the Antennae galaxies, with luminosities above 10³⁹ erg s^-1and up to 10⁴⁰ erg s^-1.

These discoveries have raised difficulties in the interpretation of these sources. Even though it is statistically possible to have some individual cases of off-centre black holes with masses of the order of 10³-10⁴ $M_{\odot}$ (by assuming a typical Eddington ratio of 0.1-0.01; cf. Nowak 1995), it is very difficult to explain the high number of sources detected so far within this scenario. Dynamical friction should have caused the objects to spiral to the nucleus of the galaxy. Several other hypotheses have been suggested about the nature of ULX: anisotropic emission from accreting black holes (King et al. 2001), emission from jets in microblazars (Körding et al. 2002), emission from accreting Kerr black holes (Makishima et al. 2000), and inhomogeneities in radiation-pressure dominated accretion disks (Begelman 2002). However, the lack of sufficient information has not allowed us to distinguish between the different models proposed. The search for optical counterparts has not yet yielded much data: to date only one ULX appears to have a plausible counterpart (Roberts et al. 2001), and other ULX may be associated with planetary nebulae or H II regions (Pakull & Mirioni 2002; Wang 2002).

Our team has been awarded about 250 ks of XMM-EPIC guaranteed time, and we started a distance-limited survey of Seyfert galaxies. We selected 28 objects in the northern hemisphere with $B_{{\rm T}}<12.5$ mag and d<22 Mpc (Di Cocco et al. 2000; Cappi et al. 2002) from the Palomar survey of Ho et al. (1997a). The distances were estimated according to Ho et al. (1997a), and we adopt the same convention in the present paper.

Here we present the results from a study of the discrete sources detected in the galaxies, which are neither the nucleus nor background objects. To date we have obtained 13 objects in our sample, but three observations were heavily corrupted by soft-proton flares and it was not possible to extract any useful information. Here we present part of a study of the discrete source population in the remaining 10 Seyfert galaxies (Table 1), specifically the catalog of ULX sources. Some preliminary results have been presented in Foschini et al. (2002).

Previous detections of ULX have been largely confined to late-type galaxies (e.g., IC 342, M 82, NGC 3628, and NGC 5204) or interacting systems undergoing a starburst phase (e.g., the Antennae). Although the objects studied here technically have Seyfert nuclei, the level of nuclear activity is extremely low, and for the present purposes they can be considered "typical'' nearby galaxies. The one selection effect to bear in mind is that most of the Palomar Seyferts tend to be relatively bulge-dominated disk galaxies (see, e.g., Ho et al. 1997b), and so late-type galaxies are underrepresented in our sample.

 

 

Table 1: Main characteristics of the observed host galaxies. Columns: (1) Name of the host galaxy from the New General Catalog; (2) optical coordinates of the nucleus from Cotton et al. (1999); (3) Hubble type; (4) spectral classification of the nucleus; (5) distance (Mpc); (6) major axis of the D₂₅ ellipse [arcmin]; (7) Galactic absorption column density (10²⁰ cm^-2); (8) date of XMM-Newton observation (year-month-day); (9) effective exposure time, i.e. cleaned from soft-proton flares [ks]. Data for Cols. (3)-(6) are taken from Ho et al. (1997a).

Galaxy	RA, Dec (J2000)	Hubble Type	Sp. Class.	d	D25	$N_{\rm H}$	Date	Exp.
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
NGC 1058	02:43:30.2, +37:20:27.2	SA(rs)c	S2	9.1	3.02	6.65	2002-02-01	6.0
NGC 3185	10:17:38.7, +21:41:17.2	SB(r)0/a	S2	21.3	2.34	2.12	2001-05-07	9.1
NGC 3486	11:00:24.1, +28:58:31.6	SAB(r)c	S2	7.4	7.08	1.9	2001-05-09	4.2
NGC 3941	11:52:55.4, +36:59:10.5	SB(s)0	S2	18.9	3.47	1.9	2001-05-09	5.0
NGC 4138	12:09:29.9, +43:41:06.0	SA(r)0+	S1.9	17.0	2.57	1.36	2001-11-26	10.0
NGC 4168	12:12:17.3, +13:12:17.9	E2	S1.9	16.8	2.75	2.56	2001-12-04	17.4
NGC 4501	12:31:59.3, +14:25:13.4	SA(rs)b	S2	16.8	6.92	2.48	2001-12-04	2.8
NGC 4565	12:36:21.1, +25:59:13.5	SA(s)b	S1.9	9.7	15.85	1.3	2001-07-01	10.0
NGC 4639	12:42:52.5, +13:15:24.1	SAB(rs)bc	S1.0	16.8	2.75	2.35	2001-12-16	9.7
NGC 4698	12:48:23.0, +08:29:14.8	SA(s)ab	S1.9	16.8	3.98	1.87	2001-12-16	9.2

 

 

Table 2: ULX in the present catalog (significance greater than $4\sigma$). Columns: (1) Name of the host galaxy; (2) ULX number; (3) coordinates of the ULX; (4) angular separation from the optical centre of the galaxy (arcsec); (5) ULX name according to XMM-Newton rules.

