The TDSS/SPIDERS/eBOSS Pilot Survey

Contact

For TDSS: Paul Green
Harvard-Smithsonian Center for Astrophysics
pgreen@cfa.harvard.edu

For SPIDERS: Andrea Merloni
Max-Planck-Institute for Extraterrestrial Physics
am@mpe.mpg.de

Summary

A pilot survey during SDSS-III designed to find targets in optical and X-ray imaging useful for planning subsequent phases of the SDSS

Finding Targets

An object whose ANCILLARY_TARGET2 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name)	Bit number	Target Description	Number of Fibers
TDSS_PILOT	24	Objects identified from PanSTARRS imaging using the selection criteria described below	857
SPIDERS_PILOT	25	Objects identified from XMM-Newton x-ray imaging using the selection criteria described below	367
TDSS_SPIDERS_PILOT	26	Objects identified using both sets of selection criteria	109
TDSS_PILOT_PM	28	Objects identified using X-ray imaging that has high proper motions	129
TDSS_PILOT_SNHOST	29	Objects identified in PanSTARRS imaging that showed transient behavior in extended objects	7

Description

This program carried out pilot observations during SDSS-III of two fields, in preparation for two components of the subsequent SDSS-IV eBOSS survey: the Time Domain Spectroscopic Survey (TDSS) and the SPectroscopic IDEntification of eROSITA Sources (SPIDERS) survey.

The quest for efficient, reliable and unbiased selection of quasars for large spectroscopic surveys remains a crucial endeavor for a variety of studies. Very wide field, sensitive optical cameras such as PanSTARRS-1 and PTF (and in the future, Gaia and LSST), as well as the eROSITA all-sky X-ray survey, promise to bring variability and X-ray selection to improve the efficiency of quasar targeting efforts.

Target Selection

This SDSS-III program targeted quasars in two fields. The first field encompasses existing XMM-Newton Large-Scale Survey (XMM-LSS), deep multi-band CFHTLS field imaging, and a Pan-STARRS1 (PS1; Kaiser et al. 2002, Kaiser et al. 2010) medium deep survey field (MD01) with hundreds of epochs. The second field is also a PS1 medium deep field (MD03), located in the Lynx/IfA Deep Field. Both fields have 3-4 times as many epochs from PS1 as the Stripe 82 survey area had from SDSS (Annis et al. 2014), and PS1 was continuing to monitor these fields at the time that BOSS spectroscopy of these plates was carried out.

There were five target selection algorithms on these plates, described below.

Target Selection

Five sample selections were tested on these plates, as follows:

Objects with the TDSS_PILOT target class were selected from PS1 photometry calibrated as described in Schlafly et al. (2012). Targets were selected by variability within each of the gri filters, with the requirement of a median PS1 magnitude 17 < mag_x < 20.5 and at least 30 observed epochs within that filter.

Objects were required to be pointlike in SDSS, with the difference between their PSF and model magnitude less than 0.05 in each filter, and with no detectable proper motions. Lightcurves for objects that passed a variability threshold in at least one filter [following Kim et al. (2011)] were visually inspected in all three filters.

We assign each object a priority based on the number of passed criteria summed over filters, the source brightness, and whether or not a BOSS spectrum already exists for the object.

Objects identified as TDSS_PILOT_PM were selected in the same way, but this target flag value marks objects with significant (> 3σ) total proper motion as measured by the SDSS.

Objects identified by the TDSS_PILOT_SNHOST target flag showed transient behavior in extended objects in the PS1 medium deep photometry, as described in Chornock et al. (2013).

Objects identified by the SPIDERS_PILOT target flag were selected as X-ray sources with clear optical counterparts in SDSS DR8 imaging. The X-ray selection was performed on a source catalog constructed from public XMM-Newton data in the XMM-LSS survey area following the procedure described in Georgakakis & Nandra (2011). The sample was flux-limited in soft X-rays (0.5-2 keV) to the expected limit of the eROSITA deep field survey (~6 x 10^-15 erg s^-1 cm^-2), and were required to have 17 < r_PSF < 22.5 and not to have been spectroscopically observed by BOSS as of DR9. Objects with higher soft X-ray flux were given higher priority in fiber assignment.

Objects targeted by both the SPIDERS and TDSS algorithms were given higher priority, and were assigned the TDSS_SPIDERS_PILOT target class.