5 A first set of variable objects

Figure 8: R-band magnitude difference between the two epochs October 1999 and February 2000 versus combined R-magnitude (grey dots = all objects). Objects selected as variable are shown as black crosses, but strongly variable sources lie outside the plot. Twenty objects have been omitted from the sample after visual inspection since they were affected by scattered light or diffraction spikes of 9th magnitude stars.

   

Table 3: A first sample of variable objects drawn from R-band photometry of two epochs. While most objects are likely to be quasars at z<3, some objects might be Seyfert galaxies or even stars. We marked the objects where we assume a Supernova has caused the variability. The object numbers refer to the published catalog and the object names encode the object positions.

cat. No.	object name	$\alpha_{{\rm J2000}}$	$\delta_{{\rm J2000}}$	$B_{{\rm Oct99}}$	$V_{{\rm Oct99}}$	$R_{{\rm Oct99}}$	$R_{{\rm Feb00}}$	comment
4050	COMBO-0332203-280215	$03^{{\rm h}} 32^{{\rm m}} 20\hbox{$.\!\!^{\rm s}$ }315$	$-28\hbox{$^\circ$ }02\hbox{$^\prime$ }14\hbox{$.\!\!^{\prime\prime}$ }66$	21.146	21.013	20.698	20.605
4427	COMBO-0331278-280156	$03^{{\rm h}} 31^{{\rm m}} 27\hbox{$.\!\!^{\rm s}$ }766$	$-28\hbox{$^\circ$ }01\hbox{$^\prime$ }55\hbox{$.\!\!^{\prime\prime}$ }96$	24.537	24.213	23.084	22.903
4809	COMBO-0331362-280150	$03^{{\rm h}} 31^{{\rm m}} 36\hbox{$.\!\!^{\rm s}$ }247$	$-28\hbox{$^\circ$ }01\hbox{$^\prime$ }49\hbox{$.\!\!^{\prime\prime}$ }59$	22.695	22.525	22.258	22.046
5826	COMBO-0332470-280119	$03^{{\rm h}} 32^{{\rm m}} 46\hbox{$.\!\!^{\rm s}$ }953$	$-28\hbox{$^\circ$ }01\hbox{$^\prime$ }19\hbox{$.\!\!^{\prime\prime}$ }38$	24.805	23.736	22.906	22.812
7139	COMBO-0332354-280041	$03^{{\rm h}} 32^{{\rm m}} 35\hbox{$.\!\!^{\rm s}$ }368$	$-28\hbox{$^\circ$ }00\hbox{$^\prime$ }41\hbox{$.\!\!^{\prime\prime}$ }19$	24.338	24.225	23.629	23.328
7902	COMBO-0332302-280020	$03^{{\rm h}} 32^{{\rm m}} 30\hbox{$.\!\!^{\rm s}$ }200$	$-28\hbox{$^\circ$ }00\hbox{$^\prime$ }19\hbox{$.\!\!^{\prime\prime}$ }95$	22.308	21.901	21.809	21.972
13244	COMBO-0333148-275749	$03^{{\rm h}} 33^{{\rm m}} 14\hbox{$.\!\!^{\rm s}$ }849$	$-27\hbox{$^\circ$ }57\hbox{$^\prime$ }48\hbox{$.\!\!^{\prime\prime}$ }96$	23.992	24.026	23.473	23.087
15278	COMBO-0331208-275649	$03^{{\rm h}} 31^{{\rm m}} 20\hbox{$.\!\!^{\rm s}$ }762$	$-27\hbox{$^\circ$ }56\hbox{$^\prime$ }48\hbox{$.\!\!^{\prime\prime}$ }97$	20.983	20.829	20.719	20.835
15396	COMBO-0332161-275644	$03^{{\rm h}} 32^{{\rm m}} 16\hbox{$.\!\!^{\rm s}$ }133$	$-27\hbox{$^\circ$ }56\hbox{$^\prime$ }44\hbox{$.\!\!^{\prime\prime}$ }12$	23.064	22.916	22.533	22.440
