Properties of the GDR2 short timescale, suspected periodic candidates

In this Section, I explore the contents and properties of the GDR2 short timescale can-didates list: where are they located in the sky and in the HR diagram? What are their characteristic light-curve shapes, amplitudes, periods, detection timescales, etc... ?

As a starting point, I explore in more details the known variables present in the short timescale sample of 3018 sources. It includes 138 known variables from the reference crossmatched source list presented in Section 4.1.2: 71 with P_ext ≤ 0.5d, 32 with 0.5 <

P_ext ≤ 1d, 27 with P_ext > 1d, and 8 with noP_ext information from literature but whose type is compatible with short timescale periodic variability. None of the constant sources and non-periodic variables from the reference crossmatched sample remain in the final short timescale candidates list. This represents a completeness of the sample of about 12%

relatively to the 439 + 382 short period variables in the input sample (see Section 4.1.2).

From this crossmatch, the contamination would be assessed around 19%, but coming only from longer period variability.

To go further in the completeness and contamination analysis, we decide to focus in areas covered by theOGLEsurvey, thus restricted essentially to the region of the Magel-lanic Clouds, and compare theGaiaand OGLEvariability results. From theOGLEIII and IV catalogues of variable stars, 45,966 sources are either in the LMC or in the SMC, and haveOGLEperiodsP_ext ≤ 1 d. Only 24 of them are within the published short timescale sample, hence the global completeness in this area is as low as 0.05%. I remind that com-pleteness was not the main goal of the GDR2 short timescale analysis, and that it should be significantly improved in future Data Releases. In total, 48OGLEvariables are identi-fied as bona fide short timescale suspected periodic candidates in the Magellanic Clouds (24 withP_OGLE ≤1d, 24 withP_OGLE >1d), thus resulting in a significant contamination from longer period variables around 50%. To check if longer period variables are the only source of contamination remaining in the GDR2 short timescale sample, I crossmatch it

102CHAPTER 4. THEGAIADATA RELEASE 2: FIRST SAMPLE OF SHORT TIMESCALE VARIABLE CANDIDATES IDENTIFIED BYGAIA

Table 4.1: Excerpt of thevari_short_timescaletable, from theGaiaDR2 archive.

0.0 0.5 1.0 1.5

18.518.017.517.016.516.0

Gaia DR2 source_id: 1191504471436192512

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Figure 4.13: Phase-folded Gaia light-curve of the PCEB NN Ser (left), and zoom on the fading transit visible at phase around0.3(right).Gaiasource_idis 1191504471436192512.

with the OGLEII photometric data base, covering parts of the LMC, and compare theI -bandOGLElight-curves to theGaiaGlight-curves for a few tens of sources. If the features seen byGaiaare reproduced and compatible with theOGLEII photometry, then the vari-ability is confirmed, otherwise the short timescale candidate is considered as spurious.

From this analysis, we end with a contamination level from spurious variables between 10 and 20% in the LMC. Note that this region is quite dense in sources, and hence more likely to be affected by contamination between neighbor stars than e.g. the Galactic halo.