Host Galaxy	Object	RA, Dec (J2000.0)	Separation	XMM ID
(1)	(2)	(3)	(4)	(5)
NGC 1058	ULX1	02:43:23.5, +37:20:38	77	XMMU J024323.5+372038
	ULX2	02:43:28.3, +37:20:23	19	XMMU J024328.3+372023
NGC 3185	ULX1	10:17:37.4, +21:41:44	30	XMMU J101737.4+214144
NGC 3486	ULX1	11:00:22.4, +28:58:18	23	XMMU J110022.4+285818
NGC 3941	ULX1	11:52:58.3, +36:59:00	38	XMMU J115258.3+365900
NGC 4168	ULX1	12:12:14.5, +13:12:48	45	XMMU J121214.5+131248
NGC 4501	ULX1	12:32:00.1, +14:22:28	166	XMMU J123200.1+142228
	ULX2	12:32:00.8, +14:24:42	40	XMMU J123200.8+142442
NGC 4565	ULX1	12:36:05.2, +26:02:34	289	XMMU J123605.2+260234
	ULX2	12:36:14.8, +26:00:53	127	XMMU J123614.8+260053
	ULX3	12:36:17.3, +25:59:51	59	XMMU J123617.3+255951
	ULX4	12:36:17.4, +25:58:54	51	XMMU J123617.4+255854
	ULX5	12:36:18.8, +26:00:34	83	XMMU J123618.8+260034
	ULX6	12:36:27.8, +25:57:34	139	XMMU J123627.8+255734
	ULX7	12:36:30.6, +25:56:50	197	XMMU J123630.6+255650
NGC 4639	ULX1	12:42:48.3, +13:15:41	61	XMMU J124248.3+131541
	ULX2	12:42:51.4, +13:14:39	50	XMMU J124251.4+131439
NGC 4698	ULX1	12:48:25.9, +08:30:20	73	XMMU J124825.9+083020

 

 

Table 3: X-ray data on the ULX of the present catalog. Columns: (1) Host galaxy; (2) ULX number; (3) count rate in counts per second, as calculated with eboxdetect; (4) likelihood of the detection (cf. Ehle et al. 2001); (5) absorbing column density (10²¹ cm^-2); (6) photon index of the power law; (7) X-ray luminosity in the 0.5-10 keV energy band (10³⁸ erg s^-1), calculated from the count rates of Col. 3 and converted with the factor of $3\times 10^{11}$ cnt$\cdot$ cm²/erg, which in turn is derived by using the power-law model with $\Gamma =2.0$ and an average Galactic $N_{{\rm H}}=3\times 10^{20}$ cm^-2 (cf. Ehle et al. 2001). When the parameters $N_{\rm H}$ and $\Gamma$ are present, it means that the statistics are sufficient to perform a spectral fitting and uncertainties in the parameters estimate are at the $90\%$ confidence limits. For $N_{{\rm H}}=N_{{\rm H, gal}}$, it means that no additional absorption is found to be significant. The luminosities were calculated using distances from Ho et al. (1997a), who considered an infall velocity of 300 km s^-1 for the Local Group, H₀=75 km s^-1Mpc^-1, and the distance of the Virgo Cluster of 16.8 Mpc (cf. Table 1, Col. 5).

Host Galaxy	Object	Count Rate (10-3)	Likelihood	$N_{\rm H}$	$\Gamma$	$L_{0.5-10 \rm ~keV}$
(1)	(2)	(3)	(4)	(5)	(6)	(7)
NGC 1058	ULX1	$13\pm 2$	79	$N_{{\rm H, gal}}$	$1.1\pm 0.3$	11
	ULX2	$6\pm 1$	22	-	-	2.3
NGC 3185	ULX1	$5\pm 1$	27	-	-	13
NGC 3486	ULX1	$20\pm 4$	( $^{{\rm *}}$)	$N_{{\rm H, gal}}$	$2.2\pm 0.5$	5.0
NGC 3941	ULX1	$43\pm 4$	320	$N_{{\rm H, gal}}$	$1.9\pm 0.2$	74
NGC 4168	ULX1	$4.0\pm 0.7$	36	-	-	6.0
NGC 4501	ULX1	$11\pm 3$	23	-	-	17
	ULX2	$29\pm 4$	93	$N_{{\rm H, gal}}$	$2.3\pm 0.4$	37
NGC 4565	ULX1	$7\pm 1$	44	-	-	3.4
	ULX2	$10\pm 1$	90	$6\pm 5$	$1.7\pm 0.6$	16
	ULX3	$3.9\pm 0.9$	21	-	-	2.0
	ULX4	$51\pm 3$	821	$N_{{\rm H, gal}}$	$1.9\pm 0.1$	25
	ULX5	$7\pm 1$	57	-	-	3.4
	ULX6	$13\pm 2$	121	$N_{{\rm H, gal}}$	$1.5\pm 0.3$	9.0
	ULX7	$4\pm 1$	23	-	-	2.0
NGC 4639	ULX1	$5\pm 1$	29	-	-	8.0
	ULX2	$3\pm 1$	( $^{{\rm *}}$)	-	-	5.0
NGC 4698	ULX1	$16\pm1$	168	$N_{{\rm H, gal}}$	$2.0\pm 0.2$	30

$\parbox{12.5cm}{ \begin{list}{}{} \item[$^{{\rm *}}$ ] Data from manual analysis. \end{list}}$