arXiv subjects

Andy Lawrence

Publications and source records attributed to Andy Lawrence.

At least 19 recordsLinked to original sources

arXiv2025

The Second and Third Data Releases from the UKIRT Hemisphere Survey

This paper describes the second and third data releases (DR2 and DR3, respectively) from the ongoing United Kingdom Infrared Telescope (UKIRT) Hemisphere Survey (UHS). DR2 is primarily comprised of the $K$-band portion of the UHS survey, and was released to the public on June 1, 2023. DR3 mainly includes the $H$-band portion of the survey, with a public release scheduled for September 2025. The $H$- and $K$-band data releases complement the previous $J$-band data release (DR1) from 2018. The survey covers approximately 12,700 square degrees between declinations of 0 degrees and $+$60 degrees and achieves median 5$\sigma$ point source sensitivities of 19.0 mag and 18.0 mag (Vega) for $H$ and $K$, respectively. The data releases include images and source catalogs which include $\sim$581 million $H$-band detections and $\sim$461 million $K$-band detections. DR2 and DR3 also include merged catalogs, created by combining $J$- and $K$-band detections (DR2) and $J$-, $H$-, and $K$-band detections (DR3). The DR2 merged catalog has a total of $\sim$513 million sources, while the DR3 merged catalog contains $\sim$560 million sources.

astro-ph.GA↗

arXiv2025

Long Term Reverberation Mapping of Iron Coronal Lines in MKN 110

We present flux measurements of the coronal lines [Fe VII] and [Fe X] spanning three decades, in the highly variable Active Galactic Nucleus (AGN) MKN 110. These coronal lines are sensitive to the spectral energy distribution (SED) of AGNs in the extreme ultraviolet (EUV). Neither [Fe VII] nor [Fe X] demonstrates variability in the short term on a weekly or monthly timescale. However, by taking advantage of a long term decrease in the continuum flux of MKN 110 on the order of years, we were able to track the [Fe VII] and [Fe X] fluxes as they respond to the continuum. We were able to detect a lag for [Fe VII] relative to the continuum at 5100 {\AA}, with a modal lag of 652 days, but were unable to detect a significant lag in the [Fe x] flux, though there exist significant uncertainties in the [Fe X] fit. These two lag results are not consistent and the line widths for the two line species also do not match. This provides strong evidence for stratification within the coronal line region (CLR). There is also evidence of a non-varying component within the coronal line flux, probably a result of a more extended region of origin. Taken together, these results suggest a CLR where the bulk of the [Fe VII] originates on parsec scales, but a portion of the [Fe VII] flux originates further out, at or beyond a 10 pc scale. These results also indicate the limitations of single-cloud models in describing the physical conditions of the CLR.

astro-ph.GA↗

arXiv2024

Enabling Science from the Rubin Alert Stream with Lasair

Lasair is the UK Community Broker for transient alerts from the Legacy Survey of Space and Time (LSST) from the Vera C. Rubin Observatory. We explain the system's capabilities, how users can achieve their scientific goals, and how Lasair is implemented. Lasair offers users a kit of parts that they can use to build filters to concentrate their desired alerts. The kit has novel lightcurve features, sky context, watchlists of special sky objects and regions of the sky, dynamic crossmatching with catalogues of known astronomical sources, and classifications and annotations from other users and partner projects. These resources can be shared with other users, copied, and modified. Lasair offers real-time machine-to-machine notifications of filtered transient alerts. Even though the Rubin Observatory is not yet complete, Lasair is a mature system: it has been processing and serving data from the similarly formatted stream of the Zwicky Transient Facility (ZTF) alerts.

