On the Variable Nature of Galactic and Extragalactic Objects with Sources from the Faint Sky Variability Survey

Abstract

The Faint Sky Variability Survey (FSVS) has cataloged approximately 23 deg in BVI filters from ∼16 to 25 mag to investigate variability in faint sources at moderate to high Galactic latitudes, encompassing a wide range of goals as is discussed in P. J. Groot et al. (2003, MNRAS, 339, 427). Multiepoch (10–30) observations were made in V spanning minutes to years. Amplitude sensitivity limits were determined from simulated variability tests to be ∼0.015–0.075 mag in V over 18–22 mag. The fraction of point sources from the survey found to be variable reaches 8% between 18 and 22.0 mag in V over both moderate and high Galactic latitudes of 30 and 90 . Overall, the dominant population of variable sources are bluer than a B V of 0.65 and have main-sequence colors, likely reflecting larger populations of RR Lyrae, SX Phe, g Doradus, and W UMa variables than expected. The population of ultracool dwarfs has been investigated using V I color selection to identify the potential for variability studies. An L2 dwarf is the coolest source thus far spectroscopically identified, with candidates roughly predicted to temperatures as cool as L8. Variability information for ultracool dwarfs is mostly lost as the cooler sources become exceedingly faint in V. Thus, variability information is found to be limited to an amplitude sensitivity of mag reaching only to DV ∼ 0.1 late M type dwarfs (M8–M9). Follow-up photometry in the I band has revealed variability in an M8–M9 and an L2 dwarf, with the M8–M9 dwarf showing a 3 hr periodicity likely attributable to rotational modulation of surface features. The population of cataclysmic variables (CVs) has been investigated using a B V, V I color and variability selection criteria to additionally search for a “missing” population of evolved, low mass transfer types as discussed in S. B. Howell, L. A. Nelson, & S. Rappaport (2001, ApJ, 550, 897). Two known CVs, GO Com and V394 Lyr, were observed in the FSVS and identified using the CV selection criteria in the survey. In conjunction with the FSVS photometry, two first possible periods are proposed for V394 Lyr of 2.2 and 5.7 hr. An estimate of the number of CVs expected has been made in comparison to the candidates found and shows an overabundance of candidates by a factor of 3–300 over the V magnitude range of the survey, making additional colors or spectroscopy necessary to identify candidates in a larger sample. An initial sample of eight candidates out of over 1000 had follow-up observations obtained and were identified to be pulsational variables (e.g., RR Lyrae, SX Phe), a W UMa binary, an M dwarf flare star, and a quasar, identifying a sample of the types of contaminating sources. As quasars overlap the color space of CVs and variability can provide limits on theoretical models for the unresolved interior processes, the population of quasars has been investigated for variability with a sample of known and newly discovered sources. The initial sample is predominately radioquiet—only one out of 27 is identified as radio-loud as determined from overlapping radio surveys—and potentially very large (∼700 to mag) given quasar space densities B ∼ 21 found in the 2dF redshift survey (S. M. Croom et al. 2001, MNRAS, 322, L29). Few quasars (∼10%) are found with intranight and day timescale variability, whereas the entire sample shows long month to year timescale variability at greater amplitudes (typically greater than 0.1 mag), not too dissimilar to that found in other studies (e.g., G. E. Romero, S. A. Cellone, & J. A. Combi 1999, AA G. Gopal-Krishna et al. 2000, MNRAS, 314, 815; M. R. S. Hawkins 2000, A&AS, 142, 465). The sample of radio-quiet, low-redshift, low-luminosity quasars classified as point sources are found to have brightness variations correlated with changes in the seeing FWHM of ∼0.03 mag arcsec , thus showing a false variability signature. Such a correlation is attributable to a detectable contribution of the host galaxy and provides a warning in studies of variability on a seemingly pointlike source at these amplitude levels. Overall, the FSVS provides a discovery survey data set with suggestions for future time-sampled, wide-field imaging surveys. In regards to other surveys, the FSVS provides a faint sample of variability to large-sky photometric surveys (e.g., SDSS) and extending brighter variability surveys (e.g., ROTSE, LONEOS, BSVS). Further detailed follow-up studies are in progress to extend the science results.