[GRB] First filter to extract orphan GRB

[JulienPeloton]

@bregeon @anaismoller @karpov-sv

Based on the work of @bregeon, I recently pushed a new filter to extract orphan GRB candidates:

1. [Recent object] No more than a month between first and last detection
2. [Faint object] Max magnitude at 18
3. [Slow transient] At least 3 detections in 10 days
4. [Fading] The last measurement must be lower than the previous one
5. The difference between the g-band and r-band must be almost constant (~0.3) and positive
6. The alert should not be an identified Solar System Objects

Based on current ZTF data, it yields about 50-100 candidates per month (out of >2,000,000 incoming alerts). You can find the implementation for Fink here: https://github.com/astrolabsoftware/fink-filters/pull/74/files.

As an example, here are the 64 candidates for August 2021 (0.003%), and their classification if they were not considered as orphans (a posteriori):

objectId	candid	class	link
ZTF21abshvst	1688478550815010001	Unknown	http://134.158.75.151:24000/ZTF21abshvst
ZTF21abpoqoh	1673272524615015031	SN candidate	http://134.158.75.151:24000/ZTF21abpoqoh
ZTF21abpjeds	1673350271615015011	SN candidate	http://134.158.75.151:24000/ZTF21abpjeds
ZTF21abpiqsc	1673306493115015009	SN candidate	http://134.158.75.151:24000/ZTF21abpiqsc
ZTF21abowurd	1673216534715015014	SN candidate	http://134.158.75.151:24000/ZTF21abowurd
ZTF21abpoufi	1673323481715015006	SN candidate	http://134.158.75.151:24000/ZTF21abpoufi
ZTF21absxrfq	1684480532115010011	SN candidate	http://134.158.75.151:24000/ZTF21absxrfq
ZTF21abrskgx	1684236481315015010	SN candidate	http://134.158.75.151:24000/ZTF21abrskgx
ZTF21abrzwwo	1690468653115010010	SN candidate	http://134.158.75.151:24000/ZTF21abrzwwo
ZTF21abtykmm	1690468650415015020	QSO	http://134.158.75.151:24000/ZTF21abtykmm
ZTF21abulrsl	1699484660615015003	SN candidate	http://134.158.75.151:24000/ZTF21abulrsl
ZTF21abvaqke	1699169582415015029	SN candidate	http://134.158.75.151:24000/ZTF21abvaqke
ZTF21abqsndo	1680239210515015005	SN candidate	http://134.158.75.151:24000/ZTF21abqsndo
ZTF21abquzym	1680371310715015000	SN candidate	http://134.158.75.151:24000/ZTF21abquzym
ZTF21abqftqr	1675310935315015027	SN candidate	http://134.158.75.151:24000/ZTF21abqftqr
ZTF21abqqeuo	1674448621415015006	SN candidate	http://134.158.75.151:24000/ZTF21abqqeuo
ZTF21abpkgco	1674349296215015012	Unknown	http://134.158.75.151:24000/ZTF21abpkgco
ZTF21abrxpcs	1681400061115015018	SN candidate	http://134.158.75.151:24000/ZTF21abrxpcs
ZTF21abqoivk	1681374545215015008	SN candidate	http://134.158.75.151:24000/ZTF21abqoivk
ZTF21abpvqaa	1676367471415015008	SN candidate	http://134.158.75.151:24000/ZTF21abpvqaa
ZTF21abrcqhg	1676416334415015006	Unknown	http://134.158.75.151:24000/ZTF21abrcqhg
ZTF21abrcdhl	1676419012515010014	SN candidate	http://134.158.75.151:24000/ZTF21abrcdhl
ZTF21abutvhu	1696159691815015008	Unknown	http://134.158.75.151:24000/ZTF21abutvhu
ZTF21abvbijg	1700262185415015002	Unknown	http://134.158.75.151:24000/ZTF21abvbijg
ZTF21abqqbyu	1680463152815015003	SN candidate	http://134.158.75.151:24000/ZTF21abqqbyu
ZTF21abrpanc	1681429051415015008	SN candidate	http://134.158.75.151:24000/ZTF21abrpanc
ZTF21abqvzbi	1681433846215015004	SN candidate	http://134.158.75.151:24000/ZTF21abqvzbi
ZTF21abvdvsz	1700207910415015000	SN candidate	http://134.158.75.151:24000/ZTF21abvdvsz
ZTF21abqtckk	1680199200615015004	SN candidate	http://134.158.75.151:24000/ZTF21abqtckk
