J/MNRAS/510/1791 Blazars study with ZTF DR6 and Roma-BZCAT (Negi+, 2022)
Optical flux and colour variability of blazars in the ZTF survey.
Negi V., Joshi R., Chand K., Chand H., Wiita P., Ho L.C., Singh R.S.
<Mon. Not. R. Astron. Soc. 510, 1791-1800 (2022)>
=2022MNRAS.510.1791N 2022MNRAS.510.1791N (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei ; QSOs ; Galaxies ; BL Lac objects ;
Radio sources ; Optical ; Transient ; Photometry ; Spectroscopy ;
Photometry, classification ; Redshifts ; Positional data ;
Magnitudes
Keywords: galaxies: active - BL Lacertae objects: general - galaxies: jets -
quasars: general
Abstract:
We investigate the temporal and colour variability of 897 blazars,
comprising 455 BL Lacertae objects (BL Lacs) and 442 Flat Spectrum
Radio Quasars (FSRQs), selected from the Roma-BZCAT catalogue, using
the multiband light curves from the Zwicky Transient Facility (ZTF
DR6) survey. Assessing the colour variability characteristics over ∼2 yr
time-scales, we found that 18.5 per cent (84 out of 455) BL Lacs
showed a stronger bluer-when-brighter (BWB) trend, whereas 9.0 per
cent (41 out of 455) showed a redder-when-brighter (RWB) trend. The
majority (70 per cent) of the BL Lacs showing RWB are host galaxy
dominated. For the FSRQ subclass, 10.2 per cent (45 out of 442)
objects showed a strong BWB trend and 17.6 per cent (78 out of 442)
showed a strong RWB trend. Hence, we find that BL Lacs more commonly
follow a BWB trend than do FSRQs. This can be attributed to the more
dominant jet emission in the case of BL Lacs and the contribution of
thermal emission from the accretion disc for FSRQs. In analysing the
colour behaviour on shorter time windows, we find many blazars evince
shorter partial trends of BWB or RWB nature (or occasionally both).
Some of such complex colour behaviours observed in the
colour-magnitude diagrams of the blazars may result from transitions
between the jet-dominated state and the disc-dominated state and vice
versa.
Description:
Blazars are a special subclass of active galactic nuclei (AGNs),
possessing a relativistic jet directed close to the observer's line
of sight. The blazars emit in a very wide range of wavelengths from
radio to gamma-ray bands and show violent variability on diverse
time-scales ranging from minutes to years. Blazars are broadly
classified into two categories: BL Lacertae objects (BL Lacs) and flat
spectrum radio quasars (FSRQs) based on the rest-frame equivalent
widths (EWs). The optical variability of blazars has been a topic of
intensive research since their discovery as it affords a unique
perspective on accretion disc and jet physicsn particular, optical
brightness variations in blazars are often accompanied by spectral
variations that can be revealed by the colour-magnitude (or spectral
index-magnitude) correlations.
In this work, we investigate for the first time whether there is a
universality in the colour behaviour of the blazars with the largest
homogeneous sample taken to date. We have made use of the Zwicky
transient facility (ZTF, Bellm et al. 2019PASP..131a8003M 2019PASP..131a8003M), which
scans the northern sky in g, r, and i bands with an average 3 d
cadence using a 47 deg2 wide-field imager mounted on a 48-inch
Schmidt telescope on Mount Palomar, (i.e see section Introduction).
We employ the latest version (5th edition) of the Roma-BZCAT catalogue
(Massaro et al. 2015Ap&SS.357...75M 2015Ap&SS.357...75M, Cat. VII/274), the most complete
list of all the blazars detected in multifrequency surveys to date. It
consists of a total of 3561 blazars, including 1425 BL Lacs, with 92
of them being BL Lac candidates, 1909 FSRQs, and 227 blazars of
uncertain type. Next, we searched for the point spread function (PSF)
fit based light curves from the 6th ZTF public data release for all
the objects centred at the target position within an angular
separation of 1.5 arcsec radius. Applying ZTF quality cuts in order
to take the light curve corresponding to the observation ID with the
maximum number of data points. We have considered only the
quasi-simultaneous observations to analyse the brightness variability
and the true spectral behaviour of these sources, (i.e section 2 Data
and sample selection).
Therefore, we select the sources having at least 10 data points with
quasi-simultaneous observations defined as being within 30 min in
both g and r bands. This yields a final sample of 897 blazars, out of
which 455 are BL Lacs and 442 are FSRQs, with 109 of the BL Lacs
having a dominant host galaxy component in their SED, we provide their
basic informations from Roma-BZCAT in table1.dat, (i.e section 2 Data
and sample selection). Finally, as detailed in the section 3 Analysis
and results, we produce analysis on ZTF DR6 light curves and
colour-magnitude diagrams variabilities from trend and amplitudes
parameters in order to classify blazars optical emessions and thus
classify these objects among groups. Their data values are presented
in the table3.dat.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 60 897 *Basic parameters of our blazars sample
table3.dat 60 897 Retrieved parameters and the trends from ZTF DR6
light curves for our blazars sample
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Note on table1.dat: Blazars parameters taken from Roma BZCAT Massaro et al.
(2015Ap&SS.357...75M 2015Ap&SS.357...75M, Cat. VII/274). All selected 897 blazars light curves from
the 6th ZTF public data release (https://www.ztf.caltech.edu,
Masci et al. 2019PASP..131a8003M 2019PASP..131a8003M) are available in supplementary material pdfs.
