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  • 1
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    The XMM-Newton view of the central degrees of the Milky Way (2015)
    Oxford University Press
    Details
    Publication Date: 2015-08-21
    Description: The deepest XMM–Newton mosaic map of the central 1 $_{.}^{\circ}$ 5 of the Galaxy is presented, including a total of about 1.5 Ms of EPIC-pn cleaned exposures in the central 15 arcsec and about 200 ks outside. This compendium presents broad-band X-ray continuum maps, soft X-ray intensity maps, a decomposition into spectral components and a comparison of the X-ray maps with emission at other wavelengths. Newly discovered extended features, such as supernova remnants (SNRs), superbubbles and X-ray filaments are reported. We provide an atlas of extended features within ±1° of Sgr A * . We discover the presence of a coherent X-ray-emitting region peaking around G0.1–0.1 and surrounded by the ring of cold, mid-IR-emitting material known from previous work as the ‘Radio Arc Bubble’ and with the addition of the X-ray data now appears to be a candidate superbubble. Sgr A's bipolar lobes show sharp edges, suggesting that they could be the remnant, collimated by the circumnuclear disc, of an SN explosion that created the recently discovered magnetar, SGR J1745–2900. Soft X-ray features, most probably from SNRs, are observed to fill holes in the dust distribution, and to indicate a direct interaction between SN explosions and Galactic centre (GC) molecular clouds. We also discover warm plasma at high Galactic latitude, showing a sharp edge to its distribution that correlates with the location of known radio/mid-IR features such as the ‘GC Lobe’. These features might be associated with an inhomogeneous hot ‘atmosphere’ over the GC, perhaps fed by continuous or episodic outflows of mass and energy from the GC region.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    Erratum: Finding rare AGN: XMM-Newton and Chandra observations of SDSS Stripe 82 (2016)
    Oxford University Press
    Details
    Publication Date: 2016-04-07
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    The evolution of the disc variability along the hard state of the black hole transient GX 339-4 (2015)
    Oxford University Press
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    Publication Date: 2015-10-15
    Description: We report on the analysis of hard-state power spectral density function (PSD) of GX 339-4 down to the soft X-ray band, where the disc significantly contributes to the total emission. At any luminosity probed, the disc in the hard state is intrinsically more variable than in the soft state. However, the fast decrease of disc variability as a function of luminosity, combined with the increase of disc intensity, causes a net drop of fractional variability at high luminosities and low energies, which reminds the well-known behaviour of disc-dominated energy bands in the soft state. The peak frequency of the high-frequency Lorentzian (likely corresponding to the high-frequency break seen in active galactic nuclei, AGN) scales with luminosity, but we do not find evidence for a linear scaling. In addition, we observe that this characteristic frequency is energy dependent. We find that the normalization of the PSD at the peak of the high-frequency Lorentzian decreases with luminosity at all energies, though in the soft band this trend is steeper. Together with the frequency shift, this yields quasi-constant high-frequency (5–20 Hz) fractional rms at high energies, with less than 10 per cent scatter. This reinforces previous claims suggesting that the high-frequency PSD solely scales with black hole mass. On the other hand, this constancy breaks down in the soft band (where the scatter increases to ~30 per cent). This is a consequence of the additional contribution from the disc component, and resembles the behaviour of optical variability in AGN.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    The evolution of the X-ray luminosity functions of unabsorbed and absorbed AGNs out to z~ 5 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-11
    Description: We present new measurements of the evolution of the X-ray luminosity functions (XLFs) of unabsorbed and absorbed active galactic nuclei (AGNs) out to z  ~ 5. We construct samples containing 2957 sources detected at hard (2–7 keV) X-ray energies and 4351 sources detected at soft (0.5–2 keV) energies from a compilation of Chandra surveys supplemented by wide-area surveys from ASCA and ROSAT . We consider the hard and soft X-ray samples separately and find that the XLF based on either (initially neglecting absorption effects) is best described by a new flexible model parametrization where the break luminosity, normalization, and faint-end slope all evolve with redshift. We then incorporate absorption effects, separately modelling the evolution of the XLFs of unabsorbed (20 〈 log N H  〈 22) and absorbed (22 〈 log N H  〈 24) AGNs, seeking a model that can reconcile both the hard- and soft-band samples. We find that the absorbed AGN XLF has a lower break luminosity, a higher normalization, and a steeper faint-end slope than the unabsorbed AGN XLF out to z  ~ 2. Hence, absorbed AGNs dominate at low luminosities, with the absorbed fraction falling rapidly as luminosity increases. Both XLFs undergo strong luminosity evolution which shifts the transition in the absorbed fraction to higher luminosities at higher redshifts. The evolution in the shape of the total XLF is primarily driven by the changing mix of unabsorbed and absorbed populations.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Fifteen years of XMM-Newton and Chandra monitoring of Sgr A*: evidence for a recent increase in the bright flaring rate (2015)
    Oxford University Press
    Details
    Publication Date: 2015-10-08