16155	COMBO-0332042-275626	$03^{{\rm h}} 32^{{\rm m}} 04\hbox{$.\!\!^{\rm s}$ }157$	$-27\hbox{$^\circ$ }56\hbox{$^\prime$ }26\hbox{$.\!\!^{\prime\prime}$ }39$	24.130	23.860	23.029	22.814
16404	COMBO-0332226-275622	$03^{{\rm h}} 32^{{\rm m}} 22\hbox{$.\!\!^{\rm s}$ }604$	$-27\hbox{$^\circ$ }56\hbox{$^\prime$ }22\hbox{$.\!\!^{\prime\prime}$ }48$	24.001	23.360	22.339	22.616	SN candidate
20787	COMBO-0333053-275409	$03^{{\rm h}} 33^{{\rm m}} 05\hbox{$.\!\!^{\rm s}$ }312$	$-27\hbox{$^\circ$ }54\hbox{$^\prime$ }09\hbox{$.\!\!^{\prime\prime}$ }11$	22.525	21.951	21.576	21.442
27080	COMBO-0333042-275103	$03^{{\rm h}} 33^{{\rm m}} 04\hbox{$.\!\!^{\rm s}$ }206$	$-27\hbox{$^\circ$ }51\hbox{$^\prime$ }02\hbox{$.\!\!^{\prime\prime}$ }77$	24.619	23.535	22.332	22.455
28275	COMBO-0331525-275027	$03^{{\rm h}} 31^{{\rm m}} 52\hbox{$.\!\!^{\rm s}$ }508$	$-27\hbox{$^\circ$ }50\hbox{$^\prime$ }27\hbox{$.\!\!^{\prime\prime}$ }48$	24.062	23.586	22.456	22.613
29793	COMBO-0333104-274945	$03^{{\rm h}} 33^{{\rm m}} 10\hbox{$.\!\!^{\rm s}$ }389$	$-27\hbox{$^\circ$ }49\hbox{$^\prime$ }44\hbox{$.\!\!^{\prime\prime}$ }75$	24.112	23.775	23.202	22.051
30792	COMBO-0332432-274914	$03^{{\rm h}} 32^{{\rm m}} 43\hbox{$.\!\!^{\rm s}$ }239$	$-27\hbox{$^\circ$ }49\hbox{$^\prime$ }14\hbox{$.\!\!^{\prime\prime}$ }11$	22.442	22.474	22.288	22.801
32254	COMBO-0333263-274831	$03^{{\rm h}} 33^{{\rm m}} 26\hbox{$.\!\!^{\rm s}$ }301$	$-27\hbox{$^\circ$ }48\hbox{$^\prime$ }31\hbox{$.\!\!^{\prime\prime}$ }14$	23.329	23.352	22.957	22.712
34357	COMBO-0332087-274734	$03^{{\rm h}} 32^{{\rm m}} 08\hbox{$.\!\!^{\rm s}$ }669$	$-27\hbox{$^\circ$ }47\hbox{$^\prime$ }34\hbox{$.\!\!^{\prime\prime}$ }20$	19.422	19.141	18.765	18.907
35677	COMBO-0332142-274647	$03^{{\rm h}} 32^{{\rm m}} 14\hbox{$.\!\!^{\rm s}$ }246$	$-27\hbox{$^\circ$ }46\hbox{$^\prime$ }47\hbox{$.\!\!^{\prime\prime}$ }44$	24.293	24.051	23.477	23.685
36683	COMBO-0333343-274621	$03^{{\rm h}} 33^{{\rm m}} 34\hbox{$.\!\!^{\rm s}$ }343$	$-27\hbox{$^\circ$ }46\hbox{$^\prime$ }20\hbox{$.\!\!^{\prime\prime}$ }94$	25.560	23.748	23.329	25.408	SN candidate
37487	COMBO-0332391-274602	$03^{{\rm h}} 32^{{\rm m}} 39\hbox{$.\!\!^{\rm s}$ }084$	$-27\hbox{$^\circ$ }46\hbox{$^\prime$ }01\hbox{$.\!\!^{\prime\prime}$ }82$	21.101	20.944	20.696	20.897
38551	COMBO-0332300-274530	$03^{{\rm h}} 32^{{\rm m}} 29\hbox{$.\!\!^{\rm s}$ }982$	$-27\hbox{$^\circ$ }45\hbox{$^\prime$ }29\hbox{$.\!\!^{\prime\prime}$ }84$	21.692	21.480	21.102	21.249
38905	COMBO-0333036-274519	$03^{{\rm h}} 33^{{\rm m}} 03\hbox{$.\!\!^{\rm s}$ }616$	$-27\hbox{$^\circ$ }45\hbox{$^\prime$ }18\hbox{$.\!\!^{\prime\prime}$ }71$	23.744	23.362	22.833	22.962