Let us go back now to the 138 crossmatched sources from the reference sample in the published short timescale candidates list. More particularly, variables of very interesting types are recovered among those listed in Chapter 1, such as some Post-Common Enve-lope Binaries (PCEB), or Cataclysmic Variables (CV). The first striking example of known PCEB within the short timescale candidate I want to highlight is the case of NN Ser (Gaia source_id1191504471436192512), whose orbital period is of3.12h (Haefner 1989). The Gaia short timescale period found for this source is P_{ST S} ≈ 0.13 d ≈ 3.12h, hence re-covering exactly the period from literature. This binary system is also known to have two exoplanets orbiting around it, NN Ser c and NN Ser d, which are suspected to have masses of a fewMJ up and orbital periods of15.5and7.7yrs (Beuermann et al. 2010). My interest for this object is motivated by the curiosity to see if any planetary transit is visible in itsG-CCD light-curve (Figure 4.13). As can be seen, NN Ser has only one strongly fad-ing FoV transit within itsGaiaGphotometry, losing more than1mag over40s. Though it is very deep, could this be due to a transit of one of the two exoplanets surrounding this PCEB? According to the ephemeris from Beuermann et al. (2010), the time of this decay rather corresponds to an eclipse of the binary system. Note that this tremendous fading transit is removed from the G-FoV time series by the CU7 cleaning operator chain, be-cause of its high photometry uncertainty (resulting directly from the spread of the CCD measurements within the transit). This example demonstrates the fundamental need and interest for short timescale variability search to make use ofGphotometry at the per-CCD level. By the end of the nominal mission, the analysis of all the Gper-CCD light-curves, without any selection on the fraction of noisy transit nor on the magnitude, should enable to spot tens of such fast and smooth variation phenomena, be it due to eclipses in binary systems, transiting exoplanets or occulting asteroids.

Figures 4.14 and 4.15 represent phase-folded Gaia light-curves and variograms,

to-4.3. TheGaiaData Release 2 short timescale sample

gether with the phase-folded Catalina light-curve from literature³, for another known PCEB (CSS J210017.4-141125, hereafter CSS J210017) and one known cataclysmic variable (CSS J231330.8+165416, hereafter CSS J231330) respectively. TheirGaiasource identifiers are 6888269309535155456 and 2818311909906928384. The latter is an AM Her variable star, also referred to as polar variable, i.e. a cataclysmic binary composed of an accreting white dwarf and a low mass donor component, the system exhibiting no accretion disk:

the accretion goes through an accretion stream directly onto the white dwarf component.

Conversely, the DQ Her variables, or intermediate polar variables, are cataclysmic bina-ries with similar components but this time with the formation of an accretion disk around the white dwarf. For both CSS J210017 and CSS J231330,P_{AAV SO}andP_{ST S} are similar, and their phase-foldedGaialight-curves are convincing and coherent in bothG,GBP andGRP

bands. Their Gaia observational variograms exhibit features compatible with a periodic behavior at the period found, i.e. 3.5h and 81min respectively. In the case of the PCEB CSS J210017, despite the sparse scanning law and the reduced timespan of the analyzed data when compared to the nominal mission duration (22months instead of5years), the deep and fast eclipse of the system is already sampled and visible inGaiaphotometry.

Moreover, with my analysis, I obtain quite reliable period information, with satisfactory and coherent phase-folded time series in both three bands, for some variables which had no such information in the literature, e.g. for the CV from theAAVSOVariable Star Index⁴ presented in Figure A.2. All in all, those known variables, investigated as kind of school cases, demonstrate the potential of Gaiafor short timescale variability studies, as already inferred by simulations (Chapter 3), even with intermediate photometry.

As detailed in Chapter 3, by quantifying the averaged variation rate of the considered light-curve, the variogram detection timescaleτ_detand associated variogram valueγ(τ_det) give clues for future ground-based follow-up of the published GaiaDR2 short timescale candidates. For example, the CV CSS J231330 has a detection timescale τ_det = 19.4s and associated value γ(τ_det) = 0.00337 mag² ≈ (58 mmag)², which means that with a photo-metric instrument whose accuracy is around 58mmag, the follow-up observing cadence to detect the variability should be as short as∼20s. In the case of the PCEB CSS J210017, we have τ_det = 0.074d and γ(τ_det) = 0.049 mag² ≈ (0.22 mag)², hence the photometric cadence to adopt for ground-based follow-up should be of1 h 46 minif the instrument ac-curacy is around0.22mag. For an instrument of accuracy55mmag, the variogram plot of CSS J210017 shows that the required cadence would fall withinGaialag gap, somewhere between40s and1 h 46 min, certainly around a few minutes.

