3 Photometry

3.1 Long term variability

Broadband photometric observations of LS 5039 from 1973-present have been reported by a number of authors (e.g. Drilling 1975; Lahulla & Hilton 1992, Ma98) and indicate that between 1973-present LS 5039 has been remarkably stable (e.g. $\Delta V<0.1$ mag.) in the optical. This stability is also evident in the observations presented in Table 3 - obtained by us on 1996 October 3-5 with the 1.9 m telescope of the South African Astronomical Observatory (SAAO) and 2000 August 17-24 with the 0.7 m telescope of the Crimean Astrophysical Observatory (CrAO). With the exception of the single V band datum from Kilkenny et al. 1993 these observations are consistent with no variability.

However JHK band observations made with the Telescope Carlos Sanchez (TCS) between 1995-2000 (Table 4) appear to show significant variability (${\sim}0.4$ mag) in H and K bands respectively. The same standard star (BS 7236) and reduction procedures were adopted for all observations, and given the proximity of BS 7236 to LS 5039 the possibility of differential extinction due to dust close to the horizon can also be eliminated as a cause of such variability. The source of such variability is not clear. The wind from the O6.5V((f)) primary is not expected to contribute significantly at these wavelengths (Runacres, priv. comm.), while extrapolation of the radio flux to near-IR wavelengths yields a flux 3 orders of magnitude too small to explain the observed variability.

 

 

Table 3: Comparison of our 2 epochs of broad band photometry to archival observations presented by Kilkenny et al. (1993, K93), Spencer Jones et al. (1993, SJ93), both made with the SAAO & Ma98 (1998 data).

Observation	B	V	R	I	J	H	K	L
K93, SJ93	-	$11.24\pm 0.01$	$10.59\pm 0.01$	$9.88\pm 0.01$	$9.02\pm 0.01$	$8.79\pm 0.02$	$8.57\pm 0.03$	-
1996 Oct.	$12.18\pm 0.02$	$11.33\pm 0.02$	$10.65\pm 0.02$	$9.87\pm 0.02$	$9.05\pm 0.02$	$8.75\pm 0.02$	$8.60\pm 0.02$	$8.69\pm 0.05$
1998 June 7	-	$11.35\pm 0.03$	$10.64\pm 0.03$	$9.90\pm 0.03$	-	-	-	-
1998 June 8	-	$11.39\pm 0.03$	$10.69\pm 0.03$	$9.95\pm 0.03$	-	-	-	-
2000 Sept.	$12.17\pm 0.03$	$11.32\pm 0.01$	$10.61\pm 0.01$	$9.91\pm 0.01$	-	-	-	-

 

 

Table 4: Broad band JHK photometry obtained with the TCS in the period 1995-2000.

Date	J	H	K
14/10/95	$9.06\pm 0.01$	$9.04\pm 0.01$	$8.93\pm 0.03$
31/7/96	$9.05\pm 0.03$	$8.93\pm 0.03$	$8.85\pm 0.03$
19/7/97	$9.06\pm 0.01$	$8.91\pm 0.01$	$8.88\pm 0.01$
21/7/97	$9.07\pm 0.02$	$8.90\pm 0.02$	$8.83\pm 0.02$
26/7/99	$9.04\pm 0.02$	$8.91\pm 0.02$	$8.82\pm 0.01$
28/7/99	$9.10\pm 0.02$	$9.04\pm 0.02$	$9.01\pm 0.02$
31/7/99	$8.97\pm 0.02$	$8.71\pm 0.02$	$8.58\pm 0.02$
2/10/99	$9.01\pm 0.02$	$8.76\pm 0.02$	$8.63\pm 0.02$
7/10/99	$9.02\pm 0.02$	$8.74\pm 0.02$	$8.55\pm 0.02$
6/7/00	$8.99\pm 0.02$	$8.75\pm 0.02$	$8.65\pm 0.04$
17/10/00	$9.05\pm 0.02$	$8.81\pm 0.02$	$8.65\pm 0.01$
18/10/00	$8.97\pm 0.02$	$8.68\pm 0.02$	$8.53\pm 0.01$

3.2 Medium term variability

In order to search for medium term, day-to-day variability - possibly indicative of orbital modulation - photometric observations of LS 5039 were made using the 1.3 m telescope at the Skinakas Observatory (Crete, Greece). Standard Johnson B and V filters were used in combination with a tektronix $1024 \times 1024$ CCD with 24 $\mu$m pixels. The source was observed on 21 nights during the period May 12, 2000 to June 24, 2000. The seeing during the observation run varied between ${\sim} 1''$ and ${\sim} 3''$. Standard image processing (bias subtraction and flat-fielding) was applied to all images.

To improve the accuracy of the results we performed relative photometry of the source with respect to three reference stars in the CCD frame. We checked the constancy of these three stars by obtaining the difference between one of them and the average of the other two. The standard deviation of the individual light curves throughout the period was ${\leq} 0.01$. An idea of the accuracy of our relative photometry can be obtained by taking the standard deviation of the global light curve of the three reference stars: $\sigma_B=0.009$ and $\sigma_V=0.01$ for the B and V bands, respectively.

LS 5039 shows variations within a night with an amplitude of a few hundreths of magnitude. These low amplitude variations also occur from one night to the next but on a longer time scale no particular periodic or quasiperiodic trend is apparent. From these observations we can exclude the presence of (periodic) orbital modulation between 3-15 days, noting that if periodic variability were present at the level observed in HDE 226868 (Brocksopp et al. 2000) - the mass donor in Cygnus X-1 - our observations would have detected it. Aperiodic photometric variability at this level is not unknown in O type stars. Balona (1992) surveyed 16 bright O stars and found variability in all the supergiants observed, and a subset of the main sequence stars, including HD 101190, an O6V((f)) star (for which no periodicity was identifiable). Balona (1992) suggests that either variations in mass loss rates or non-radial pulsations could cause the observed variability but is unable to discriminate between the two possibilities.