    Description: We present a study of the X-ray flaring activity of Sgr A * during all the 150 XMM–Newton and Chandra observations pointed at the Milky Way centre over the last 15 years. This includes the latest XMM–Newton and Chandra campaigns devoted to monitoring the closest approach of the very red Br emitting object called G2. The entire data set analysed extends from 1999 September through 2014 November. We employed a Bayesian block analysis to investigate any possible variations in the characteristics (frequency, energetics, peak intensity, duration) of the flaring events that Sgr A * has exhibited since their discovery in 2001. We observe that the total bright or very bright flare luminosity of Sgr A * increased between 2013 and 2014 by a factor of 2–3 (~3.5 significance). We also observe an increase (~99.9 per cent significance) from 0.27 ± 0.04 to 2.5 ± 1.0 d –1 of the bright or very bright flaring rate of Sgr A * , starting in late summer 2014, which happens to be about six months after G2's pericentre passage. This might indicate that clustering is a general property of bright flares and that it is associated with a stationary noise process producing flares not uniformly distributed in time (similar to what is observed in other quiescent black holes). If so, the variation in flaring properties would be revealed only now because of the increased monitoring frequency. Alternatively, this may be the first sign of an excess accretion activity induced by the close passage of G2. More observations are necessary to distinguish between these two hypotheses.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    A spectroscopic survey of X-ray-selected AGNs in the northern XMM-XXL field (2016)
    Oxford University Press
    Details
    Publication Date: 2016-01-21
    Description: This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a ~ 18 deg 2 area of the equatorial XMM-XXL north field. A sample of 8445 point-like X-ray sources detected by XMM–Newton above a limiting flux of $F_{\rm 0.5{\rm -}10\, keV} 〉 10^{-15} \rm \,erg\, cm^{-2}\, s^{-1}$ was matched to optical (Sloan Digital Sky Survey, SDSS) and infrared (IR; WISE ) counterparts. We followed up 3042 sources brighter than r = 22.5 mag with the SDSS Baryon Oscillation Spectroscopic Survey (BOSS) spectrograph. The spectra yielded a reliable redshift measurement for 2578 AGNs in the redshift range z = 0.02–5.0, with 0.5-2 keV luminosities ranging from 10 39 -10 46 erg s – 1 . This is currently the largest published spectroscopic sample of X-ray-selected AGNs in a contiguous area. The BOSS spectra of AGN candidates show a distribution of optical line widths which is clearly bimodal, allowing an efficient separation between broad- and narrow-emission line AGNs. The former dominate our sample (70 per cent) due to the relatively bright X-ray flux limit and the optical BOSS magnitude limit. We classify the narrow-emission line objects (22 per cent of the full sample) using standard optical emission line diagnostics: the majority have line ratios indicating the dominant source of ionization is the AGN. A small number (8 per cent of the full sample) exhibit the typical narrow line ratios of star-forming galaxies, or only have absorption lines in their spectra. We term the latter two classes ‘elusive’ AGN, which would not be easy to identify correctly without their X-ray emission. We also compare X-ray ( XMM–Newton ), optical colour (SDSS) and and IR ( WISE ) AGN selections in this field. X-ray observations reveal, by far, the largest number of AGN. The overlap between the selections, which is a strong function of the imaging depth in a given band, is also remarkably small. We show using spectral stacking that a large fraction of the X-ray AGNs would not be selectable via optical or IR colours due to host galaxy contamination. A substantial fraction of AGN may therefore be missed by these longer wavelength selection methods.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    The clustering amplitude of X-ray-selected AGN at z ~ 0.8: evidence for a negative dependence on accretion luminosity (2016)
    Oxford University Press
    Details
    Publication Date: 2016-03-03
    Description: The northern tile of the wide-area and shallow XMM-XXL X-ray survey field is used to estimate the average dark matter halo mass of relatively luminous X-ray-selected active galactic nucleus (AGN) [ $\rm log\, {\it L}_X (\rm 2{\rm -}10\,keV)= 43.6^{+0.4}_{-0.4}\,erg\,s^{-1}$ ] in the redshift interval z  = 0.5–1.2. Spectroscopic follow-up observations of X-ray sources in the XMM-XXL field by the Sloan telescope are combined with the VIMOS Public Extragalactic Redshift Survey spectroscopic galaxy survey to determine the cross-correlation signal between X-ray-selected AGN (total of 318) and galaxies (about 20 000). We model the large scales (2–25 Mpc) of the correlation function to infer a mean dark matter halo mass of $\log M / (\mathrm{M}_{{\odot }} \, h^{-1}) = 12.50 ^{+0.22} _{-0.30}$ for the X-ray-selected AGN sample. This measurement is about 0.5 dex lower compared to estimates in the literature of the mean dark matter halo masses of moderate-luminosity X-ray AGN [ L X (2-10 keV) 10 42 -10 43 erg s – 1 ] at similar redshifts. Our analysis also links the mean clustering properties of moderate-luminosity AGN with those of powerful ultraviolet/optically selected QSOs, which are typically found in haloes with masses few times 10 12 M . There is therefore evidence for a negative luminosity dependence of the AGN clustering. This is consistent with suggestions that AGN have a broad dark matter halo mass distribution with a high mass tail that becomes subdominant at high accretion luminosities. We further show that our results are in qualitative agreement with semi-analytic models of galaxy and AGN evolution, which attribute the wide range of dark matter halo masses among the AGN population to different triggering mechanisms and/or black hole fuelling modes.
