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J/other/ApSS/365.89 Gaia Alerts with LAMOST and SDSS (Huo+, 2020)
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Beginning of ReadMe :J/other/ApSS/365.89 Gaia Alerts with LAMOST and SDSS (Huo+, 2020)
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Characterizing some Gaia Alerts with LAMOST and SDSS.
Huo Z., Dennefeld M., Liu X., Pursimo T., Zhang T.
<Astrophys. Space Sci., 365, 89 (2020)>
=2020Ap&SS.365...89H (SIMBAD/NED BibCode)
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ADC_Keywords: Stars, variable ; Galaxies ; Supernovae ; QSOs
Keywords: Gaia Alerts - stars - galaxies - supernovae - quasars - Astrophysics -
Astrophysics of Galaxies
Abstract:
The ESA-Gaia satellite is regularly producing Alerts on objects where
photometric variability has been detected after several passages over
the same region of the sky. The physical nature of these objects has
often to be determined with the help of complementary observations
from ground-based facilities. We have compared the list of Gaia Alerts
(from the beginning in 2014 to Nov. 1st, 2018) with archival LAMOST
and SDSS spectroscopic data. A search radius of 3" has been adopted.
In using survey data, the date of the ground-based observation rarely
corresponds to the date of the Alert, but this allows at least the
identification of the source if it is persistent, or the host galaxy
if the object was only transient like a supernova (SN). Some of the
objects have several LAMOST observations, and we complemented this
search by adding also SDSS DR15 data in order to look for long-term
variability. A list of Gaia Nuclear Transients (GNT) from
Kostrzewa-Rutkowska et al. (2018. 2018MNRAS.481..307K, Cat.
J/MNRAS/481/307), has been included in this search also. We found 26
Gaia Alerts with spectra in LAMOST+SDSS labelled as stars, among which
12 have multi-epoch spectra. A majority of them are Cataclysmic
Variables (CVs). Similarly, 206 Gaia Alerts have associated spectra
labelled as galaxies, among which 49 have multi-epoch spectra. Those
spectra were generally obtained on a date widely different from the
Alert date, and are mostly emission-line galaxies with no
particularity (except a few Seyferts), leading to the suspicion that
most of the Alerts were due to a SN. As for the GNT list, we found 55
associated spectra labelled as galaxies, among them 13 with
multi-epoch spectra. In these two galaxy samples, in only two cases,
Gaia17aal and GNTJ170213+2543, was the date of the spectroscopic
observation close enough to the Alert date: we find a trace of the SN
itself in their LAMOST spectrum, both being now classified here as a
type Ia SN. Compared to the galaxy sample from the Gaia alerts, the
GNT sample has a higher proportion of AGNs, suggesting that some of
the detected variations are also due to the AGN itself. Similarly for
Quasars, we found only 30 Gaia Alerts but 68 GNT cases associated with
single epoch quasar spectra in the databases. In addition to those, 12
plus 23 are quasars where multi-epoch spectra are available. For ten
out of these 35, their multi-epoch spectra show appearance or
disappearance of the broad Balmer lines and also variations in the
continuum, qualifying them as "Changing Look Quasars" and therefore
significantly increasing the available sample of such objects.
Description:
We have selected all Alerts appearing in the Gaia Alerts website from
the beginning (first one Gaia14aaa, detected on August 30th, 2014), to
the end of October 2018 (last one Gaia18dge, detected on October 31st,
2018), that is a total of 6308 candidates. Kostrzewa-Rutkowska et al.
(2018, Cat. J/MNRAS/481/307) have investigated the detectability of
nuclear transients by Gaia using a method different from the one used
for the standard Alerts (AlertPipe) and found 482 candidates in the
period ranging from June 2016 to June 2017, only 5 of which were also
detected by the standard Gaia AlertPipe system. We have included in
our search all candidates from this GNT list. As these authors have,
by definition, targeted galaxies only, by cross-matching on source
position a Gaia source with the sample of SDSS-DR12 (Alam et al.,
2015, Cat. V/147) catalogued galaxies or quasars, each object from
their sample already has one associated SDSS entry, but not
necessarily a spectrum for classification (they have only 142 spectral
classifications, out of 482 objects). We therefore extended the search
to find possibly other spectra and characteristics of their objects,
and look for long term variability.
Our sample has been cross-correlated with the LAMOST DR5 database
(http://dr5.lamost.org, Luo et al., 2015, Cat. V/164).