Image of the Week

Gaia's uncalibrated BP/RP spectra

Figure 1. Observation of an extreme nuclear transient (ENT) by Gaia. Left panel shows the observation in G magnitude over the years. Right panel shows the uncalibrated Gaia BP and RP spectra presented on pixel space. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO.

Gaia observed the whole sky for 10.5 years according to its scanning law and has seen sources about once or twice per month on average, making Gaia particularly suited to the detection of slowly varying objects. The Gaia Photometric Science Alerts System first noticed a brightening in a faint Gaia source on 12 August 2018. It had become over 10 times brighter in the space of 1.5 years, but was not associated with any previously classified source.

The above movie shows the time evolution of the Gaia observations of the transient event Gaia18cdj, as published by Gaia's Photometric Science Alerts. Each spectrum in the right hand panel was measured with Gaia only seconds after the corresponding G-band observation (highlighted in orange) in the left-hand panel.

All Gaia sources, if they are bright enough, will have both a Blue Photometer (BP) and a Red Photometer (RP) spectrum. Though when the sources are faint this spectrum can look very noisy. In the video one can see that as the source brightened, the spectra became less noisy, but also that the flux increases more in the BP than the RP, i.e. changing colour to become more blue. A source becoming bluer suggests that it is becoming hotter.

The spectra shown in the above video are uncalibrated Gaia spectra. It can be seen that the spectral shape changes but not the full spectrum. These spectra are uncalibrated, which means they are presented in pixel instead of wavelength space, so there is no direct relation seen with the wavelength. If interested, more information can be found from Carrasco+21. A small gap is seen between the blue Gaia BP spectrum and the red Gaia RP spectrum. This is as expected and is caused by sampling on pixel space.

These public data were used by Hinkle+24 to trigger follow-up ground-based spectroscopic observations leading to the identification of a new class of extremely luminous transients (see also Bao+24, Wiseman+25). A story on this discovery was published earlier this year "New transient event class found with Gaia: Extreme Nuclear Events".

Figure 2. GIF created by T. Roegiers based on the artwork of the University of Hawaii by Nancy Hulbirt. The life of an ENT explained. Published here.

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Gaia Data Release 4 is expected in December 2026 and will include time series of individual BP and RP spectra in addition to the mean, i.e., time-averaged BP and RP spectra as were already published with Gaia Data Release 3. More details on the expected contents of Gaia DR4 are here.

Further reading:

• Paper: The most energetic transients: Tidal disruptions of high-mass stars - Hinkle+24
• Paper: ​​​​​​​Gleeok's Fire-breathing: Triple Flares of AT 2021aeuk within Five Years from the Active Galaxy SDSS J161259.83+421940.3 - Bao+24
• Paper: A systematically selected sample of luminous, long-duration, ambiguous nuclear transients - Wiseman+25
• Paper: Internal calibration of Gaia BP/RP low-resolution spectra - Carrasco+21
• Story: New transient event class found with Gaia: Extreme Nuclear Transients

Credits: ESA/Gaia/DPAC, Simon Hodgkin

​​​​​​​Story written by Simon Hodgkin

[Published: 06/10/2025]