astro-ph.IM↗

arXiv2023

Astronomy, Doughnuts, and Carrying Capacity

I examine the applicability of ecological concepts in discussing issues related to space environmentalism. Terms such as "ecosystem"", "carrying capacity"", and "tipping point" are either ambiguous or well defined but not applicable to orbital space and its contents; using such terms uncritically may cause more confusion than enlightenment. On the other hand, it may well be fruitful to adopt the approach of the Planetary Boundaries Framework, defining trackable metrics that capture the damage to the space environment. I argue that the key metric is simply the number of Anthropogenic Space Objects (ASOs), rather than for example their reflectivity, which is currently doubling every 1.7 years; we are heading towards degree scale separation. Overcrowding of the sky is a problem astronomers and satellite operators have in common.

astro-ph.IM↗

arXiv2022

Systematic light curve modelling of TDEs: statistical differences between the spectroscopic classes

With the sample of observed tidal disruption events (TDEs) now reaching several tens, distinct spectroscopic classes have emerged: TDEs with only hydrogen lines (TDE-H), only helium lines (TDE-He), or hydrogen in combination with He II and often N III/O III (TDE-H+He). Here we model the light curves of 32 optically-bright TDEs using the Modular Open Source Fitter for Transients (MOSFiT) to estimate physical and orbital properties, and look for statistical differences between the spectroscopic classes. For all types, we find a shallow distribution of star masses, compared to a typical initial mass function, between $\sim 0.1-1$ M$_\odot$, and no TDEs with very deep ($\beta \gg 1$) encounters. Our main result is that TDE-H events appear to come from less complete disruptions (and possibly lower SMBH masses) than TDE-H+He, with TDE-He events fully disrupted. We also find that TDE-H events have more extended photospheres, in agreement with recent literature, and argue that this could be a consequence of differences in the self-intersection radii of the debris streams. Finally, we identify an approximately linear correlation between black hole mass and radiative efficiency. We suggest that TDE-H may be powered by collision-induced outflows at relatively large radii, while TDE-H+He could result from prompt accretion disks, formed more efficiently in closer encounters around more massive SMBHs.

astro-ph.HE↗

arXiv2019

Behaviour of the MgII 2798AA Line Over the Full Range of AGN Variability

We investigate the responsiveness of the 2798AA Mg II broad emission line in AGN on timescales of several years. Our study is based on a sample of extremely variable AGN as well as a broad population sample. The observed response of the line in previous studies has been mixed. By focussing on extreme variability ($|\Delta g|>$ 1) we find that Mg II clearly does respond to the continuum. However, the degree of responsiveness varies strikingly from one object to another; we see cases of Mg II changing by as much as the continuum, more than the continuum, or very little at all. In 74% of the highly variable sample the behaviour of Mg II corresponds with that of H$\beta$, with 30% of the objects showing large variations in both lines. We do not detect any change in the line width that would correspond to Broad Line Region `breathing', in accordance with results from literature. Some of the objects in our highly variable sample show a clear asymmetry in the Mg II profile. This skewness can be both to the blue and the red of the line centre. Results from our broad population sample show that highly variable quasars have lower Eddington ratios. This result holds for the variability of the continuum, but the correlation is significantly reduced for the variability of the Mg II line. For the first time, we present an overview of the value of the intrinsic Baldwin Effect for Mg II in a large sample.

astro-ph.GA↗

arXiv2019

The 'Big Dipper': The nature of the extreme variability of the AGN SDSS J2232-0806

SDSS J2232-0806 (the 'Big Dipper') has been identified as a 'slow-blue nuclear hypervariable': a galaxy with no previously known active nucleus, blue colours and large-amplitude brightness evolution occurring on a timescale of years. Subsequent observations have shown that this source does indeed contain an active galactic nucleus (AGN). Our optical photometric and spectroscopic monitoring campaign has recorded one major dimming event (and subsequent rise) over a period of around four years; there is also evidence of previous events consistent with this in archival data recorded over the last twenty years. Here we report an analysis of the eleven optical spectra obtained to date and we assemble a multiwavelength data set including infrared, ultraviolet and X-ray observations. We find that an intrinsic change in the luminosity is the most favoured explanation of the observations, based on a comparison of continuum and line variability and the apparent lagged response of the hot dust. This source, along with several other recently-discovered 'changing-look' objects, demonstrate that AGN can exhibit large-amplitude luminosity changes on timescales much shorter than those predicted by standard thin accretion disc models.