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See also:
VII/274 : The Roma BZCAT - 5th edition (Massaro+, 2015)
IX/67 : Incremental Fermi LAT 4th source cat. (4FGL-DR3)
(Fermi-LAT col., 2022)
J/ApJS/268/23 : Magnetic fields & emission regions of Fermi blazars (Fan+,2023)
I/284 : The USNO-B1.0 Catalog (Monet+ 2003)
J/ApJS/249/18 : The ZTF catalog of periodic variable stars (Chen+, 2020)
https://irsa.ipac.caltech.edu/Missions/ztf.html : ZTF DR6 home page data
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- ID Object ID number (Object_ID)
5- 18 A14 --- Name Source name based on J2000 position as
JHHMMSS+DDMMSS (Name) (G1)
20- 21 I2 h RAh Right ascension (J2000)
23- 24 I2 min RAm Right ascension (J2000)
26- 30 F5.2 s RAs Right ascension (J2000)
32 A1 --- DE- Declination sign (J2000)
33- 34 I2 deg DEd Declination (J2000)
36- 37 I2 arcmin DEm Declination (J2000)
39- 43 F5.2 arcsec DEs Declination (J2000)
45- 50 A6 --- Class Source classification (Type) (G2)
52- 55 F4.1 mag rmag The Roma BZCAT r-band SDSS magnitude of the
blazar taken from USNO-B1 catalogue (Rmag) (1)
57- 60 F4.2 --- z Heliocentric redshift (Redshift)
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Note (1): Red magnitudes from USNO-B1 catalogue, Monet at al.
(2003AJ....125..984M 2003AJ....125..984M, Cat. I/284) when available (or from SDSS DR10).
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- ID Object ID number (Object_ID)
5- 18 A14 --- Name Source name based on J2000 position as
JHHMMSS+DDMMSS (Name) (G1)
20- 25 A6 --- Class Source classification (Type) (G2)
27- 30 A4 --- Trend Colour trend on ZTF DR6 r-g versus r
colour-magnitude diagrams (Trend) (1)
32- 38 A7 --- Duration [partial whole] Trend duration of MJD time
observations on colour variation diagrams
(Duration) (2)
40- 43 F4.2 mag Psir ? Amplitude of variability Ψr computed
ZTF DR6 SDSS r band magnitudes (Ψr) (3)
45- 52 F8.5 --- DFvar ? Difference of fractional variability
amplitudes in g and r bands
as Fvarg - Fvarr (ΔFvar) (4)
54- 60 F7.5 --- e_DFvar ? Computed error σ of DFvar
(σΔFvar) (4)
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Note (1): Colour trend for our sample is made in groups as follows:
BWB = 195 blazars in our sample have this trend where colour becoming
bluer with increasing brightness
RWB = 286 blazars in our sample have this trend where colour becoming
redder with increasing brightness
both = 12 blazars in our sample have shown a complex colour behaviour
consisting of both partial BWB and RWB trends for different
lengths of time
none = 404 blazars in our sample showing no trend across the whole
duration
For the 897 sources in our sample, we generated the colour-magnitude
(r - g versus r) diagrams using only the quasi-simultaneous data.
In addition, the colour-magnitude diagrams for all the sources
showing some trends in our sample are presented in the online material
pdfs. For a robust measurement of possible BWB or RWB trends in the
light curves, we first calculated the Pearson correlation coefficients
by randomly redistributing the values of colour index (CI) and r mag
within the range from P - Pe to P + Pe for about 104 realization,
where P and Pe are the parameter value and the corresponding error,
respectively (i.e section 3.2 Colour variability for more details on
Pearson correlation coefficient ρ and the rejection probability
p values related to coulour trend groups).
Note (2): Trend durations are divived in groups as follows:
partial = 245 blazars in our sample showed colour variation
trends on a partial MJD time observations
whole = 248 blazars in our sample showed colour variation
trends on the whole MJD time observations
Note (3): As explained in the section 3.1 Brightness variability, we compare
the brightness variations in the BL Lacs and FSRQs in our ZTF sample.
To do so, we computed two frequently employed quantities. First, the
amplitude of variability, Ψ, introduced by Heidt & Wagner
(1996A&A...305...42H 1996A&A...305...42H) as Ψr using equation 1 in this section
for the whole data set in the ZTF DR6 SDSS r band.
Note (4): As described in the section 3.1 Brightness variability, secondly,
we calculated the fractional variability amplitude Fvar described
by Vaughan et al. (2003MNRAS.345.1271V 2003MNRAS.345.1271V) of both types of blazars in
the available ZTF DR6 SDSS g and r bands using the equation 2 in this
section. This parameter gives a measure of intrinsic variability
amplitude and represents the averaged amplitude of observed
variations, corrected for the effects of measurement noise.
Next, the error in Fvar is given by the equation 3 of the same
section, we propagate it for the ΔFvar = Fvarg - Fvarr
values.
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Global notes:
Note (G1): Source blazars from Roma BZCAT - 5th edition catalogue,
Massaro et al. (2015Ap&SS.357...75M 2015Ap&SS.357...75M, Cat. VII/274). Name's format are
made as (5BZB/Q JHHMM+DDMM).
Note (G2): Classification type has two groups as follows:
BL Lac = 455 blazars in our sample
FSRQ = 442 blazars in our sample
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History:
From electronic version of the journal
(End) Luc Trabelsi [CDS] 22-Oct-2024