Draft version December 4, 2018 Typeset using LATEX preprint2 style in AASTeX61 THE BENEFIT OF SIMULTANEOUS SEVEN-FILTER IMAGING: 10 YEARS OF GROND OBSERVATIONS J. Greiner1 1Max-Planck-Institut für extraterrestrische Physik, 85740 Garching, Germany ABSTRACT A variety of scientific results have been achieved over the last 10 years with the GROND simul- taneous 7-channel imager at the 2.2m telescope of the Max-Planck Society at ESO/La Silla. While designed primarily for rapid observations of gamma-ray burst afterglows, the combination of si- 0 0 0 0 multaneous imaging in the Sloan g r i z and near-infrared JHKs bands at a medium-sized (2.2 m) telescope and the very flexible scheduling possibility has resulted in an extensive use for many other astrophysical research topics, from exoplanets and accreting binaries to galaxies and quasars. Keywords: instrumentation: detectors, techniques: photometric arXiv:1812.00636v1 [astro-ph.HE] 3 Dec 2018 [email protected] 2 1. INTRODUCTION (5) mapping of galaxies to study their stel- An increasing number of scientific questions lar population; (6) multi-color light curves of require the measurement of spatially and spec- supernovae to, e.g., recognize dust formation trally resolved intensities of radiation from as- (Taubenberger et al. 2006); (7) differentiat- trophysical objects. Over the last decade, tran- ing achromatic microlensing events (Paczyn- sient and time-variable sources are increasingly ski 1986) from other variables with similar moving in the focus of present-day research light curves; (8) identifying objects with pe- (with its separate naming of “time-domain as- culiar SEDs, e.g. photometric redshift surveys tronomy”), recently boosted spectacularly by for high-z active galactic nuclei, or identifying the follow-up of gravitational wave sources. brown dwarfs; (9) observations of transiting If the spatial scale of such a study is small extrasolar planets to infer orbital periods, mul- (few arcmin), integral field spectrographs such tiplicity of planets, or characteristics of their as PMAS (3.6m Calar Alto) or MUSE (VLT) atmospheres (Jha et al. 2000); or (10) map- or ESI/OSIRIS (Keck) are the instruments of ping of reflectance of solar system bodies as a choice. If crowding is not an issue, (objective) function of their rotation to map their surface prism spectroscopy is a valuable option (Teplitz chemical composition (Jewitt 2002). et al. 2000). For large scales, simultaneous Instruments with simultaneous imaging ca- multi-channel imaging is applied. The phys- pability in different filter bands prior to the ical measurement goals often request a com- GROND development include ANDICAM (De- promise between spatial, temporal or spectral poy 1998), BUSCA (Reif et al. 1999), HIPO resolution, which adds to the challenges of the (Dunham et al. 2004), MITSuME (Kotani et measurement principle. al. 2005), TRISPEC (Watanabe et al. 2005), Simultaneous imaging in different filter-bands SQIID (Ellis et al. 1993), and ULTRACAM (whether Johnson UBV RIJHK or Sloan (Dhillon et al. 2007). GROND-inspired instru- u0g0r0i0z0 or anything else) is of interest in a va- ments include the 6-channel RATIR (Butler riety of astrophysical themes. The primary aim et al. 2012) and the 4-channel ROS2 (Spano is to measure the spectral energy distribution et al. 2010) instruments. Further projects for (SED) or its evolution in variable astrophysi- simultaneous multi-band instruments are the cal objects, in order to uncover the underlying 8-channel OCTOCAM (Gorosabel & Ugarte emission mechanism. Examples are, among Postigo 2010), selected as part of the Gemini others, (1) monitoring of all kinds of variable instrumentation program in 2017 (Roming et stars (flare stars, cataclysmic variables, X-ray al. 2018), the 4-channel SPARC4 (Rodrigues binaries) to determine the outburst mechanisms et al. 2012) planned for installation at the 1.6 and differentiate between physical state changes m telescope of the Pico dos Dias Observatory and changes induced by geometrical variations, (Brazil) (Bernardes et al. 2018), an unnamed like eclipses; (2) follow-up of gamma-ray burst 8-channel imager for the IRTF (Connelley et (GRB) afterglows for e.g. rapid redshift es- al. 2013), and the SIOUX project (Christille et timates, mapping the SED evolution to mea- al. 2016). In comparison, the GROND instru- sure circumburst parameters, or the search for ment (Greiner et al. 2008) at the 2.2 m telescope dust destruction; (3) monitoring of AGN to of the Max-Planck Society (MPG) in La Silla understand the physical origin of the observed (ESO/Chile) with its 7 simultaneous channels variability; (4) determining the inclination of so far still delivers the largest degree of multi- X-ray heated binaries (Orosz & Bailyn 1997); plexing at such a telescope size. 3 the visual, plus three (standard JHKs) bands in the near-infrared (NIR). The separation of the different photometric bands was achieved using dichroic beamsplitters (in the converg- ing beam), whereby the short wavelength part of the light is always reflected off the dichroic, while the long-wavelength part passes through (Fig.1). The use of dichroics implies that adja- cent bands do have identical 50% transmission wavelengths, making the Sloan filter system (Fukugita et al. 1996) the obvious choice for Figure 1. Scheme of the optical beam path of the visual bands. GROND with the optical components and the de- The field-of-view (FOV) of the camera was tectors labeled. [From Greiner et al.