NGC 4698 – Line of X-ray sources

It was found by Foschini et al. (2002a) that there is an ultraluminous X-ray source (ULX) (Object 2 in Figure 1) within the disk of NGC 4698. They have in that paper a note added in proof that says:

After acceptance of this manuscript, we obtained the VLT–FORS1 spectrum of NGC 4698–ULX1, that allowed us to identify this source as a background BL Lac object at z = 0.43, thus rejecting the classification of that source as a ULX (see Foschini et al., in preparation).

That spectral identification is discussed in Foschini et al. (2002b), and the situation is also noted in Foschini et al. (2002c). Foschini et al. (2002b) also mention another X-ray source in the field. It is shown in Foschini et al. (2002b) Figure 1 marked as “1RXS J124828.1+083103”. They say:

The three X-ray sources identified to date, however, have three redshifts: XMMU J124825.9+083020 has z = 0.43, NGC 4698 has z = 0.0033, and the ROSAT source 1RXS J124828.1+083103 has been recently identified with a Seyfert nucleus at z = 0.12 (Xu et al. 2001). Therefore, these three sources are not members of a single cluster.

So, what we have here is a straight line of three objects; the nucleus of NGC 4698 (z = 0.003), object 2 (z = 0.43), object 3 (z = 0.12). Foschini et al. (2002a) Figure 2 shows the X-ray contours drawn over an optical image of NGC 4698. NGC 4698 nucleus is an X-ray source, and then there is another X-ray source marked as “ULX 1”, it’s the object 2. Further out you can see stronger X-ray source, it’s the object 3. Notice how straight the alignment is between these three X-ray sources.

Burbidge et al. (2003) also briefly discuss the ULX in NGC 4698. Check out this nice image of this system. The field is quite crowded with apparent background galaxies. Notice how the object 2 seems clearly reddened (compare the small objects within the NGC 4698 area and outside of it, there’s a general difference in coloring of the objects), so it might very well be a background object.

Objects near NGC 4698
Figure 1. The objects with measured redshifts near NGC 4698. Size of the image is 7 x 7 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

Objects and their data

1 NGC 4698 SAab, Sy2 0.00334 11.46 0
2 XMMU J124825.9+083020 BLLAC 0.43000 20.5 1.3
3 RXJ1248.4+0831 Sy 0.12000 18.0 (g) 2.4

NED object descriptions: Object 1, Object 2, Object 3

SDSS SkyServer object descriptions: Object 1, Object 3

SDSS SkyServer image of the system


Burbidge et al., 2003, A&A, 400, 17, “The nature of the ultraluminous X-ray sources inside galaxies and their relation to local QSOs”

Foschini et al., 2002a, A&A, 392, 817, “XMM-Newton observations of ultraluminous X-ray sources in nearby galaxies”

Foschini et al., 2002b, A&A, 396, 787, “BL Lac identification for the ultraluminous X-ray source observed in the direction of NGC 4698”

Foschini et al., 2002c, iorb. conf, 88, “Search for ultraluminous X-ray sources in nearby galaxies”

May 23, 2009 – Updated: I fought with HTML-tables, and finally won. 🙂

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