astro-ph.GA↗

arXiv2018

Changing-Look Quasar Candidates: First Results from Follow-up Spectroscopy of Highly Optically Variable Quasars

Active galactic nuclei (AGN) that show strong rest-frame optical/UV variability in their blue continuum and broad line emission are classified as "changing-look" AGN, or at higher luminosities changing look quasars (CLQs). These surprisingly large and sometimes rapid transitions challenge accepted models of quasar physics and duty cycles, offer several new avenues for study of quasar host galaxies, and open a wider interpretation of the cause of differences between broad and narrow line AGN. To better characterize extreme quasar variability, we present follow-up spectroscopy as part of a comprehensive search for CLQs across the full SDSS footprint using spectroscopically confirmed quasars from the SDSS DR7 catalog. Our primary selection requires large-amplitude (|\Delta g|>1 mag, |\Delta r|>0.5 mag) variability over any of the available time baselines probed by the SDSS and Pan-STARRS 1 surveys. We employ photometry from the Catalina Sky Survey to verify variability behavior in CLQ candidates where available, and confirm CLQs using optical spectroscopy from the William Herschel, MMT, Magellan, and Palomar telescopes. For our adopted S/N threshold on variability of broad H\beta emission, we find 17 new CLQs, yielding a confirmation rate of >~ 20%. These candidates are at lower Eddington ratio relative to the overall quasar population which supports a disk-wind model for the broad line region. Based on our sample, the CLQ fraction increases from 10% to roughly half as the continuum flux ratio between repeat spectra at 3420 Angstroms increases from 1.5 to 6. We release a catalog of over 200 highly variable candidates to facilitate future CLQ searches.

astro-ph.GA↗

arXiv2018

Quasar Viscosity Crisis

Recent observations of extreme variability in Active Galactic Nuclei have pushed standard viscous accretion disc models over an edge. "Extreme reprocessing" where an erratically variable central quasi-point source is entirely responsible for heating an otherwise cold and passive low-viscosity disc, may be the best route forward.

astro-ph.HE↗

arXiv2017

The broad band SEDs of four 'hypervariable' AGN

We present an optical to X-ray spectral analysis of four 'hypervariable' AGN (HVAs) discovered by comparing Pan-STARRS data to that from SDSS over a 10 year baseline (Lawrence et al 2016). There is some evidence that these objects are X-ray loud for their corresponding UV luminosities, but given that we measured them in a historic high state, it is not clear whether to take the high-state or low-state as typical of the properties of these HVAs. We estimate black hole masses based on MgII and H$\alpha$ emission line profiles, and either the high and low state luminosities, finding mass ranges $\log(M_{\rm BH}/M_{\odot}) = 8.2-8.8$ and $\log(M_{\rm BH}/M_{\odot}) = 7.9-8.3$ respectively. We then fit energy conserving models to the SEDs, obtaining strong constraints on the bolometric luminosity and $\alpha_{\rm OX}$. We compare the SED properties with a larger, X-ray selected AGN sample for both of these scenarios, and observe distinct groupings in spectral shape versus luminosity parameter space. In general, the SED properties are closer to normal if we assume that the low-state is representative. This supports the idea that the large slow outbursts may be due to extrinsic effects (for example microlensing) as opposed to accretion rate changes, but a larger sample of HVAs is needed to be confident of this conclusion.

astro-ph.HE↗

arXiv2017

First Discoveries of z>6 Quasars with the DECam Legacy Survey and UKIRT Hemisphere Survey