Now that I have dug on the side of known variables, what about the properties of the new short timescale candidates brought to light by this analysis? Figure 4.16 shows the sky distribution of the 3018 GDR2 short timescale candidates. Most of them are close to the Galactic plane, accordingly to the overall density distribution within our Galaxy. The sources located in the Galaxy halo are found in the sky regions more densely sampled by Gaiaover its first 22months of operations (see e.g. Figure 1 of Holl et al. 2018), which is coherent with our decision to investigate only sources with more than 20 FoV transits, and with the known dependency of the variogram method efficiency relatively to the number of observations. Note also that, given the fact that the short timescale variability search has been limited to sources with more than 20 FoV transits, the GDR2 short timescale candidates distribution reproduces the expected lack of sources observed more than 20

3CatalinaData Release 2 photometry is available athttp://nesssi.cacr.caltech.edu/DataRelease/

4https://www.aavso.org/vsx/

104

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18.518.017.517.016.516.0

Gaia DR2 source_id: 6888269309535155456

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Figure 4.14: Phase-folded light-curves and variogram of the PCEB CSS J210017. From left to right and from top to bottom: Catalinaphase-foldedV light-curve with period from Drake et al. (2014c)

; Gaia phase-folded light-curve with period from short timescale analysis in G, G_BP and G_RP bands; Gaiavariogram from short timescale analysis (the orange dashed line indicates the Gaia short timescale periodP_{ST S}).Gaiasource_idis 6888269309535155456.

106CHAPTER 4. THEGAIADATA RELEASE 2: FIRST SAMPLE OF SHORT TIMESCALE VARIABLE CANDIDATES IDENTIFIED BYGAIA

0.0 0.5 1.0 1.5

19.018.017.0

Gaia DR2 source_id: 2818311909906928384

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Figure 4.15: Same as Figure 4.14 for the cataclysmic variable of type AM Her CSS J231330, but this time withPextfrom Margon et al. (2014).Gaiasource_idis 2818311909906928384.

Figure 4.16: Sky density map of the 3018 published short timescale candidates, in galactic coordi-nates, and in counts per square degree.

times around the Galactic center, as a consequence of theGaiascanning law.

In the southern galactic hemisphere, two slight overdensities correspond to the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). A very rough box search around the Magellanic Clouds (MCs) positions:

LMC:α= 05 : 23 : 34.6 δ=−69 : 45 : 22 SMC:α= 00 : 52 : 38.0 δ=−72 : 48 : 01

with width of5^◦and1^◦respectively, resulted in a set of about 40 short timescale suspected periodic candidates within the MCs, most of them being known variables, as a result of the high scientific interest and extensive exploration of those regions.

Figure 4.17 represents the frequency - amplitude diagram (AG−CCD versusf_{ST S}) of the final short timescale variables sample for GDR2. First, it appears that the performed anal-ysis results in large amplitude candidates as well as relatively low amplitudes ones, down to about0.1mag inGband. Then, similarly to Figure 4.5, aliasing features are clearly visi-ble in this diagram, with clumps at higher frequencies near multiples of4 d⁻¹which corre-sponds to the rotation period ofGaia. As explained in Section 4.1.3, though they may not be short period variables per se, we are confident that those candidates with aliased peri-ods are reliable variable sources, probably with longer period, but with sufficient average variation rate in their light-curves to justify their detection at the short timescale level. Fig-ure A.3 shows an example of such aliased candidate, with frequencyf_{ST S} ∼80 d⁻¹(period P_{ST S} ∼18 min): though the period seems spurious from the phase-folded light-curves, the variogram clearly indicates some periodic features at least at timescales of a few hours, and variations in both threeGaiabands are coherent (rG−BP ∼0.60andrG−RP ∼0.68).

Figure A.4 presents an example of low amplitude short timescale candidate, with AG−CCD ≈ 0.10 mag, and period PST S ≈ 3.7 h. Here, as one can see, the period found

4.3. TheGaiaData Release 2 short timescale sample

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Figure 4.17: Frequency- amplitude distribution of the 3018 published short timescale candidates.