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    Topics: Physics
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  • 8
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    Non-linearity and environmental dependence of the star-forming galaxies main sequence (2015)
    Oxford University Press
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    Publication Date: 2015-11-27
    Description: Using data from four deep fields (COSMOS, AEGIS, ECDFS, and CDFN), we study the correlation between the position of galaxies in the star formation rate (SFR) versus stellar mass plane and local environment at z 〈 1.1. To accurately estimate the galaxy SFR, we use the deepest available Spitzer /MIPS 24 and Herschel /PACS data sets. We distinguish group environments ( M halo ~ 10 12.5–14.2 M ) based on the available deep X-ray data and lower halo mass environments based on the local galaxy density. We confirm that the main sequence (MS) of star-forming galaxies is not a linear relation and there is a flattening towards higher stellar masses ( M * 〉 10 10.4–10.6 M ), across all environments. At high redshift (0.5 〈 z 〈 1.1), the MS varies little with environment. At low redshift (0.15 〈 z 〈 0.5), group galaxies tend to deviate from the mean MS towards the region of quiescence with respect to isolated galaxies and less-dense environments. We find that the flattening of the MS towards low SFR is due to an increased fraction of bulge-dominated galaxies at high masses. Instead, the deviation of group galaxies from the MS at low redshift is caused by a large fraction of red disc-dominated galaxies which are not present in the lower density environments. Our results suggest that above a mass threshold (~10 10.4 –10 10.6 M ) stellar mass, morphology and environment act together in driving the evolution of the star formation activity towards lower level. The presence of a dominating bulge and the associated quenching processes are already in place beyond z ~1. The environmental effects appear, instead, at lower redshifts and have a long time-scale.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger (2018)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2018-08-03
    Description: Tidal disruption events (TDEs) are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole (SMBH). We have observed a transient source in the western nucleus of the merging galaxy pair Arp 299 that radiated 〉1.5 x 10 52 erg at infrared and radio wavelengths but was not luminous at optical or x-ray wavelengths. We interpret this as a TDE with much of its emission reradiated at infrared wavelengths by dust. Efficient reprocessing by dense gas and dust may explain the difference between theoretical predictions and observed luminosities of TDEs. The radio observations resolve an expanding and decelerating jet, probing the jet formation and evolution around a SMBH.
    Keywords: Astronomy
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 10
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    X-ray spectral properties of the AGN sample in the northern XMM-XXL field (2016)
    Oxford University Press
    Details
    Publication Date: 2016-04-27
    Description: In this paper we describe and publicly release a catalogue consisting of 8445 point-like X-ray sources detected in the XMM-XXL north survey. For the 2512 AGN which have reliable spectroscopy from SDSS-III/BOSS, we present the X-ray spectral fitting which has been computed with a Bayesian approach. We have also applied an X-ray spectral stacking method to different sub-samples, selected on the basis of the AGN physical properties ( L 2–10 keV , z, M BH , Edd and N H ). We confirm the well-known Iwasawa–Taniguchi effect in our luminosity–redshift sub-samples, and argue that such an effect is due to a decrease in the covering factor of a distant obscuring ‘torus’ with increasing X-ray luminosity. By comparing the distribution of the reflection fraction, the ratio of the normalization of the reflected component to the direct radiation, we find that the low-luminosity, low-redshift sub-sample had systematically higher reflection fraction values than the high-redshift, high-luminosity one. On the other hand, no significant difference is found between samples having similar luminosity but different redshift, suggesting that the structure of the torus does not evolve strongly with redshift. Contrary to previous works, we do not find evidence for an increasing photon index at high Eddington ratio. This may be an indication that the structure of the accretion disc changes as the Eddington ratio approaches unity. Comparing our X-ray spectral analysis results with the optical spectral classification, we find that ~20 per cent of optical type-1 AGN show an X-ray absorbing column density higher than 10 21.5 cm – 2 , and about 50 per cent of type-2 AGN have an X-ray absorbing column density less than 10 21.5 cm – 2 . We suggest that the excess X-ray absorption shown in the high-luminosity optical type-1 AGN can be due to small-scale dust-free gas within (or close to) the broad-line region, while in the low-luminosity ones it can be due to a clumpy torus with a large covering factor.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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