DESY, Zeuthen · Summer Student Programme 2018

Monitoring the variability of TeVBlazars

Exploring the Universe at the highest energies

Serena Di PedeUniversità di Bologna · Italy

SupervisorsDr Elisa PueschelDr Tarek Hassan

September 3, 2018

Abstract

Blazars are characterized by rapid variability at virtually all wavelengths fromradio to TeV gamma-rays. The challenge since their discovery has been tounderstand the origin of their luminous, non-thermal, nuclear emission.

Using data from the last four VERITAS blazar catalog, we tried to investigate theseso far objects, monitoring their so unpredictable behaviour and trying to hunt out

new hints about the gamma-ray sources in the extra-galactic space.Exploiting the power of multi-wavelength astronomy, we studied the "Alerts"

received by VERITAS from different instruments trying to assess which ones weremore useful.

Performing Fermi anaylsis we searched for how many of these "Alerts" could havebeen self-triggered using LAT data.

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Contents

1 Physics behind the project 11.1 Blazar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Multi-wavelength approach 22.1 VERITAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 FermiLAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Project goal 3

4 Results 34.1 Blazar ToOs from the last four seasons . . . . . . . . . . . . . . . . . . . . . 34.2 Light curves with FermiLAT data . . . . . . . . . . . . . . . . . . . . . . . . 6

5 Conclusion 8

Appendix 9

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1 Physics behind the project

The physics behind this project is about the most luminous objects of the Universe: ActiveGalaxies. Active Galactic Nuclei (AGNs) are astrophysical objects that consist of a super-massive black hole in the center, an hot accretion disk and a larger dust torus surrodingit, two relativistic jets that emerge perpendicular to the accretion disk and large lobeswhere the jets terminates. They can emit thermal radiation, originating from in-fallingmatter strongly heated, that produces mostly in optical, UV and X-ray bands, or non-thermal radiation, produced by highly energetic particles accelerated in jets of material,that encompasses the entire electromagnetic spectrum, from radio to gamma-rays. [2]

The wide variety of AGN types seen is largely a function of viewing angle and ofgeometry rather than the physics. When the jets make an acute angle with the line ofsight, the radio and the optical emission from the core can be seen, and the object iscalled Quasar, which are radio weak AGN with a continuum emission in the optical range.Instead, when the jet is the most feature, the object is called Blazar. [1]

1.1 Blazar

Blazars are the most luminous persistent gamma-rays emitters in the Universe. Theyconstitute a subclass of radio-loud AGNs in which a jet of relativistic particles pointsalmost along the line of sight.

The emission spectra of these objects are dominated by non-thermal and variable ra-diation which spans from radio frequencies to above 1TeV. Looking at their spectrum,it can be easily recognise the typical "camel’s back" shape that consists of two distinctspectral components (see Figure 1): a low-energy synchrotron bump (responsible for emis-sion at lower frequencies), and a high-energy inverse Compton-scattered bump (at higherfrequencies, up to GeV-TeV).

Figure 1: Overall Spectral Energy Distribution (SED) of the Mrk 501. [4]

The variability was the first property to find and identify blazars. Variations in blazarsare reported on time scales from years down to less than a day. Because of this variability,the selection criteria to study them are strongly dependent on the kind of observationand on the single source characteristics. That’s why the observational details of multi-wavelength study dictate the type of blazar studied. [3]

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2 Multi-wavelength approach

We are in the "Golden age" of multi-wavelength astronomy. VERITAS blazars discoveryprogram uses multi-wavelength informations from other experiments to try to observe VHEblazars candidates when they are in high flux state.

After being triggered by a signal received from a certain source of interest, a detectormay generate an "Alert" for other instruments. Data that the instruments took in responseof a trigger are called ToOs (Target of Opportunities). The instruments that participateat this kind of programme can be very different from each other on the basis of theirsensitivity to different part of the electromagnetic spectrum. With this multi-wavelengthapproach of detecting, all together can give more and more complete informations aboutthe same object.

VERITAS blazars discovery programme consists of instruments that get informationsfrom the optical, X-ray, GeV and TeV band of the electromagnetic spectrum. In particular,FLWO and the Tuorla Observatory operate in the optical range, the SWIFT X-ray Tele-scope (XRT), FermiLAT in the GeV band, the IACTs telescopes (HESS, MAGIC, FACTin addition to VERITAS) and the Water Cherenkov Gamma-Ray Observatory HAWC inthe TeV band.