The remaining known variables in the sample are indicated by colored dots.

matches perfectly with the oscillations visible in the variogram, the phase-folded light-curves look quite convincing, and both three photometric bands are tightly correlated (rG−BP ∼0.65andrG−RP ∼0.74).

Out of 3018 published short timescale suspected periodic candidates, 59 have good enough astrometry, photometry and parallax estimates to be reliably positioned in the HR diagram (Figure 4.18, without correction of extinction). I note φ¯_BP, φ¯_RP and φ¯_G the mean fluxes in G_BP, G_RP and Gbands respectively, andσ_BP, σ_BP and σ_G the associated uncertainties ; ω is the parallax, and σ_ω the corresponding uncertainty. The selection of this “good enough” sample is also used to extract the reference Gaia HR diagram (grey dots in Figure 4.18), in the same way as in Gaia Collaboration et al. (2018b):

• ^σφ¯^BPBP <0.05, ^σφ¯^RPRP <0.05and ^σφ¯^GG <0.02, to ensure the quality of the color and magni-tude estimates for the source,

• φ¯G∗(1.2+0.03∗( ¯^{φ¯BP+ ¯φ}φ^RPBP−φ¯RP)²) <1.2to ensure the consistency betweenG,G_BP andG_RP mea-surements,

• ω >0andσω/ω <0.2to ensure the reliability of the parallax estimate,

• the excess noise, measuring the disagreement (expressed as an angle in mas) be-tween the observation of the source and the best-fitting standard five-parameters astrometric model, verifyingi <0.5,

• the visibility periods used, i.e. the number of phases when the considered source was visible byGaiacovered by the measurements used to derive the astrometric solution, strictly greater than5.

Among the 59 short timescale candidates within this HR diagram, 8 are falling on the Main Sequence, and a few ones lie in the White Dwarf area. Nevertheless, the major-ity of them are between those two regions, where not many objects are expected. Is this

108

Figure 4.18: HR diagram, without extinction correction, of 59 of the 3018 GDR2 short timescale can-didates, which are the ones with the most reliable astrometry and photometry information (blue dots). Some of the known variables within this sample are indicated with different dot shapes and colors. “AM and DQ” category stands for AM Her and DQ Her types, “Ecl” for eclipsing binaries.

The grey HR background is a subset of the sources closer than200pc from the HR diagram in Gaia Collaboration et al. (2018b).

the indication of some problem, and possible spuriousness, when dealing with the candi-date detection and selection? Hopefully, as can be seen, several known variable sources, with good astrometry and photometry, sample this zone of the HR diagram, typically PCEB systems, White Dwarf - Main Sequence binary stars, and novae. Consequently, the identified candidates are likely some extreme binary systems, involving Main Sequence components with degenerate or semi-degenerate companions.

Among the bluer and brighter short timescale candidates, in the same region of the HR diagram as the PCEB CSS J210017 (purple diamond on Figure 4.18), I find a very nice ex-ample of possible unknown PCEB system: 5646693014160460416, hereafter STS56466930, with periodPST S ≈2.7 h. The associated light-curves and variogram are presented in Fig-ure 4.19. Another example or blue and bright candidate found nearby in the HR diagram, 564551735705888384, hereafter STS56455173, is shown in Figure A.5. Again, it exhibits smooth (in this case sinusoidal-like) and coherent variability in both three photometric bands after phase-folding, and the short period found,P_{ST S} ≈2.6 his perfectly consistent with the observational variogram features. Moreover, STS56455173 is likely an eclipsing binary system as well, with only one transit (at phase∼0.2) sampling the eclipse.

