Proceedings of the Visnjan School of Astronomy 2001 - Supernovae Group Report

VSA2001 Report from the Supernova-Search Group

Gregor Kervina⁽¹⁾, Natko Bajic⁽²⁾, Nikola Strah ⁽³⁾, Sonja Milicic ⁽⁴⁾ ,

⁽¹⁾ Srednja pomorska sola, Portoroz, Slovenia; gregorkervina@hotmail.com, ⁽²⁾ III gimnazija, Split, Croatia; natkobajic@yahoo.com, ⁽¹⁾ Gimnazija Varazin, Varazdin, Croatia; nstrah@astro.hr, ⁽⁴⁾ Gimnazija Matija Mesic, Sl. Brod, Croatia; smilicic@eskola.hfd.hr

Abstract

During the VSA 2001 our group was searching for supernovae. We took images of galaxies every night, and compared them with previosly taken images from diferent Data Archives. At the end we didn't found anything, but we learned a lot of new techniques of image processing. We also learned wich technique for images comparison is the most efficient.

Introduction

A supernova is an exploding star. There are at least two types of supernovae: Type I and Type II, originated by different astrophysical mechanisms. The difference between the two is simply the presence or absence of the Hydrogen lines in the spectrum of the supernova.

Type I supernovae has no Hidrogen lines in their spectra. They aren't as well understood, but it is clear that they have nothing to do with the normal evolution of single stars. Scientists believe that type I supernovae occur in binary systems that contain a White Dwarf star. Due to the effect of gravity, material from the companion star accretes onto the white dwarf. When the white dwarf exceeds the limit of 1.44 solar masses, it will collapses, causing the carbon to ignite, which then causes an explosion.

Type II SNe, with clear Hidrogen marks in their spectra, are originated by the core collapse of a single, very massive star. SNe type I are likely to occour on almost every galaxy type (spirals, ellipticals, etc.), while SNe type II occour only in early types, like spirals, with preference for the rich star forming regions of these. Each type of SNe has also at least two important subclasses (type Ia and Ib, and type II linear and type II plateau). These distinction are mainly based on their photometrical properties.

The aim of our group at the current year Višnjan School of Astronomy (VSA 2001) was to experience how a scientific research program is planned, conducted and concluded. In the following sections we will present the results of our work.

Used Instrumentation and Comments

During our work we used a rich set of hardware and software: Visnjan Obs. 0.4m, f/4.3 Newton + AP8 CCD was the opto-electronic image acquisition system. The output of this instrumental complex is a .FITS format CCD image, spanning almost 1x1 square degree, with a pixel-scale factor equal to 2.7arcsec/px. The CCD camera itself has a 1024x1024 pixels array, each pixel being 24 micrometer square. The typical limiting magnitude with a 60 second of exposure time (cooling @+10C) in a clear and transparent night was usually around 18. In order to increase the S/N ratio of our frames, we composed (added) mos t of the times two to three images in a final one. An autoguiding system, based on a 200mm f/10 Schmidt-Cassegrain telescope + SBIG ST6 CCd camera was used. The telescope was set to the target galaxies by the Gandalf (T.M.) software, developed by M.Jurić. The image processing was performed using Astroart 1.1 software. For our photometry of SN fields, we never applied any filtering nor unsharping process. Some ancillary software was used as well, like Guide-6 for finding the targeted star fields, Simbad database for extracting data about galaxies and cluster of galaxies, POSS (Palomar Observatory Sky Survey) has provided the galaxies reference images.

During our work we had encountered a number of different problems, but most of them were successfully solved. For example, we changed our observing strategy in the course of the school, in order to increase our possibilty of finding a new supernova. In several images, we found some (possible) moving objects and we spent a lot of time trying to identify them. We thought they might be asteroids, but later we found out that they were simply spurious camera defects. There was a lot of problems with hot pixels and the background noise because every SN candidate we found turned out to be just a camera artifact. We operated the telescope manually because we had to decide on the moment if we have to make a new image or not, which depended on the brightness of the galaxy and on the image quality. We also met some problems with pointing the telescope, so we were forced to image galaxies that were on the S-E part of the sky. In spite of everything we took a lot of images. The weather was serving us nicely in the last part of VSA 2001.

Data reduction procedure

The image processing was restricted to the absolutely necessary steps: first we imported the images from the database to our directory, then we opened them on our PC using Astroart. In Astroart we subtracted the dark frame from image and devided it with flat field. Normally we recorded with telescope a few images (tipically two to three), in order to compose them together. So in the program we composed all the single images of the same field in the final one. With this step we increasesed the total exposure time, and so the limiting magnitude on our final image. After this we cheked if there were any new objects in the galaxies. We compared our pictures with the reference ones that we took from internet archive. If a sospect object was found, then we made additional pictures of the same galaxy in the following nights, in order to discriminate a possible newly discovered SN from noise effects or artifacts of our CCD camera and, if the pictured star field was in the Ecliptic area, from an asteroid passing by on the galaxy star field.

Obtained Results and Their Discussion

These SN-search observig program was very exciting, because we got some suspect candidates in a few galaxies we pictured, but unfortunately all of them were lately recognized to be CCD artifacts or background noise originated by cosmic rays. We also realized that such search is very time consuming: while a single image required only 2 minutes to be acquired, later it needed almost 30 minutes to be processed and completely analized. In this way we were unable to process all the 5 to 10 star fields we usually we recorded on each clear night. So another strategy should be considered in order to speed up the image processing and analysis. For example, the professional SN-search programs usually operate in a completely automatic way, also for the identification of any possible SN in their plates. From a statistical point of view, we consider that in the first two days our efficiency at the telescope (number of images per hour) was rather low, because we had yet to make experience with the telescope, the software, and so on. Since the third night on, we were able to collect a total number of images close to 40 (in 3 hours of oberving time), providing an average rate of almost 13 images per hour. The professional automated SN-search are able to record 50 to 70 images per hour. So we would expect that our rate of discovery would be almost four times smaller of that of a professional SN-search as the LOSS. Usually the LOSS cooperation is discovering three to five SNe each month. If our searching program has continued for other three or four weeks, we have a chance for discovering our own SN. So it is not surprising that on a total number of only three complete and good observing nights, the total number of SN discovered by us is equal to zero.

Target objects:

List of the main Galaxies and Galaxies Clusters recorded:

NGC 7177, NGC 7206, NGC 7272, NGC 7332, NGC 7335, NGC 7497, NGC 7535, NGC 7550, NGC 7726, NGC 7741, HCG 92, M31, M33, NGC 7290, NGC 6944, NGC 6951...

But there are some 100 more galaxies that are in the same field of view, but are fainther.

Conclusion

During the VSA 2001 our group has conducted a SN-Search program. We have experienced the procedure of SN search that is described in details in this paper, giving some useful addresses if any similar observing program would be established in future VSA's.

And now for some images of galaxies that we took

HCG 92

NGC 6951

Acknowledgements

We would like to thanks the ICSC-World Laboratory for the support and Višnjan Observatory staff, and in particular K. Korlević and M. Jurić, for their kind hospitality and for the continous support their gave us in the conduction of our observing program.

We would also like to thanks the other groups of the VSA2001, for their helpful collaboration of the telescope time allocation.

References

Jelena Perić, Gregor Kervina and Giovanni Sostero: Report from the Supernova-Search Group , http://www.astro.hr/vsa2000/VSA_Report/Groups/SN2000/SN_report/VSA_SNreport/vsa_snreport.html
: Supernovae, http://www2.arnes.si/~gljsentvid10/supn1.html