We need your help to observe Gaia Alerts! Gaia is great for finding transients, but not so good at monitoring them, as Gaia is constantly on the move and looking for new transients. So, we need to use other telescopes on the ground to do “follow-up” observations of Gaia discoveries. Here, you will find some information on what this entails, and details on how to take part.
If you are in a school, then you can use the LCOGT telescope network to observe Gaia Alerts. Your teacher will have the details of how to schedule observations. Access for schools in the UK and Ireland is provided through the Faulkes Telescope Project (teachers need to register for their classes to observe with the Faulkes Telescope Project).
To locate an Alert on the sky, we need to know its coordinates. The Gaia satellite gives us the “Right Ascension” and “Declination” for everything it observes. For convenience, these are often shortened to “RA” and “Dec”. The RA and Dec of an Alert are similar to latitude and longitude on a map of the Earth – the Dec changes between the poles and equator, while the RA changes as you spin the sky around the axis of the Earth.
The selected Alerts which we have listed here are generally observable right now. But some of them may not be, depending on whether you are in the northern or southern hemisphere, or what time of year it is. You can check whether an Alert can be observed based on its coordinates [here].
Along with where an Alert is, we also need to know how bright it is. Astronomers use a slightly strange system for measuring the brightness of objects in the sky, called the magnitude system. The magnitude system originally was based on what could be seen with the naked eye from a dark site. The brightest stars that could be seen were defined as having a magnitude of around 0, while the faintest stars had magnitude of 6. Since the invention of the telescope in the early 17th century, astronomers can see fainter stars, and so the magnitude scale has been extended beyond 6. The faintest stars which can be seen at present, using telescopes such as the Hubble Space Telescope, have magnitudes of around 28, which is about a billion times fainter than what we can see with our eyes.
When observing Gaia Alerts, the higher the magnitude, the fainter an Alert is. For fainter Alerts, we either need to use a larger telescope, or observe for longer with a small telescope.
Most professional telescopes are equipped with filters. These are sheets of coloured glass, which only let through light of a particular colour. Filters are useful for two main reasons: to make observations consistent across many telescopes, and to allow astronomers to estimate the temperature of an Alert object.
When we measure the magnitude of a star, we would like to be able to combine and compare measurements from many different telescopes. However, if the combination of mirrors, glass and camera in one telescope make it more sensitive to blue light, while another is more sensitive to red light, we are not comparing like with like. By using the same filter on both telescopes, we can be sure that we are always measuring light with a particular range of colours (or wavelengths).
The second reason to use filters is that by taking measurements of an Alert’s brightness in two filters, we can see how hot or cool it is. If we take an image in both a red filter and a blue filter, we can see in which filter the Alert is brighter. Hotter objects emit more blue light, while cooler object emit more red light; this is similar to how a piece of metal in a furnace will glow with different colours depending on its temperature.
For the selected Gaia Alerts, you will find a note in the list of targets specifying what filters we are using when observing.
Depending on how bright an Alert is, we will have to use a longer or shorter exposure time when taking an image of it with a camera. The exposure time is simply how long we leave the shutter open on our camera and telescope – longer exposure times let in more light, and allow us to see fainter objects. The following table gives some suggested exposure times for the 1-m Faulkes Telescopes depending on how bright an Alert is (magnitude), and what filter (g, r or i) you are using.
Gaia Alerts are transients, meaning they change in brightness over time. Depending on what sort of Alert they are, they may change over hours, days, weeks or even months. How rapidly an Alert changes determines how frequently we wish to observe it (the cadence); you can find information on this for each target on the list of Alerts.