We present the first discoveries from a survey of $z\gtrsim6$ quasars using imaging data from the DECam Legacy Survey (DECaLS) in the optical, the UKIRT Deep Infrared Sky Survey (UKIDSS) and a preliminary version of the UKIRT Hemisphere Survey (UHS) in the near-IR, and ALLWISE in the mid-IR. DECaLS will image 9000 deg$^2$ of sky down to $z_{\rm AB}\sim23.0$, and UKIDSS and UHS, which will map the northern sky at $0 6.5$ to $J_{\rm VEGA}<19.5$.

astro-ph.GA↗

arXiv2017

Discovery of 16 New z ~ 5.5 Quasars : Filling in the Redshift Gap of Quasar Color Selection

We present initial results from the first systematic survey of luminous $z\sim 5.5$ quasars. Quasars at $z \sim$ 5.5, the post-reionization epoch, are crucial tools to explore the evolution of intergalactic medium, quasar evolution and the early super-massive black hole growth. However, it has been very challenging to select quasars at redshifts 5.3 $\le z \le$ 5.7 using conventional color selections, due to their similar optical colors to late-type stars, especially M dwarfs, resulting in a glaring redshift gap in quasar redshift distributions. We develop a new selection technique for $z \sim$ 5.5 quasars based on optical, near-IR and mid-IR photometric data from Sloan Digital Sky Survey (SDSS), UKIRT InfraRed Deep Sky Surveys - Large Area Survey (ULAS), VISTA Hemisphere Survey (VHS) and Wide field Infrared Survey Explorer (WISE). From our pilot observations in SDSS-ULAS/VHS area, we have discovered 15 new quasars at 5.3 $\le z \le$ 5.7 and 6 new lower redshift quasars, with SDSS z band magnitude brighter than 20.5. Including other two $z \sim$ 5.5 quasars already published in our previous work, we now construct an uniform quasar sample at 5.3 $\le z \le$ 5.7 with 17 quasars in a $\sim$ 4800 square degree survey area. For further application in a larger survey area, we apply our selection pipeline to do a test selection by using the new wide field J band photometric data from a preliminary version of the UKIRT Hemisphere Survey (UHS). We successfully discover the first UHS selected $z \sim$ 5.5 quasar.

astro-ph.GA↗

arXiv2016

Clues to the Structure of AGN through massive variability surveys

Variability studies hold information on otherwise unresolvable regions in Active Galactic Nuclei (AGN). Population studies of large samples likewise have been very productive for our understanding of AGN. These two themes are coming together in the idea of systematic variability studies of large samples - with SDSS, PanSTARRS, and soon, LSST. I summarise what we have learned about the optical and UV variability of AGN, and what it tells us about accretion discs and the BLR. The most exciting recent results have focused on rare large-scale outbursts and collapses - Tidal Disruption Events, changing-look AGN, and large amplitude microlensing. All of these promise to give us new insight into AGN physics.

astro-ph.HE↗

arXiv2015

A Systematic Search for Changing-Look Quasars in SDSS

We present a systematic search for changing-look quasars based on repeat photometry from SDSS and Pan-STARRS1, along with repeat spectra from SDSS and SDSS-III BOSS. Objects with large, |\Delta g|>1 mag photometric variations in their light curves are selected as candidates to look for changes in broad emission line (BEL) features. Out of a sample of 1011 objects that satisfy our selection criteria and have more than one epoch of spectroscopy, we find 10 examples of quasars that have variable and/or "changing-look" BEL features. Four of our objects have emerging BELs; five have disappearing BELs, and one object shows tentative evidence for having both emerging and disappearing BELs. With redshifts in the range 0.20 < z < 0.63, this sample includes the highest-redshift changing-look quasars discovered to date. We highlight the quasar J102152.34+464515.6 at z = 0.204. Here, not only have the Balmer emission lines strongly diminished in prominence, including H$\beta$ all but disappearing, but the blue continuum $f_{\nu} \propto \nu^{1/3}$ typical of an AGN is also significantly diminished in the second epoch of spectroscopy. Using our selection criteria, we estimate that >15% of strongly variable luminous quasars display changing-look BEL features on rest-frame timescales of 8 to 10 years. Plausible timescales for variable dust extinction are factors of 2-10 too long to explain the dimming and brightening in these sources, and simple dust reddening models cannot reproduce the BEL changes. On the other hand, an advancement such as disk reprocessing is needed if the observed variations are due to accretion rate changes.