2.1 VERITAS

VERITAS (Figure 2) is one of the four ground-based gamma ray instrument currentlyoperating. It consists of an array of four optical reflectors that observe gamma-ray sourcesin the GeV-TeV range.

Figure 2: VERITAS 4-telescope situated in the southern Arizona.

Imaging Cherenkov Atmospheric telescopes (IACT) are deployed such that they havethe highest sensitivity in the VHE energy band (50GeV - 50TeV). Since VERITAS is apointed telescope that only covers almost 3 degrees of the sky in each observation (Field ofView ∼ 3.5◦), knowning when AGNs are flaring is not possible. For this reason, "Alerts"from other instruments are required for VERITAS to study interesting and bright AGNflares. [5]

2.2 FermiLAT

FermiLAT (Figure 3) is an imaging high-energy gamma-ray telescope covering the energyrange from about 20 MeV to more than 300 GeV. The LAT’s field of view covers about

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20% of the sky at any time, and it scans continuously, covering the whole sky every threehours. [6]

Figure 3: The satellite FermiLAT.

3 Project goal

My project aims to study the "Alerts" received by VERITAS. Looking at the VERITASblazar catalogs, from 2014 up to the past year catalog (2017-2018), informations about theToOs have been obtained. The object was to try to asses the time of the ToO with respectto the AGN flare and to study which "Alert" were more useful.

Then, looking at the FermiLAT public data, we try to understand how many "Alerts"could have been self-triggered using LAT data.

4 Results

4.1 Blazar ToOs from the last four seasons

56 sources have been analysed: 13 sources from the Season 2014-2015, 14 from the Season2015-2016, 14 from the Season 2016-2017, and 15 from the Season 2017-2018. We lookedat the next-day analysis [7] for each trigger received. Data have been taken from theEventdisplay analysis [8], a machine learning algorithm (Boosted Decision Tree) developedby the VERITAS Collaboration, that seeks to suppress the cosmic-ray background eventsretaining gamma-rays in order to achieve good sensitivity to faint gamma-ray sources.The data file contains informations about the observing dates, the measured flux, and themeasured flux in unit of Crab Nebula.

First, light curves of the sources were produced. The second step was to check if thetrigger resulted in a variability detection. We looked for the measurement’s deviation fromthe season’s average calculated without the ToOs in a way that it was not biased by thetrigger measurements. To quantize this deviation it was used a value called z-score:

z − score =ΦToO − Φ̄withoutToO√

(∆ΦToO)2 + (∆Φ̄withoutToO)2(1)

where ΦToO is the measured flux of the ToO; Φ̄withoutToO is the season’s average fluxcalculated without the ToOs; ∆ΦToO and ∆Φ̄withoutToO are the respective errors.

Table 1 contains the trigger dates and a list of the sources for which ToO observationsrevealed a significant flare, namely the measured flux results consistent with the hypothesis

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Trigger date Source name z − scorewithToOs z − scorewithoutToOs

58014 1ES1959+650 10.39 11.3258107 BL Lac 19.47 20.9058108 Mkr421 2.95 2.8457666 BL Lac 28.06 29.6457844 1ES1215+303 7.71 5.2357665 Mrk501 7.58 9.9957755 NGC 1275 17.06 9.6357373 1ES2344+514 13.95 11.7357507 1ES1959+650 23.14 15.28

Table 1: List of the flaring AGNs together with the Z-score calculated with and withoutToOs. The table is splitted horizontally into three parts, related to the three seasons,respectively from the top: season 2017-2018, season 2016-2017, season 2015-2016. Note:we didn’t find anything interesting for the season 2014-2015.

of detection. In the last two columns are shown the z− score calculated with the equation1, with and without the ToOs. We considered as a detection only sources with z − scorevalues bigger than 5.

Table 2 shows the same blazar’s list together with what type of AGN is (for thisclassification see [9] and the Appendix). Scrolling through the list, it can be seen that themajority of blazars result as BL Lac HBL (high frequency peaked blazars [1]) and onlythe NGC 1275 is FRI (Class I Fanaroff-Riley), a specific type of radio-loud AGN. The last

Source name Type of blazar Trigger Condition1ES1959+650 HBL Self-trigger VERITAS(TeV)

BL Lac BL Lac VERITAS(TeV)Mkr421 HBL FACT(TeV)BL Lac BL Lac VERITAS(TeV)

1ES1215+303 HBL FermiLAT(GeV)Mrk501 HBL FACT(TeV)

NGC 1275 FRI VERITAS,MAGIC(TeV)1ES2344+514 HBL SWIFT,FermiLAT(X-rays,GeV)1ES1959+650 HBL FACT,SWIFT,FermiLAT

Table 2: The same list of Table 1 with the AGN’s type and the trigger condition.

column contains the information of the trigger condition. As it is described in Section 2,VERITAS can receive different "Alerts" from different instruments. The largest part aretriggers from the IACTs and this shows the succesful of these instruments in detectinginteresting and bright AGN flares.

Choosing one blazar for each of the three season groups from the tables, light curves ofblazars 1ES1959+650, BL Lac, 1ES2344+514 are shown below in Figure 4, Figure 5 andFigure 6.

It is worth talking also about the cases of no-detection. In general, a source is detectedif the flux measurement is bigger than some threshold. If a source is detected, upperand lower flux limits (or a confidence interval) that probably contains the true flux can

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Figure 4: Light curve of 1ES1959+650 (season 2017-2018). Left: on the y-axis the measured fluxvs the observing time in Modified Julian Date (MJD). The red line is the average flux of the seasonwith its error; blue lines are the trigger date. Right: the same in unit of Crab Nebula.

Figure 5: Light curve of BL Lac (2016-2017). Left: on the y-axis the measured flux vs theobserving time in Modified Julian Date (MJD). The red line is the average flux of the season withits error; blue lines are the trigger date. Right: the same in unit of Crab Nebula.

Figure 6: Light curve of 1ES2344+514 (2015-2016). Left: on the y-axis the measured flux vs theobserving time in Modified Julian Date (MJD). The red line is the average flux of the season withits error; blue lines are the trigger date. Right: the same in unit of Crab Nebula.

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be assigned using the uncertainties. If a source is not detected the flux measurementis consistent with zero (the null-hypothesis of no-detection). In this case we proceed bydefining an upper limit on the flux. A flux upper limit ΦUL is usually assigned accordingto one’s choice of the probability or confidence that the actual source with flux = ΦUL

will be detected [10]. As an example of source that required the calculation of the fluxUpper Limit, the light curve of blazar 3C279 (season 2014-2015) is shown in Figure 7. Thez − score calculated is ' 0.65.

Figure 7: Light curve of 3C279 (season 2014-2015) which need the a flux upper limit for thedetection.

4.2 Light curves with FermiLAT data

From Table 1 a subsample of blazars has been chosen to perform Fermi analysis. Foursources with z−score ≥ 10 were choesen: BL Lac and Mrk501 from the season 2016-2017,1ES2344+514 and 1ES1959+650 from the season 2015-2016. Data have been downloadedfrom the FSSC (Fermi Science Support Center) archive [11] and the analysis was performedusing the Fermipy package [12].

To download the data, the query needs the object name or coordinates, the searchradius, the observation dates, the energy range and LAT data type you want to download.Searches are executed by locating all events which occur within the specified radius of thetarget location. For the four sources, a search radius of 10◦ was used. As for the observationdates, we only analyse data around the ToO date, not the whole mission light curve. Weused the same energy range for the four sources: from 300 MeV to 300000 MeV. Finally,Photon LAT data type was chosen because it consists of the cleaner photon selections("SOURCE", "CLEAN" and "ULTRACLEAN" classes) which provide lower backgroundcontamination at the expense of lower effective areas. Then, after downloading also thespacecraft data that contains the spacecraft position, pointing data and others instrument-specific data, the query is completed. The following figures present the comparison betweenthe light curve made with VERITAS data and the light curve produced with FERMIanalysis. Note that the temporal scale is different.

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Figure 8: BL Lac. Left: light curve from VERITAS catalog data (2016-2017). Right: light curvefrom FERMI data taken from 57483,333 to 57817,333 (MJD).

Figure 9: Mrk 501 or 7C165211.80+395026.00 [13]. Left: light curve from VERITAS catalog data(2016-2017). Right: light curve from Fermi data taken from 57482,333 to 57816,333 (MJD).

Figure 10: 1ES2344+514 or QSOB2344+514 [13]. Left: light curve from VERITAS catalog data(2015-2016). Right: light curve from Fermi data taken from 57220,333 to 57556,333 (MJD).

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Figure 11: 1ES1959+650 or QSOB1959+650 [13]. Left: light curve from VERITAS catalog data(2015-2016). Right: light curve from Fermi data taken from 57324,333 to 57661,333 (MJD).

5 Conclusion

In Table 2 we see that most of the identified blazars are BL Lacs. Some of them are alsoidentified, from the TeV online catalog ([9]), as HBL BL Lacs objects. Only NGC 1275 isFRI type: in particular, it is a misaligned radio galaxy in the Perseus cluster of galaxies.From the trigger condition showed in Table 2 it is clear that most of the triggers are dueto IACTs. However, some of them are the result of multi-wavelength campaign, involvingTeV, GeV instrument (Fermi) and the SWIFT telescope in the X-ray energy range. Thisis, for example, the case of 1ES2344+514 and 1ES1959+650.

Looking at the FERMI light curves (Figure 8, 9, 11), Fermi analysis also confirmed theblazar flare on the trigger date written in Table 1. Instead, for the blazar 1ES2344+514(Figure 10), Fermi data showed pretty flat behaviour of the spectrum during almost allthe time of observation, in contrast with SWIFT and VERITAS which had seen a flaringbehaviour in the same time interval (graph at left in Figure 10).

FermiLAT provides useful informations for bright AGN flares, but it is not as good asIACTs for short observation times or faint sources due to its small effective area. Anyway,it would be ideal if a satellite like Fermi would be available in the future, as CTA wouldenormously profit from it.

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AGN Classification

Figure 12: A general classification of Active Galactic Nuclei fromhttp://inspirehep.net/record/1422753/plots.

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Acknowledgements

I would like to thank my supervisor Dr. Elisa Pueschel and Dr. Tarek Hassan for theirguidance, support and patient while navigating in my project.

I would like to thank all the VERITAS group for sharing with me their experience andknowledge, Dr. Mireia Nievas, Dr. Maria Krause, Konstantin, Chiara Giuri,...

I would like to thank Dr. Karl Jansen for providing me the possibility to join the DESYband and to play for the first time ever with only physicists.

I would like to thank Dr. Gernot Maier and all the DESY staff that were involvedin the coordination of the program for providing the summer students with an incredibleopportunity to mature as aspiring physicists.

I would like to thank my family because without their support and encouragement Iwould surely not have been here.

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References

[1] Trevor Weeks, Very High Energy Gamma-Ray Astronomy, Institute of Physics Bristoland Philadelphia, Series in Astronomy and Astrophysics (2003).

[2] Maurizio Spurio, Particle and Astrophysics, a multi-messenger Approach, Astronomyand Astrophysics Library, University of Bologna, (2015)

[3] C. Megan Urry, An Overview of Blazar Variability, Space Telescope Science Institute,(1996)

https://arxiv.org/abs/astro-ph/9609023

[4] Theoretical Implications from the Spectral Evolution of Markarian 501 Observed withBeppoSAX

http://iopscience.iop.org/article/10.1086/321394/fulltext/

[5] Very Energetic Radiation Imaging Telescope Array System

https://veritas.sao.arizona.edu/

[6] The Fermi Large Area Telescope

http://www-glast.stanford.edu/

[7] Next-Day VERITAS Analysis

https://veritas.sao.arizona.edu/private/nextday/

[8] Maria Krause, Elisa Pueschel, Gernot Maier, Improved gamma/hadron separation forthe detection of faint gamma-ray sources using boosted decision trees

https://arxiv.org/pdf/1701.06928.pdf

[9] TeVCat online catalog for TeV Astronomy

http://tevcat.uchicago.edu/

[10] F. Masci, Computing flux upper-limits for non-detections (2011)

http://wise2.ipac.caltech.edu/staff/fmasci/UpperLimits_FM2011.pdf

[11] FERMI Science Support Center archive

https://fermi.gsfc.nasa.gov/cgi-bin/ssc/LAT/LATDataQuery.cgi

[12] Fermipy’s documentation

https://fermipy.readthedocs.io/en/latest/index.html

[13] SIMBAD Astronomical Database - CDS Strasbourg

http://simbad.u-strasbg.fr/simbad/

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