Constraining the past X-ray luminosity of AGN in clusters of galaxies: the role of resonant scattering

Only a small fraction of galactic nuclei in the nearby universe are luminous, while most of them are currently dim. We investigate the feasibility of constraining the X-ray luminosity in the recent past (up to ~10^6 years ago) of the nucleus of a cluster dominant galaxy by measuring the contribution of scattered nuclear radiation to the surface brightness of the intracluster gas dominated by thermal emission. We show that resonance X-ray lines present advantage over the continuum near the lines, because the relative contribution of scattered radiation is typically larger in the line case by a significant factor of the order of 3-10. As an example, we estimate the level of constraints that could be derived from future fine spectroscopic observations on the past X-ray luminosity of the nearby M87 and Cyg A active galaxies. For comparison we show that already available XMM-Newton and Chandra data on the continuum emission from the X-ray haloes around these galaxies enable obtaining an order of magnitude weaker upper limits on their past luminosity. A similar method can be applied to distant powerful quasars (at redshifts z>1) if they have cluster-like gaseous coronae, as suggested by Rosat and Chandra observations of active galaxies at z> kT/(1+z) (where T is the gas temperature) should be dominated by redshifted scattered radiation from the quasar. Therefore, measurements with the next generation of X-ray telescopes could give information on the lifetime of quasars and parameters of the hot gas around them

Scattering in the vicinity of relativistic jets: a method for constraining jet parameters

Relativistic jets of radio loud active galactic nuclei (AGN) produce highly directed, intense beams of radiation. A fraction of this beamed radiation scatters on the thermal plasma generally surrounding an AGN. The morphology of the scattered emission can thus provide constraints on the physical properties of the jet. We present a model to study the feasibility of constraining the parameters of a jet, especially its inclination angle and bulk Lorentz factor in this way. We apply our model to the well studied jet of M87 and the surrounding diffuse gas and find that the observational limits of the surface brightness measured in the region of the putative counterjet provide the tightest constraints on the jet parameters consistent with constraints derived by other methods. We briefly discuss the applicability of our model to other sources exhibiting relativistic motionsComment: 17 pages, 15 figures, to appear in A&A, 420, 33 (2004

Hydrogen-like nitrogen radio line from hot interstellar and warm-hot intergalactic gas

Hyperfine structure lines of highly-charged ions may open a new window in observations of hot rarefied astrophysical plasmas. In this paper we discuss spectral lines of isotopes and ions abundant at temperatures 10^5-10^7 K, characteristic for warm-hot intergalactic medium, hot interstellar medium, starburst galaxies, their superwinds and young supernova remnants. Observations of these lines will allow to study bulk and turbulent motions of the observed target and will broaden the information about the gas ionization state, chemical and isotopic composition. The most prospective is the line of the major nitrogen isotope having wavelength 5.65 mm (Sunyaev and Churazov 1084). Wavelength of this line is well-suited for observation of objects at z=0.15-0.6 when it is redshifted to 6.5-9 mm spectral band widely-used in ground-based radio observations, and, for example, for z>=1.3, when the line can be observed in 1.3 cm band and at lower frequencies. Modern and future radio telescopes and interferometers are able to observe the absorption by 14-N VII in the warm-hot intergalactic medium at redshifts above z=0.15 in spectra of brightest mm-band sources. Sub-millimeter emission lines of several most abundant isotopes having hyperfine splitting might also be detected in spectra of young supernova remnants.Comment: 12 pages, 5 figures, accepted by Astronomy Letters; v3: details added; error fixe

Interstellar gas in the Galaxy and the X-ray luminosity of Sgr A* in the recent past

Information about the X-ray luminosity of the supermassive black hole located at the Galactic center (GC), Sgr A*, and its temporal variations in the past is imprinted in the scattered emission observed today in the direction towards giant molecular clouds (GMCs) located in our Galaxy. Due to light travel time effects these clouds probe the activity of Sgr A* at different times in the past depending on their position relative to the GC and the observer. In this paper we combine results of recent ASCA observations along the Galactic plane, providing upper limits for the scattered flux in the 4-10 keV range produced in a given direction, with data from CO surveys of the same regions. These CO surveys map the position and mass of the molecular gas which the GMCs are made up of. Demanding the scattered flux to be not larger than the observed one, this data enables us to derive upper limits for the 4-10 keV luminosity of Sgr A* at certain times during the last 40.000 years down to about 8*10^40 erg/s. At other times the limits are less tight, of the order of 10^41-10^42 erg/s. The more extended and continuous HI distribution in the Galactic disk, which also scatters the radiation emitted by Sgr A*, allows us to extend the time coverage further into the past, back to about 110.000 years, albeit the limits are becoming less tight. We thereby can rule out a long term X-ray activity phase of Sgr A* at one per cent of its Eddington level ending less than about 80.000 years ago. The limits presented in this paper can be improved by observations of emission in the fluorescent iron K_alpha-line. We study the feasibility of these methods to investigate past nuclear activity in other spiral galaxies observed with the angular resolution of X-ray telescopes like Chandra and XMM-Newton

