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Survey of nuclear transient in galaxies with Gaia

(P52-019, PI: Seppo Mattila)

Abstract:

Over the next five years, the Gaia satellite will provide an unbiased survey of the entire transient sky down to magnitude 20. Gaia will thus detect a very large number of supernovae and other transient objects. Its uniqueness for this project will be its ability to detect supernovae at very small angular offsets (0.3 arcsec) from the nuclei of their host galaxies that have remained hidden in most previous surveys. To fully exploit the Gaia discoveries, their ground based spectroscopic classification is required. Here, we propose to start a new programme at the NOT to perform spectroscopic classification of nuclear transients discovered by Gaia. This will allow the estimation of the relative and absolute supernova rates for the first time within the nuclear regions of galaxies. Furthermore, in classifying a number of nuclear transients, we expect to identify rare and peculiar objects which are primary targets to be followed-up for detailed studies.


Observing instructions for ALFOSC spectroscopy and photometry:

  • Required setup: R_Bes 650_130 (#76) and WG345 356_LP (#94) filters; Grism#4, Grism#5, Grism#7 and Grism#8; narrow 0.75" and 1.0" slits. Please, note that the Atmospheric Dispersion Corrector (ADC) should always be used in the spectroscopic observations unless all the observations are carried out using the parallactic angle. The WG345 356_LP (#94) blocking filter should be used in all the observations with Grism#7.
    • NB: Also other standard filters: U_Bes 362_60 (#7), B_Bes 440_100 (#74), V_Bes 530_80 (#75), and i_int 797_157 (#12) maybe required for some targets.

  • All the scripts are available in the directory ~/scripts/52-019/

  • Obtain 11 full and 11 windowed biases during daytime before the twilight time or in the morning after the twilight either by running the script bias or the script alfosc-calibs. The bias script takes ~10 min to run.

  • Take 3 imaging flat fields in the R-band using twilight sky, either in the evening or in the morning. If not possible, we will use the archive flats.
    • NB: Also UBVi flat fields are required if imaging observations were taken in these bands.

  • The exposure time of the spectrum depends on the magnitude of the supernova:
    • If V<17mag the on-source exposure time is 1 x 600 sec
    • If V=17-18mag the on-source exposure time is 1 x 1200 sec
    • If V=18-19mag the on-source exposure time is 1 x 2400 sec
    • If V>19 the on-source exposure time is 1 x 3600 sec

    How to observe a supernova:

    • If you encounter problems with the scripts, see instructions below how to take target observations manually. Please note, that the scripts will take care of pointing to the object, acquisition on the slit, changing the optical elements and focus-delta values, and taking the necessary calibrations.

    • Unless otherwise noted, obtain a 60 sec R-band image of the supernova by running a script:
      too_sn_phot "supernova" "exptime" "exposures" "filter" (U, B, V, R or I)
      e.g.: too_sn_phot SN2015XX 60 1 R
      • NB: If mentioned, images with other standard UBVRi filters may also be required.

    • Obtain a spectrum of the supernova using the script:
      too_sn_spec "supernova" "exptime" "Grism_#x" "Slit_y" "ADC" "angle" "acqtime",
      where x is 4, 7 or 8; y is 1.0 or 0.75; ADC on or off; and angle the P.A. or parallactic      ,
      e.g.: too_sn_spec SN2015XX 1800 Grism_#4 Slit_1.0 on 95 15
      • If using a position angle different from the parallactic angle, the ADC should always be used.
      • If seeing <1.0", the narrow 0.75" slit should be used (if available), otherwise the narrow 1.0" slit should be used. Please, note that for some faint supernovae embedded in the nuclear regions of the host galaxy, the narrow 0.75" slit could be requested regardless of the seeing.

    • Observe the standard star always using the parallactic angle and the same setup (slit, grism, blocking filter, ADC) as for the supernova observations. To obtain the spectrum use the script: too_sn_spec,
      e.g.: too_sn_spec Feige34 100 Grism_#4 Slit_1.0 on parallactic 15
      • Use one of the following standards (sorted in the order of preference):

        • ING Name Name RA (J2000.0) Decl (J2000.0) Spect. exp. time Finding chart
        SP1036+433 Feige 34 10:39:36.71 +43:06:10.1 100 sec Feige34.gif
        SP1234+254 Feige 66 12:37:23.55 +25:04:00.3 50 sec Feige66.gif
        SP0501+527 G191B2B 05:05:30.6 +52:49:56 120 sec G191B2B.gif
        SP1550+330 BD+33 2642 15:51:59.9 +32:56:53 60 sec BD+33.2642.gif
        SP2148+286 BD+28 4211 21:51:11.1 +28:51:52 60 sec BD+28.4211.gif

        Visibility plot of the standards is here.


      Taking target observations manually:

      • Use only if you encounter a problems with the scripts in /scripts/52-019/

      • Set rempath /data/service/52/019/

      • Set remsave_on to save the images in a remote directory

      • To observe a supernova preset to the target with the selected or parallactic angle.

      • Obtain a 60 sec R-band image (full frame) of the supernovae before taking the spectrum.

      • If using a position angle different from the parallactic angle, preset the ADC before pointing using the continous mode tcs.adc-arm-in and tcs.adc-mode-automatic-continous and run alfosc.focus-offset to take into account the effect of the ADC on the focus.

      • For all the spectroscopic observations including calibrations use CCD window of 500 pixels in the spatial direction: xbeg 800 and xsize 500.

      • IMPORTANT. Obtain a 10 sec spectroscopic flat field before and after the on-source exposure and a 3 sec HeNe arc after the on-source exposure with the telescope pointing to the supernova and without changing the optical configuration of the telescope, e.g. moving slit or grism wheel. See: http://www.not.iac.es/observing/cookbook/current/Cookbook.php?instrument=ALFOSC#Calibs

      • Obtain a spectrum of the supernova using grism#7 (with the WG345 356_LP (#94) blocking filter) or grism#4 or grism#5 or grism#8 and the narrow 0.75" or 1.0" slit depending on the seeing and the availability of the 0.75" slit (the 0.75" one if the seeing is better than ~1", otherwise the 1.0" one). See: http://www.not.iac.es/observing/cookbook/current/Cookbook.php?instrument=ALFOSC#spectroscopy

      • During the night obtain a spectrum of a standard star using the same setup (slit, grism, blocking filter, ADC) as for the supernovae. However, for the standard star observation always use the parallactic angle.

      • After all the exposures have been taken, set remsave_off.


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