(2008)] c AAS. designed, on one hand, to cover the typical Reproduced with permission. few arcmin extent of GRB error boxes, and on the other hand have a pixel scale less than After a short description of the main features the mean seeing to allow for accurate photom- of the instrument and operational aspects ( 2), I etry. Mounted at the MPG-owned 2.2 m di- describe some of our prime scientific resultsx ob- ameter f/8 telescope on La Silla (ESO/Chile) tained via GROND observations, foremost for with an intrinsic image quality of 000:4, the GRBs ( 3) and transients ( 4), but also other x x FOV of each visual band is 5:4 5:4 arcmin2, science topics where color information on short × (2048x2048 CCD with plate scale 000:158/pixel), timescales is important ( 5 9). While this x − and 10 10 arcmin2 in the NIR using a fo- is predominantly a review, it contains hitherto × cal reducer (1024 1024 Rockwell HAWAII-1 unpublished results, e.g. on the discovery of a × array with a plate scale of 000:60/pixel). A Sum- hitherto unknown T5 brown dwarf. itomo closed-cycle cooler provides a tempera- ture of 65 K for the NIR detectors and 80 K for 2. THE GROND INSTRUMENT AND ITS the focal reducer optics, with simple damping OPERATION preventing any telescope/instrument vibrations The primary goal was to rapidly identify GRB which could degrade the image quality. The afterglows and measuring their redshift. This best GROND images have a full-width-half- led to the concept of a camera which allows maximum of 000:6, dominated by the dome see- simultaneous observations in multiple filters ing. This allows us to linearly increase sensitiv- throughout the optical and near-infrared re- ity by adding more exposure (stacking) up to 3– gion. The simultaneity is dictated by the fact 4 hrs, before becoming background-dominated that a typical GRB afterglow initially fades by (see, e.g. Fig.2). about 2–3 mag within 5–10 min after the GRB, The standard detector readout systems which and by another 3 mag in the following 50 min, were used at ESO at the time were imple- thus rendering cycling through different filters mented, i.e. FIERA (Beletic et al. 1998) for the useless. Furthermore, with the advent of Swift’s visual channels, and IRACE (Meyer et al. 1998) detection of 100 GRBs/yr, follow-up of each for the NIR channels. This makes for a very ∼ GRB with an 8 m telescope became imprac- flexible readout scheme, where e.g. NIR expo- tical, and some knowledge-based pre-selection sures continue during the CCD-readout. Since was needed. Four bands were implemented in 4 Figure 2. An exam- ple for the sensitivity of [email protected], reach- ing g0= 26.5 mag in 3.5 hrs exposure time (Yates et al. 2015), likely one of the deepest images from a ground-based 2 m class telescope. The green numbers are SDSS- calibrated g0-magnitudes. This g0-band image is 1:01 1:08; North is up, and East× to the left. the JHKs channels operate fully synchronously, At the start of each OB, the instrument is au- a 10 s exposure was adopted as a compromise tomatically focused by moving the telescope’s between not saturating the Ks-band while max- secondary mirror. imizing J-band exposure per telescope dither Best possible instrument efficiency has been position. In addition, a separate internal dither the main driver during the design and devel- mechanism was implemented in the Ks-band; opment of GROND. As a result, the total ef- full details can be found in Greiner et al.(2008). ficiency in the visual bands is about 70% (ex- While originally foreseen to only operate in cept the z0 band), and is still above 50% for the robotic target-of-opportunity mode for chas- three NIR bands (Greiner et al. 2008), despite ing GRB afterglows, the GROND operation the eleven lenses per channel and the compara- scheme was designed flexibly enough to allow tively low quantum efficiency of the 2001-built also visitor-mode style “manual” observations. HAWAII detectors. Thus, even in a single filter, All parameters for GROND observations can GROND is the most sensitive instrument at a be adapted through standard ESO-style ob- 2 m-class telescope.