astro-ph.GA↗

arXiv2015

Euro-VO - Coordination of Virtual Observatory activities in Europe

The European Virtual Observatory Euro-VO has been coordinating European VO activities through a series of projects co-funded by the European Commission over the last 15 years. The bulk of VO work in Europe is ensured by the national VO initiatives and those of intergovernmental agencies. VO activities at the European level coordinate the work in support of the three "pillars" of the Virtual Observatory: support to the scientific community, take-up by the data providers, and technological activities. Several Euro-VO projects have also provided direct support to selected developments and prototyping. This paper explains the methodology used by Euro-VO over the years. It summarizes the activities which were performed and their evolutions at different stages of the development of the VO, explains the Euro-VO role with respect to the international and national levels of VO activities, details the lessons learnt for best practices for the coordination of the VO building blocks, and the liaison with other European initiatives, documenting the added-value of European coordination. Finally, the current status and next steps of Euro-VO are briefly addressed.

astro-ph.IM↗

arXiv2013

On the progenitor of the Type IIP SN 2013ej in M74

We use natural seeing imaging of SN 2013ej in M74 to identify a progenitor candidate in archival {\it Hubble Space Telescope} + ACS images. We find a source coincident with the SN in the {\it F814W}-filter, however the position of the progenitor candidate in contemporaneous {\it F435W} and {\it F555W}-filters is significantly offset. We conclude that the "progenitor candidate" is in fact two physically unrelated sources; a blue source which is likely unrelated to the SN, and a red source which we suggest exploded as SN 2013ej. Deep images with the same instrument onboard {\it HST} taken when the supernova has faded (in approximately two years time) will allow us to accurately characterise the unrelated neighbouring source and hence determine the intrinsic flux of the progenitor in three filters. We suggest that the {\it F814W} flux is dominated by the progenitor of SN 2013ej, and assuming a bolometric correction appropriate to an M-type supergiant, we estimate that the mass of the progenitor of SN 2013ej was between 8 -- 15.5 M$_{\odot}$.

astro-ph.CO↗

arXiv2013

The Effect of Partial Obscuration on the Luminosity Dependence of the Obscured Fraction in Active Galactic Nuclei

Surveys of Active Galactic Nuclei (AGN) in different observational regimes seem to give different answers for the behaviour of the obscured fraction with luminosity. Based on the complex spectra seen in recent studies, we note that partial covering could significantly change the apparent luminosities of many AGN, even after apparent X-ray absorption correction. We explore whether this effect could reproduce the observed dependence of the obscured fraction on the apparent X-ray luminosities seen between 2--10 keV. We can reproduce the observed trend in a model where 33 per cent of AGN are unobscured, 30 per cent are heavily buried, and 37 per cent have a range of intermediate partial coverings. Our model is also tentatively successful at reproducing observed trends in the X-ray vs. infrared luminosity ratio for AGN.

astro-ph.CO↗

arXiv2013

The Distribution of AGN Covering Factors

We review our knowledge of the most basic properties of the AGN obscuring region - its location, scale, symmetry, and mean covering factor - and discuss new evidence on the distribution of covering factors in a sample of ~9000 quasars with WISE, UKIDSS, and SDSS photometry. The obscuring regions of AGN may be in some ways more complex than we thought - multi-scale, not symmetric, chaotic - and in some ways simpler - with no dependence on luminosity, and a covering factor distribution that may be determined by the simplest of considerations - e.g. random misalignments.

astro-ph.CO↗