Traces of past activity in the Galactic Centre

The Milky Way centre hosts a supermassive Black Hole (BH) with a mass of ~4*10^6 M_Sun. Sgr A*, its electromagnetic counterpart, currently appears as an extremely weak source with a luminosity L~10^-9 L_Edd. The lowest known Eddington ratio BH. However, it was not always so; traces of "glorious" active periods can be found in the surrounding medium. We review here our current view of the X-ray emission from the Galactic Center (GC) and its environment, and the expected signatures (e.g. X-ray reflection) of a past flare. We discuss the history of Sgr A*'s past activity and its impact on the surrounding medium. The structure of the Central Molecular Zone (CMZ) has not changed significantly since the last active phase of Sgr A*. This relic torus provides us with the opportunity to image the structure of an AGN torus in exquisite detail.Comment: Invited refereed review. Chapter of the book: "Cosmic ray induced phenomenology in star forming environments" (eds. Olaf Reimer and Diego F. Torres

Bookselling online: an examination of consumer behaviour patterns.

Based upon empirical research, and using a range of methods, this paper examines the behaviour and experiences of consumers in online bookselling settings and offers comparison between online and offline (traditional) bookselling. The research finds that while the convenience of online bookshops is important, the key factors enticing consumers online are a combination of breadth of range, ease of access to obscure titles, as well as personalised recommendations and customer reviews. The research is of value to the book trade, highlighting consumer responses to widely adopted online marketing approaches. The research also contributes to scholarly knowledge in the fields of consumer behaviour, e-marketing and e-commerce in online bookselling, as well as providing findings which can be tested in other online settings, informing future theoretical research

The effectiveness of celebrities in conservation marketing

Celebrities are frequently used in conservation marketing as a tool to raise awareness, generate funding and effect behaviour change. The importance of evaluating effectiveness is widely recognised in both marketing and conservation but, to date, little research into the effectiveness of celebrity endorsement as a tool for conservation marketing has been published. Using a combination of interviews and an online choice survey instrument, we investigated the extent to which a sample of UK-based conservation organisations, and other charities, evaluate their own usage of celebrity endorsement, and then carried out an experimental evaluation of a hypothetical marketing campaign. This experiment compared participants' willingness-to-engage (WTE) with, and recall of, a conservation message presented in versions of an advert featuring one of three prominent UK celebrities (David Beckham, Chris Packham or HRH Prince William) or a non-celebrity control treatment (featuring Crawford Allan, a director of TRAFFIC USA). We find that the organisations we interviewed did not routinely evaluate their marketing campaigns featuring celebrities. Furthermore, our experiment provides evidence that celebrity endorsement can produce both positive and negative effects. Participants were more willing to engage when presented with an advert featuring one of the three celebrities than the non-celebrity control, and WTE varied according to the characteristics of the celebrity and the respondent. However, celebrities were less effective at generating campaign message recall than non-celebrities. These findings suggest that celebrity endorsement should be used carefully. Further work is required to fully understand the role celebrity endorsers can play in conservation but, drawing on best practice from the field of marketing, this study introduces an approach to evaluation which could be applied more widely to improve the effectiveness of conservation marketing

Interstellar gas in the Galaxy and the X-ray luminosity of Sgr A

Information about the X-ray luminosity of the supermassive black hole located at the Galactic center (GC), Sgr ${\rm A}^{*}$, and its temporal variations in the past is imprinted in the scattered emission observed today in the direction towards giant molecular clouds (GMCs) located in our Galaxy. Due to light travel time effects these clouds probe the activity of Sgr ${\rm A}^{*}$ at different times in the past depending on their position relative to the GC and the observer. In this paper we combine results of recent ASCA observations along the Galactic plane, providing upper limits for the scattered flux in the 4–10 keV range produced in a given direction, with data from CO surveys of the same regions. These CO surveys map the position and mass of the molecular gas which the GMCs are made up of. Demanding the scattered flux to be not larger than the observed one, this data enables us to derive upper limits for the 4–10 keV luminosity of Sgr A* at certain times during the last $40\,000$ years down to about $8\times10^{40}\,{\rm erg\,s}^{-1}$. At other times the limits are less tight, of the order of $10^{41}{-}10^{42}\,{\rm erg\,s}^{-1}$. For two periods of time of about 2000 and 4000 years duration 8000 and $14\,000$ years ago the currently available CO data is insensitive to any enhanced activity of the GC. Flares lasting longer than 3000 years fill these time gaps and therefore can be excluded to have occurred during the last $40\,000$ years with a luminosity larger than a few $10^{42}\,{\rm erg\,s}^{-1}$. The more extended and continuous HI distribution in the Galactic disk, which also scatters the radiation emitted by Sgr A*, allows us to extend the time coverage further into the past, back to about $110\,000$ years, albeit the limits are becoming less tight. We thereby can rule out a long term X-ray activity phase of Sgr A* at one per cent of its Eddington level ending less than about $80\,000$ years ago. The limits presented in this paper can be improved by observations of emission in the fluorescent iron Kα-line. We study the feasibility of these methods to investigate past nuclear activity in other spiral galaxies observed with the angular resolution of X-ray telescopes like Chandra and XMM-Newton