Image of the Week

Discovery of a year long superoutburst in a white dwarf binary

Figure 1: Light curve of SDSS J080710.33+485259.6 (or Gaia19atk) with data from Gaia, ZTF and Swift-UVOT. The horizonal line denotes the average quiescent value determined from Gaia data, additional Gaia points are not plotted. The orange arrow indicates the duration of the superoutburst. ZTF and UVOT magnitudes are given in AB system. A constant factor has been added to the magnitudes of ZTF-r to match the quiescent level of Gaia measurements. The vertical lines accompanying the Swift UVOT measurements indicate whether X-rays were observed or not by Swift XRT. The period with no data during the superoutburst was caused by the binary being occulted. Credit: Rivera Sandoval et al. 2020

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SDSS J080710.33+485259.6 is an accreting ultracompact white dwarf binary (AM CVn) with an orbital period of approximately 53 minutes. On 8 April 2016 Gaia first observed the source at a magnitude of about 20.8. On average Gaia observes a star about 70 times over a 5-year period. These many data points gathered allow to see a brightening if the source goes into outburst. The Gaia Photometric Science Alerts team keeps an eye out for the sources that suddenly dim or brighten and then sends out alerts to allow the astronomical community to plan a follow-up on the interesting sources.

Such an alert, for this source called Gaia19atk, was announced on 26 February 2019, when the star was observed at a Gaia magnitude of 18.51, being clearly brighter now which suggested the binary to be in outburst. With the alert sent out, the Gaia epoch data for the source was then published on the Gaia Photometric Science Alerts page for Gaia19atk and some follow-up was performed. Looking at the observations from Gaia, one could see that the outburst already started earlier, in November 2018 when the source was seen to be one magnitude brighter than its quiescent level (Kupfer et al. - ATel 12558).

For most sources, the epoch data as obtained by Gaia are not yet available. Gaia epoch data for all Gaia sources are planned to be released with Gaia Data Release 4, and the Gaia Data Processing and Analysis Consortium is continuously developing pipelines, improving calibrations and scaling up the processing to allow for the vast amount of data to be published. But for these sources that qualify for an alert, the preliminarily calibrated Gaia epoch data is published when the alert is announced.

A team of Texas Tech University took a closer look at Gaia19atk and its outburst, which is actually the first ever detected for this source. The outburst started in November 2018 and lasted for a year, making it the longest superoutburst known so far for any short orbital period accreting white dwarf. The team published their results in their paper titled "A year long superoutburst from an ultracompact white dwarf binary reveals the importance of donar star irradiation" which was just accepted by ApJL.

To determine the presence of an outburst, it is very important to know the quiescent level of a source. Thanks to Gaia's great sensitivity, the observations made from 2016 to early 2018 formed a great base to determine the 20.8 mags quiescent level of Gaia19atk. Given the faintness of the object in its quiescent level, the source was not easily detectable through small ground-based telescopes, and if detected, the photometric errors were often large. Only a few other facilities, like the Zwicky Transient Facility (ZTF), provided extra data for this case. The observations by Gaia and ZTF can be found in Figure 1 above. The lightcurve composed of Gaia observations only, can be found in Figure 2.

 

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Figure 2: Light curve of Gaia19atk, taken from the Gaia Photometric Science Alerts page. Image credit: ESA/Gaia/DPAC and the Gaia Photometric Science Alerts team.

 

The data provided by Gaia were essential to determine the amplitude, the duration, as well as the rise and decline rates of the superoutburst of this source. These observables are important factors to help understanding the outburst mechanism. Rivera Sandoval et al. show that the outburst of Gaia19atk lasted for a year, with extremely slow rise and decline rates which are atypical features under disk instability models. In fact, the extreme observed values for this binary have not been observed before in any accreting white dwarf either with Hydrogen or He-rich donors. These findings challenge the commonly invoked disk instability model for He-rich disks. Instead, the results indicate the need for irradiation-induced mass transfer, a mechanism which has been suggested to be important in these ultrashort period binaries, but never unambiguously proved.

Looking at the last Gaia data points of June 2020, one can see that the source is still brighter than its initial quiescent level. This extra brightness near the end of the outburst triggered the research team led by Rivera Sandoval to perform some X-ray and UV follow-up observations with the Swift observatory, also seen in Figure 1. A variable X-ray behaviour was spotted during this post-outburst cooling phase.

The findings about Gaia19atk will hopefully help to understand how the accretion process in these binaries occurs. These results may also be relevant for space-based gravitational wave observatoires, such as the upcoming LISA mission. This because the irradiation-induced mass transfer mechanism may substantially influence the evolution of these binaries, potentially affecting the expected number of individually detected ultracompact white dwarf binaries.

Gaia has been operating now for more than 6 years, and is expected to scan the sky for several years to come, adding more and more observations to the lightcurves of its sources with each extra year of operations. With Gaia Data Release 3, a data set called the Gaia Andromeda Photometric Survey consisting of the photometric time series for all Gaia sources located in a 5.5 degree radius field centred on the Andromeda galaxy can be expected. Epoch data for all Gaia sources will become available with Gaia Data Release 4.

 

Credits: Rivera Sandoval L.E., Maccarone T.J., Pichardo Marcano M. from Texas Tech University, Gaia Photometric Science Alerts team

[Published: 04/09/2020]

 

Image of the Week Archive

2020
29/10: Gaia EDR3 passbands
15/10: Star clusters are only the tip of the iceberg
04/09: Discovery of a year long superoutburst in a white dwarf binary
12/08: First calibrated XP spectra
22/07: Gaia and the size of the Solar System
16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models
30/06: Gaia's impact on Solar system science
14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data
20/03: The chemical trace of Galactic stellar populations as seen by Gaia
09/01: Discovery of a new star cluster: Price-Whelan1
08/01: Largest ever seen gaseous structure in our Galaxy
2019
20/12: The lost stars of the Hyades
06/12: Do we see a dark-matter like effect in globular clusters?
12/11: Hypervelocity star ejected from a supermassive black hole
17/09: Instrument Development Award
08/08: 30th anniversary of Hipparcos
17/07: Whitehead Eclipse Avoidance Manoeuvre
28/06: Following up on Gaia Solar System Objects
19/06: News from the Gaia Archive
29/05: Spectroscopic variability of emission lines stars with Gaia
24/05: Evidence of new magnetic transitions in late-type stars
03/05: Atmospheric dynamics of AGB stars revealed by Gaia
25/04: Geographic contributions to DPAC
22/04: omega Centauri's lost stars
18/04: 53rd ESLAB symposium "the Gaia universe"
18/02: A river of stars
2018
21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
2017
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
 
Please note: Entries from the period 2003-2010 are available in this PDF document.