39432	COMBO-0332302-274505	$03^{{\rm h}} 32^{{\rm m}} 30\hbox{$.\!\!^{\rm s}$ }218$	$-27\hbox{$^\circ$ }45\hbox{$^\prime$ }04\hbox{$.\!\!^{\prime\prime}$ }54$	22.545	22.344	21.977	22.089
41159	COMBO-0332109-274415	$03^{{\rm h}} 32^{{\rm m}} 10\hbox{$.\!\!^{\rm s}$ }924$	$-27\hbox{$^\circ$ }44\hbox{$^\prime$ }14\hbox{$.\!\!^{\prime\prime}$ }73$	23.073	22.931	22.350	22.247
41247	COMBO-0331534-274412	$03^{{\rm h}} 31^{{\rm m}} 53\hbox{$.\!\!^{\rm s}$ }360$	$-27\hbox{$^\circ$ }44\hbox{$^\prime$ }12\hbox{$.\!\!^{\prime\prime}$ }11$	24.048	23.752	22.952	22.622
41776	COMBO-0333358-274400	$03^{{\rm h}} 33^{{\rm m}} 35\hbox{$.\!\!^{\rm s}$ }841$	$-27\hbox{$^\circ$ }44\hbox{$^\prime$ }00\hbox{$.\!\!^{\prime\prime}$ }07$	23.408	23.287	22.981	23.145
42601	COMBO-0332591-274340	$03^{{\rm h}} 32^{{\rm m}} 59\hbox{$.\!\!^{\rm s}$ }067$	$-27\hbox{$^\circ$ }43\hbox{$^\prime$ }39\hbox{$.\!\!^{\prime\prime}$ }53$	22.463	22.176	21.482	21.380
43151	COMBO-0332004-274319	$03^{{\rm h}} 32^{{\rm m}} 00\hbox{$.\!\!^{\rm s}$ }365$	$-27\hbox{$^\circ$ }43\hbox{$^\prime$ }19\hbox{$.\!\!^{\prime\prime}$ }45$	22.726	22.436	22.062	22.216
46562	COMBO-0332479-274148	$03^{{\rm h}} 32^{{\rm m}} 47\hbox{$.\!\!^{\rm s}$ }915$	$-27\hbox{$^\circ$ }41\hbox{$^\prime$ }47\hbox{$.\!\!^{\prime\prime}$ }88$	23.254	22.887	22.096	21.996
47501	COMBO-0331187-274121	$03^{{\rm h}} 31^{{\rm m}} 18\hbox{$.\!\!^{\rm s}$ }698$	$-27\hbox{$^\circ$ }41\hbox{$^\prime$ }21\hbox{$.\!\!^{\prime\prime}$ }30$	22.768	22.291	21.989	21.556
48870	COMBO-0333289-274044	$03^{{\rm h}} 33^{{\rm m}} 28\hbox{$.\!\!^{\rm s}$ }941$	$-27\hbox{$^\circ$ }40\hbox{$^\prime$ }43\hbox{$.\!\!^{\prime\prime}$ }68$	22.550	22.537	22.477	22.738
52103	COMBO-0331256-273908	$03^{{\rm h}} 31^{{\rm m}} 25\hbox{$.\!\!^{\rm s}$ }611$	$-27\hbox{$^\circ$ }39\hbox{$^\prime$ }08\hbox{$.\!\!^{\prime\prime}$ }48$	25.298	24.421	23.274	24.475	SN candidate
52280	COMBO-0333211-273912	$03^{{\rm h}} 33^{{\rm m}} 21\hbox{$.\!\!^{\rm s}$ }079$	$-27\hbox{$^\circ$ }39\hbox{$^\prime$ }11\hbox{$.\!\!^{\prime\prime}$ }92$	20.691	20.529	20.212	20.388
57527	COMBO-0333184-273641	$03^{{\rm h}} 33^{{\rm m}} 18\hbox{$.\!\!^{\rm s}$ }358$	$-27\hbox{$^\circ$ }36\hbox{$^\prime$ }41\hbox{$.\!\!^{\prime\prime}$ }34$	22.785	22.331	21.360	21.260
58758	COMBO-0333037-273611	$03^{{\rm h}} 33^{{\rm m}} 03\hbox{$.\!\!^{\rm s}$ }722$	$-27\hbox{$^\circ$ }36\hbox{$^\prime$ }10\hbox{$.\!\!^{\prime\prime}$ }90$	20.924	20.755	20.298	20.423
59821	COMBO-0332443-273403	$03^{{\rm h}} 32^{{\rm m}} 44\hbox{$.\!\!^{\rm s}$ }276$	$-27\hbox{$^\circ$ }34\hbox{$^\prime$ }03\hbox{$.\!\!^{\prime\prime}$ }31$	22.741	22.564	22.265	22.019