It is also important to mention that the GDR2 short timescale candidates sample over-laps with the candidates lists produced by some of the other CU7 SVD - SOS workpack-ages (see Section 2.5). Hence, 72 of our candidates are also identified as RR Lyrae

vari-110CHAPTER 4. THEGAIADATA RELEASE 2: FIRST SAMPLE OF SHORT TIMESCALE VARIABLE CANDIDATES IDENTIFIED BYGAIA

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Figure 4.19: From left to right and from top to bottom: phase-folded light-curves inG-CCD,GBP, GRP, and variogram for the short timescale candidate STS56466930. Phase-folding is done using theGaiashort timescale periodP_{ST S}. As usual, on the variogram plot, the purple star indicates the point triggering the detection and the orange dashed line indicates the periodP_{ST S}.

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Figure 4.20: Same as Figure 4.19 for the curious eclipsing binary STS56378276.

ables by the SOS RR Lyrae module, 5 are flagged by the Cepheids workpackage, and 3 are also rotational modulation variable candidates. However, this overlap between different modules is not problematic, in the sense that RR Lyrae stars, Cepheid stars and rotational modulation variables, with their characteristic periods and amplitudes, can either enter the short timescale definition adopted for the GaiaDR2 or be part of the acceptable con-tamination of the sample by longer period variables.

By visually inspecting the light-curves of all the 3018 short timescale candidates pub-lished in GDR2, I also spot some peculiar and interesting variables. In particular, I want to bring into light the case of 5637827617537477504 (STS56378276, Figure 4.20), exhibit-ing very deep eclipses of more than 1−1.5 magin both G, G_BP and G_RP bands, as well as significant out-of-eclipse variability. This overall light-curve shape is similar to what is expected e.g. for AM CVn stars. In the HR diagram, this candidate is positioned be-tween the Main Sequence and the White Dwarf sequence, nearby the known polars and intermediate polars (magenta squares), which favors the hypothesis of degenerate star with a low-mass Main Sequence companion. Nevertheless, the period of STS56378276 is P_{ST S} ≈ 3.4 h, hence longer than the orbital periods of known AM CVn stars which rank between5and65min (see e.g. Levitan et al. 2015). Further modeling of this very interest-ing object may brinterest-ing clues on the properties of its components and on its geometry.

The few examples of binary candidates presented in this Section illustrate the per-sistence of some eclipsing binary systems within the published short timescale sample, despite the exclusion of the best candidates from the eclipsing binary module transferred to CU4 for further investigation. Actually, the eclipsing binary analysis is based on the

4.3. TheGaiaData Release 2 short timescale sample

period search result from the CU7 characterization module, and on modeling of the light-curves according to what is expected for binary systems. The remaining binaries within the short timescale sample have periods of about1−2 h, at the limit of the period range explored by the characterization module, and very peculiar shapes, which may explain why they have been missed as bona fide eclipsing binaries, and demonstrates the comple-mentarity of the short timescale module with the other ones.

4.3.3 Conclusion

All in all, by combining the variogram analysis with a specific high frequency search, and thanks to empirical selection criteria based on various statistical metrics from Gaia photometry, the CU7 short timescale variability module identified, forGaiaDR2, a set of 3018 short timescale suspected periodic variable candidates, with limited contamination from false positive and spurious phenomena, around 10-20% in the LMC, a dense area where photometric pollution between nearby stars is more likely to occur. Non-periodic variability is expected to be efficiently excluded. The contamination from periodic vari-ables with periods longer than 1d is more important, varying between 20 and 50%

All in all, by combining the variogram analysis with a specific high frequency search, and thanks to empirical selection criteria based on various statistical metrics from Gaia photometry, the CU7 short timescale variability module identified, forGaiaDR2, a set of 3018 short timescale suspected periodic variable candidates, with limited contamination from false positive and spurious phenomena, around 10-20% in the LMC, a dense area where photometric pollution between nearby stars is more likely to occur. Non-periodic variability is expected to be efficiently excluded. The contamination from periodic vari-ables with periods longer than 1d is more important, varying between 20 and 50%

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