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Study of the stellar-mass to super-massive Black Hole connection

Final Report Summary - GBH-AGN CONNECTION (Study of the stellar-mass to super-massive Black Hole connection)

The aim of the project was to study the spectral variability in the two most important classes of accreting black holes (BHs): stellar mass galactic black holes (GBH) and the super-massive black holes in active galactic nuclei (AGN). The primary objective was to find which properties are similar/connected and which properties differ between these two classes of accreting sources. The main goal was to derive the scaling relations (if any exist) between the two classes of accreting BH, and therefore to gain a deeper insight into the accretion process in general, discerning those properties of accretion onto BHs that are fundamental from those processes that are peculiar to each class of object.

During the proposal phase it appeared that the most promising experimental technique to achieve this goal would be the study of the spectral variability of the reflection component. However, between the proposal submission and the start of the fellowship several important new results were published regarding absorption in GBH and AGN and its connection to jet activity. Therefore, at the start of the fellowship (about one year after the submission of the proposal) the project advisor and researcher decided to refocus the research project, placing a stronger effort on the study of absorption and its variations.

Toward the end of the project, and after fulfilling the vast majority of the planned objectives, we realised that it would be interesting to expand our study to accreting neutron star systems. Thus the last few months of the fellowship were spent also investigating this class of object.

We have achieved an exceptionally high publication rate of around one paper per month during the fellowship. The vast majority of the work and studies performed has been or are in process of being published. A more detailed description of the progress and results achieved can be obtained directly from those papers, however, we summarise here the main results.

The research carried out during the two years of the fellowship was intended to study both spectral variability in AGN and GBHs, and it can most naturally be divided into five sub-projects.

WINDS in GBHs:

The most important (and the one requiring the highest level of learning) was the study of the absorber-winds and their variability in GBHs (see midterm report). Whilst carrying out this project we discovered the presence of an ubiquitous equatorial accretion disc wind in the soft state of GBHs (and possibly the same applies to AGN). Significantly, these absorption features are not observed during the hard spectral state. In Ponti et al. 2012 we show that the anti-correlation between the presence of the jet and of the wind is not only present in one peculiar source (GRS1915+105), as already known, but it is a property common to all observed GBHs. This result has been recognised by the community as an important step forward in our understanding of accretion onto (and ejection from) BHs. Moreover, this work opened new important questions. At the moment we are investigating whether the wind disappearance in the hard state can be due to the effects of the change of the GBH spectral energy distribution (which is associated with the object's accretion state). The harder spectral shape might over-ionise the wind, thus hiding it in the accessible X-ray waveband. Another important result could be obtained by checking if the same phenomenon also applies to accreting neutron stars. The final months of the fellowship have been devoted to this investigation.

The preliminary results from our most recent research show that, indeed, this is the case. Whilst carrying out the work on the GBH objects, we observed an inclination dependance of the ratio between the source emission in different energy bands. The results of the investigation (led by the future Marie Curie IEF fellow, Munoz-Darias) indicate that the variation is due to the effect of observer viewing angle (including general relativistic effects) on the accretion disc emission. The expertise gained by the researcher has been useful to help-train a PhD student to perform a survey of the FeK emission line variations in GBHs (several papers will be published from this work).

AGN variability and the AGN-GBH connection:

We have measured the variability (excess variance) of all radio quiet X-ray un-obscured AGN that have been observed by the XMM-Newton space observatory (greater than 150 objects). We have shown that the variability is consistent with being the same in each AGN (once rescaled by BH mass and, to a smaller extent by accretion rate), and this supports that idea that in many ways AGN behave as scaled up versions of GBHs (Ponti et al. 2012). We measured, in agreement with previous results, a strong correlation between variability amplitude and BH mass.