For variability studies we calculated a magnitude change between the two epochs October 1999 and February 2000 and a related error as:

$$\Delta R = R_{{\rm Feb00}} - R_{{\rm Oct99}},$$ (2)

and

$$\sigma_{{\Delta R}} = \sqrt{\sigma_{R_{{\rm Oct99}}}^2 + \sigma_{R_{{\rm Feb00}}}^2}.$$ (3)

Figure 8 shows the magnitude difference of all objects with R<25 versus the combined R-magnitude. The distribution underlines that the photometric errors are realistic and the photometry is essentially consistent among the two epochs.

We selected a first sample of variable objects using the following criteria:

1.

faint cutoff: at least one detection must have R<24;

2.

absolute variability threshold: $\Delta R > 0.1$;

3.

variability significance threshold: $\Delta R /\sigma_{\Delta R} > 5$.

This way 57 entries are selected in the catalogue which were made subject to visual inspection on the sum frames of the two epochs. We eliminated 20 faint objects from the sample which were affected by scattered light or diffraction spikes of 9th magnitude stars. We have confidence in the variability of the remaining 37 objects and list their positions and original photometry from the two epochs in Table 3. The number of variable objects increases strongly with fainter magnitude but faintwards of $R \sim 22.5$ the significance criterion becomes stronger than the absolute threshold, thereby reducing the numbers again. The published catalogue of course provides the possibility to identify objects with a smaller amplitude of variability.

We show the location of the variables in a (B-V) vs. (V-R) color diagram in Fig. 9. The red magnitude is here only taken from October 1999 images to keep the color indices free of skewing by long-term variability. The October frames have been taken during 9 days, so short-term variability could still have changed the color indices, especially for RR Lyrae stars given their typical periods of $\sim$1 day. Also shown are expected colors for stars from the Pickles atlas (compare with Fig. 7) and for a quasar color library derived from quasar spectra modelled for $z=0\ldots6$ as a combination of power-law spectra, an emission-line contour (Francis et al. 1991) and Lyman-forest.

If we assume an absence of short-term variability, we would classify most objects as quasars or Seyfert galaxies, while a few could be stars. Of course, the stellar sample could contain some RR Lyrae stars whose variability time scale of $\sim$1 day could offset their colors. These color offsets could move the stars in any directions (+/-) along any of the color axes with equal probability, and a scattered sample should appear with a comparable fraction of stars below and above the original color sequence.

In Fig. 9 we see that only three objects show up below the stellar sequence, while $\sim$30 are located above. Therefore we conclude that most of the variable objects presented are indeed quasars while some objects are Seyfert-I galaxies where bright host galaxies dominate the colors. The COMBO-17 group has meanwhile reduced all data on the Chandra field and prepared lists of stars, galaxies and quasars at , which are analysed in forthcoming papers.

Among the variable objects are also three candidates for Supernovae (see Table 3), which had their bright phase in October 1999 and are undetectable to visual inspection in the February co-added frame. The first two showed up as a significant off-center brightening of a tiny faint galaxy (objects 16404 and 52103 in Table 3).

The third candidate has a rather red B-V color (object 36683 in Table 3). It is a point source located only  $2\hbox{$.\!\!^{\prime\prime}$ }0$North and  $0\hbox{$.\!\!^{\prime\prime}$ }9$ East of a small galaxy with $R=22.93\,\pm\,0.02$ (no calibration errors included). There is still non-zero flux measured at the location of the transient object in February 2000 formally yielding $R=25.41\pm 0.19$, but it is not clear, whether the light originates from the outskirts of the neighboring galaxy or still from the Supernova.

In October it was measured on Oct. 10 with $V=23.75\pm0.04$, on Oct. 13 with $B=25.56\pm0.17$ and on Oct. 19 and 20 consistently with $R=23.33\pm0.02$. Again, we can not decide on the B band sum frame with $1\hbox{$.\!\!^{\prime\prime}$ }2$ PSF whether the B flux is contributed from the galaxy or from the candidate. If it was a Supernova, the B measurement bracketed by V and R imaging could not be explained by variability. Therefore, the object must be unusually red with and it is not entirely clear whether the Supernova hypothesis can account for this color.

We note that the Supernova 1999gu reported by Cappellaro et al. (2000) and first observed with the WFI on Dec. 29, 1999 at $\alpha_{{\rm J2000}} = 3^{{\rm h}}33^{{\rm m}}00\hbox{$.\!\!^{\rm s}$ }1$ and $\delta_{{\rm J2000}} = -27\hbox{$^\circ$ }51\hbox{$^\prime$ }40\hbox{$^{\prime\prime}$ }$ in a galaxy at z=0.147 is clearly visible in the images of February 2000 with R=21.5, but since our object list was defined on the deep sum frame of October 1999, it is not contained in the catalogue presented here.

Figure 9: Color-color diagram of variable objects (black crosses) compared with the expected stellar sequence (black dots) and a quasar color library (grey dots). Assuming the absence of short-term variability, most objects would be AGNs. Two Supernova (SN) candidates are within the limits of this diagram.