ZTF21abstsyg	1686383832115015030	Unknown	http://134.158.75.151:24000/ZTF21abstsyg
ZTF21abtmmxq	1686400471415010194	Unknown	http://134.158.75.151:24000/ZTF21abtmmxq
ZTF21abpjsrn	1676317030315015000	SN candidate	http://134.158.75.151:24000/ZTF21abpjsrn
ZTF21absucop	1687291733215015028	SN candidate	http://134.158.75.151:24000/ZTF21absucop
ZTF21abvtymv	1701289941415015006	SN candidate	http://134.158.75.151:24000/ZTF21abvtymv
ZTF21abqvwzb	1681374542415015013	SN candidate	http://134.158.75.151:24000/ZTF21abqvwzb
ZTF21abvhwsn	1701237343515015006	SN candidate	http://134.158.75.151:24000/ZTF21abvhwsn
ZTF21abuuvqz	1694253650815015001	SN candidate	http://134.158.75.151:24000/ZTF21abuuvqz
ZTF21abvcugy	1699454622715015002	SN candidate	http://134.158.75.151:24000/ZTF21abvcugy
ZTF21abpvsmu	1674388912415015010	SN candidate	http://134.158.75.151:24000/ZTF21abpvsmu
ZTF21abvrrmy	1699285352215015023	SN candidate	http://134.158.75.151:24000/ZTF21abvrrmy
ZTF21absjryg	1685464222715015023	SN candidate	http://134.158.75.151:24000/ZTF21absjryg
ZTF21abpyalf	1680436365715015006	SN candidate	http://134.158.75.151:24000/ZTF21abpyalf
ZTF21abqqveh	1680476423015015001	SN candidate	http://134.158.75.151:24000/ZTF21abqqveh
ZTF21abralod	1676347196315015012	SN candidate	http://134.158.75.151:24000/ZTF21abralod
ZTF21absvlrr	1684461850315015005	SN candidate	http://134.158.75.151:24000/ZTF21absvlrr
ZTF21abrpeix	1682444063515015001	SN candidate	http://134.158.75.151:24000/ZTF21abrpeix
ZTF21abrhujh	1682365742415015012	SN candidate	http://134.158.75.151:24000/ZTF21abrhujh
ZTF21abqwiwd	1682462851115015024	SN candidate	http://134.158.75.151:24000/ZTF21abqwiwd
ZTF21abrpddz	1682479603815015001	SN candidate	http://134.158.75.151:24000/ZTF21abrpddz
ZTF21abrdbbe	1682422063215015011	SN candidate	http://134.158.75.151:24000/ZTF21abrdbbe
ZTF21abrfbso	1682463802615010001	SN candidate	http://134.158.75.151:24000/ZTF21abrfbso
ZTF21abqhkjd	1674399761115015015	SN candidate	http://134.158.75.151:24000/ZTF21abqhkjd
ZTF21absjlur	1682474890715015030	SN candidate	http://134.158.75.151:24000/ZTF21absjlur
ZTF21abrpkzp	1682462382715015021	SN candidate	http://134.158.75.151:24000/ZTF21abrpkzp
ZTF21abulrgl	1692484374615015009	SN candidate	http://134.158.75.151:24000/ZTF21abulrgl
ZTF21abqixak	1676383660015015002	SN candidate	http://134.158.75.151:24000/ZTF21abqixak
ZTF21abprtvt	1674323055415015003	Galaxy	http://134.158.75.151:24000/ZTF21abprtvt
ZTF21abtnruv	1689447364315015154	SN candidate	http://134.158.75.151:24000/ZTF21abtnruv
ZTF21abqwgbz	1675460404215015006	SN candidate	http://134.158.75.151:24000/ZTF21abqwgbz
ZTF21abspqhd	1684371786115015007	SN candidate	http://134.158.75.151:24000/ZTF21abspqhd
ZTF21abqvzsa	1684444662615015003	SN candidate	http://134.158.75.151:24000/ZTF21abqvzsa
ZTF21abqzbvv	1676296771115010093	SN candidate	http://134.158.75.151:24000/ZTF21abqzbvv
ZTF21abrofrx	1684470045815015011	SN candidate	http://134.158.75.151:24000/ZTF21abrofrx
ZTF21abqqalc	1676460335415015005	SN candidate	http://134.158.75.151:24000/ZTF21abqqalc

Grouped by class:

class
Galaxy           1
QSO              1
SN candidate    55
Unknown          7

Which makes sort of sense.

Once the tracklet computation is merged (astrolabsoftware/fink-broker#478), I will also add a new condition to remove those (there should be very few though).

Let me know if you have any comment, else, I will deploy it in the broker. Of course, we can iterate on it, and refine it later.