Our study (Ponti et al. 2012) confirms previous results based on Power Spectral Density (PSD) analysis that show that the PSD break frequency scales with BH mass and accretion rate (McHardy et al. 2006; Koerding et al. 2007). We also discovered that not only the PSD break frequency, but also the PSD normalisation depends on accretion rate. We are now collaborating with Prof. McHardy to present this result in a new paper. Interestingly, when we consider only the AGN with reverberation mapping measurements of the BH mass (the most accurate measurements of BH mass that are available), we find that (Ponti et al. 2012) the correlation becomes much tighter (only a factor of 3, i.e.approximately 0.3 dex). This has strong implications for future BH mass measurements. In fact BH masses are generally derived from single epoch of optical spectroscopy, a method that has much larger uncertainties (greater than 0.7 dex). Accurate BH mass measurement can thus be performed reliably using only X-ray variability techniques. We are now investigating the full implications of this. Can AGN be used as standard candles (estimating BH mass from variability and using the single epoch spectra relations to infer the intrinsic luminosity)? Together with the eROSITA team we are applying this technique to estimate BH masses from X-ray surveys (this is an important area of expertise that helped the researcher get his present job). We have also been performing another project aiming at studying the AGN X-ray variability of the Swift-BAT sample, thus measuring the variability properties above 10 keV.

Time lags:

We took full advantage of the research project being hosted at Southampton University by using part of the funding, and part of the time, to train an Italian post doc (Dr. Barbara DeMarco) in the study of AGN timing and spectral variability. Working in close collaboration with her we discovered the first high frequency soft lag detected in a high mass AGN (DeMarco, Ponti et al. 2012). This lag corresponds to the light crossing time of a few gravitational radii and allows us to place very strong constraints on the geometry of the accretion system. The lag is thought to be produced by the additional light-travel-time that the reflected-thermalised emission component takes to travel from the primary source to the accretion disc (where the reflection and-or thermalisation occurs). If this is indeed the case, then a dependence with BH mass is also expected (because rg depends on BH mass). After this first successful result we searched for similar lags in other sources and we found 7 new lags (DeMarco, Ponti et al. 2012). We also found a 4 sigma correlation between the lag amplitude and BH mass (DeMarco, Ponti et al. 2012). Interestingly we observe that every single AGN we studied (about 30) is consistent with having a lag time equivalent to the light crossing time of a few (1-8) gravitational radii, suggesting a very similar geometry of the corona-disc system in all these AGN. We expect that a similar geometry should be present in any BH accreting source, including GBHs and ULXs. We have recently found a similar lag in a ULX and a GBH, and derived a scaling relation between the two. Two papers on this topic are in preparation.

Warm absorber variability in AGN:

To compare the wind properties of GBHs with those in AGN we have performed several spectral variability studies of AGN. The papers: Giustini et al. 2011; Matt et al. 2011 and Cerruti, Ponti et al. 2011 show some examples of this.

MRK509:

The researcher was in the team who carried out a long multiwavelength campaign to study the warm absorber variability in one of the brightest known AGN, MKN509. This observing campaign represents a significant milestone in the study of warm absorbers in AGN, measuring, for the first time, the distance (ranging from a few parsecs to several hundred parsecs) of the absorber from the nucleus. Therefore, it seems the most probable physical origin for the absorber component is through photo-evaporation of the molecular torus surrounding the AGN. Our MKN509 campaign produced many other results that have been already published: 12 refereed papers and 5 more to be submitted soon. The researcher led the study of the Fe K emission. The 600 ks XMM-Newton monitoring in conjunction with the 300 ks of previous data allowed the researcher to apply, for the first time, the reverberation mapping technique to the Fe Kalpha emission line (Ponti et al. 2012, sub.). This study allowed us to locate the origin of the Fe Kalpha emission. We found that the bulk of the resolved line emission is most probably produced in the inner broad line region (Ponti et al. 2012).

Galactic center (GC):

After the publication of our work on the variable (sometimes superluminal) FeK emission from molecular clouds in the GC (Ponti et al. 2010; Terrier, