{"id":61,"date":"2019-08-23T15:06:22","date_gmt":"2019-08-23T13:06:22","guid":{"rendered":"http:\/\/www.issibern.ch\/teams\/studyultraviovar\/?page_id=61"},"modified":"2019-08-23T15:06:22","modified_gmt":"2019-08-23T13:06:22","slug":"proposal","status":"publish","type":"page","link":"https:\/\/www.issibern.ch\/teams\/studyultraviovar\/proposal\/","title":{"rendered":"Proposal"},"content":{"rendered":"\n

There are several ways to test prevailing models of AGN, and in particular those related to the properties of the accretion disc, and the physical processes operating therein.  These investigations come under two generic headings.  Detailed monitoring of a few AGN, mostly associated with reverberation mapping projects eg.  Mathur et al. (2017) and  Edelson et al (2019).  It is clear that for low-redshift AGN the amplitude of variability increases at shorter wavelengths, hence the importance of UV studies, see figure 1(Left) for NGC5548.  The second category statistical studies. For example the regular monitoring of specific fields eg. Sloan Digital Sky Survey (SDSS) Stripe 82, see Annis et al. (2014), and the recent cross-matching of the photometry of SDSS quasars with the same objects identified in the Dark Energy Survey (Rumbaugh et al, 2018).  These studies address different aspects of the problem. The detailed high time cadence studies reveal time lags between the optical\/UV and X-ray energy bands, which pose problems for models of reprocessing by an illuminating source. Whereas the statistical programs look for correlations between the UV variability and other basic properties such at the black hole mass and Eddington ratio, and can identify candidates for the so-called Changing Look AGN.  These objects have undergone extreme variations, often detected by their optical spectra (figure 2 Right), but which could easily be found in our proposed study. Indeed, the UV region is far better suited for their discovery than the optical because of the greater amplitude intrinsic variability, and it is much less contaminated by the presence of the constant starlight component from the host galaxy. Note: the Eddington ratio<\/strong> is a fundamental AGN parameter. It is the ratio of the observed bolometric luminosity to the maximum possible luminosity reached when radiation pressure outwards equals gravity acting inwards. Its value is determined by the accretion physics. High values may indicate mass outflows, and its range may define the  \u201clow state\u201d and \u201chigh state\u201d of an AGN (see figure 3). <\/p>\n\n\n\n

\"\"
Figure 1 Left: UV variability of an AGN over 250 days, showing greater amplitude\nat shorter wavelengths in (2670, 1813,1337 angstroms, upwards). Right: data of an\nAGN in shown in red. UV points are near to the peak emission. Also shown is an\naccretion disc model fit with a cartoon (above) showing one side of a symmetric\naccretion disc, and the black hole as a black circle adapted from Kubota and Done et al (2018).<\/figcaption><\/figure>\n\n\n\n

Our\nproposed study fails under the second heading. \nIt requires techniques related to \n\u201cBig Data\u201d mining.  The first\nstage will provide us with the sample of UV variable AGN.  Of course all AGN are variable, but some are\nmore variable than others. The extreme variable cases, using criteria we will\ndefine in the course of the study (could be a factor 2 in flux), will be the\nsubject of more detailed investigations such, as spectroscopic follow-up, and\nalso spectra energy distribution (SED) modelling to derive their bolometric\nluminosities.  In figure 1 (Right) we\nshow the SED of a typical AGN. This illustrates the observational gap imposed\nby neutral atom absorption in our own Galaxy, separating the UV emission and\nthe X-rays. Above this is a cartoon showing the regions responsible for this\nemission (Done et al. 2012). Models such as this will be employed to interpret\ncases of extreme UV variability.<\/p>\n\n\n\n

Ultra-violet Data-bases, from space and\nground-based facilities <\/strong><\/p>\n\n\n\n

The\nUV photometry for this project will be extracted from archive data-bases of the\nfollowing space missions; The XMM-Newton OM instrument (an ESA Cornerstone\nmission), the Neils Gehrels Swift Observatory UVOT instrument (a joint Italian\nNASA MIDEX mission, with Italian and UK involvement) and GALEX (a NASA small\nexplorer mission).   We will also utilise\nblue-optical (u-band) photometry from the SDSS and the Pan-STARRs survey.<\/p>\n\n\n\n

To\nemphasize the sheer amount of data waiting to be exploited , the latest (2018) XMM\nOM Serendipitous Ultraviolet Source Survey catalogue  contains 5,503,765 sources, of which 1,035,453\nhave different observations, which we can use for variability studies.  Furthermore, the Swift\/UVOT Serendipitous\nSource catalogue has 6,200,016 sources of which 2,027,265 have more than one\nobservation. GALEX has made an all-sky UV survey in two UV filters. The number\nof sources\nin the GALEX \u201cclean\u201d field of view, is  69,772,677.\n<\/p>\n\n\n\n

Of\ncourse only a small fraction of these UV sources are AGN, and so we will cross\ncorrelate them with the SDSS data release 14 quasar catalogue, Paris et al. (2018),\n which contains 526,356 quasars. The\ncross-correlation will result in many 1000\u2019s of hits. The first part of our\nproject will be to data mine these huge data-bases, and to assemble a sample of\nall AGN with multiple UV observations. <\/p>\n\n\n\n

Limitations <\/strong><\/p>\n\n\n\n

It\nis clear that our data will be only sparsely sampled in time, and so it will\nnot be possible to study time lags between the UV and emission at other\nenergies. However we will find example of extreme UV variability.  We will pay special attention to \u201cquality\ncontrol\u201d such cases, since they may be artefacts.  To quote Carl Sagan \u201cextraordinary claims\nrequire extraordinary evidence.\u201d In such cases human intervention will be\nneeded. For example, to examine the 2-D images from which the photometry is\nderived. There will be field stars near to the AGN, and the reality of extreme\nvariability can be tested by simple inspection with the constant flux field\nstars. <\/p>\n\n\n\n

A Pilot Study <\/strong><\/p>\n\n\n\n

The team leader (Ward) has commenced a pilot study of\nthis proposed project, carried out with a 4th<\/sup> year undergraduate\nstudent.  This involved only a small sub-sample\nof low redshift AGN having multiple SDSS optical spectra, and also multiple\nepoch UV photometry. We have identified several interesting objects, one of\nthese is shown in figure 2 (Left). This AGN appears to have varied in the UV by\nmore than a factor of 2. The timescales are important, and these will be statistically\nanalysed in our project.  Clearly this limited\npilot study has only sampled the \u201ctip of the iceberg\u201d of all the UV data\navailable in these catalogs, and so the time is ripe for a coordinated major\ninvestigation such as we propose here.<\/p>\n\n\n\n

\"\"
Figure 2 Left: Object of interest from our pilot study. This demonstrates that\nvariability occurs mostly in the UV. The bar symbols on the left are UV\nobservations at different dates, indicating strong variability. Right: an example of\na Changing Look AGN, (Shappee et al). These objects pose major problems for\naccretion disc models. We should discover many more of such examples.<\/figcaption><\/figure>\n\n\n\n

Phase 1:<\/strong>  The kick-off meeting in Bern will aim to scope the project. Identify the most efficient ways to mine these huge data-bases, and decide who is responsible for different parts of the work. Note: some of the basic \u201cdata-mining\u201d will be done by graduate students, with oversight by members of our team.                                                                                 <\/p>\n\n\n\n

Phase 2:<\/strong>  We will now have our sample of UV variable AGN.  We must be concerned about quality control (are the variations real, and in particular for the extreme cases?).  See the section on Limitations. We will assemble all available multi-wavelength data for a sub-set of the most interesting cases, and undertake SED modelling of their high and low states. This will used to plot their positions on the UV\/X-ray, versus Eddington ratio plot, shown in figure 3.  In parallel with this, we will begin a program of follow-up spectroscopy of the most promising variable objects.                                                                           <\/p>\n\n\n\n

Phase 3:  <\/strong>We would expect to present our final results at international conferences, and to publish in the top astronomy journals eg. The Astrophysical Journal, MNRAS and Astronomy and Astrophysics. If we discover an exceptionally interesting case, we will consider submitting it to the journal Nature or Science.  We will continue to monitor our best candidates spectroscopically, using medium aperture telescopes to which we have access.  <\/p>\n\n\n\n

Outputs<\/strong><\/p>\n\n\n\n

This\nwill be the first large scale statistical investigation of AGN UV variability\nusing several very large and mostly under-exploited satellite data-bases eg.\nXMM-Newton (OM) , Swift (UVOT) and GALEX. \nIn addition we will enhance the multi-wavelength element by using other ground-based\nfacility archives eg. SDSS, Pan-STARRS and the Catalina Survey. <\/p>\n\n\n\n

Our\ninvestigation will be complementary to previous studies of the optical\nvariability  of quasars, for which the\nspectra and photometry cover their accretion disc emission by virtue of the\nwavelength redshift.  Our sample will\nfocus on medium and low redshift \nAGN  (z < 1), where we need the\nspace based UV data to cover the wavelengths of the dominant disc\nemission.  All targets for which there is\nUV data from XMM and Swift, will also have near-simultaneous X-ray data, and so\nwe can measure the UV\/X-ray over a range of luminosities for the same object,\nfor 1000\u2019s of AGN. <\/p>\n\n\n\n

For\nthe first time we will determine the UV variability (near to the peak of the\naccretion disc emission) ranging from weeks to decades, for a large sample of\nmedium and low redshift AGN. This, when combined with Eddington ratios, will be\nused to test new ideas to explain the cross-over region between Low\/Hard and\nHigh\/Soft states of AGN,  see figure 3\nfrom Ruan et al (2019). This will help us to understand what factors influence\nthe emission properties of the accretion disc.<\/p>\n\n\n\n

Finally,\nit is statistically highly probable that we will discover some AGN with extreme\nUV variability, which will be strong candidates for Changing Look objects,  ideal for follow up optical spectroscopy. Our international\nteam has access to medium aperture telescopes which are suitable for this task.\n<\/p>\n\n\n\n

Legacy:<\/strong> Our project will provide a major resource for the\nextragalactic community. In\naddition to publishing our results we will also produce a catalogue of all AGN\nwith multiple UV observations, giving the fluxes and dates of the observations.\n<\/p>\n\n\n\n

\"\"
Figure 3. Showing UV\/X-ray index vs. Eddington ratio (from Ruan et al (2019). We\nwill test this new idea for explaining high and low states of AGN. Note: only very\nfew actual data points exist (light blue points are only predictions).<\/figcaption><\/figure>\n\n\n\n

References<\/strong><\/p>\n\n\n\n

Annis, J et al. ApJ., 794,120 (2014)<\/p>\n\n\n\n

Done, C et al. MNRAS., 420, 1848 (2012)<\/p>\n\n\n\n

Edelson, R et al. ApJ., 870,123  (2019)<\/p>\n\n\n\n

Lawrence, A  Nature, Astronomy.,\n2, 102 (2018)<\/p>\n\n\n\n

Kubota A., Done C., 2018, MNRAS, 480, 1247<\/p>\n\n\n\n

Mathur, S et al. ApJ., 846, 55 (2017)<\/p>\n\n\n\n

Paris, I et al. A & A., 613, 51  (2018)<\/p>\n\n\n\n

Ruan, J et al. preprint arXiv:1903.02553v1  (March 2019) <\/p>\n\n\n\n

Rumbaugh, N et al ApJ., 854,160 (2018)<\/p>\n\n\n\n

Shappee et al.  ApJ. 788,48\n(2014)<\/p>\n","protected":false},"excerpt":{"rendered":"

There are several ways to test prevailing models of AGN, and in particular those related to the properties of the accretion disc, and the physical processes operating therein.  These investigations come under two generic headings.  Detailed monitoring of a few … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-61","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.issibern.ch\/teams\/studyultraviovar\/wp-json\/wp\/v2\/pages\/61","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.issibern.ch\/teams\/studyultraviovar\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.issibern.ch\/teams\/studyultraviovar\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.issibern.ch\/teams\/studyultraviovar\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.issibern.ch\/teams\/studyultraviovar\/wp-json\/wp\/v2\/comments?post=61"}],"version-history":[{"count":0,"href":"https:\/\/www.issibern.ch\/teams\/studyultraviovar\/wp-json\/wp\/v2\/pages\/61\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.issibern.ch\/teams\/studyultraviovar\/wp-json\/wp\/v2\/media?parent=61"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}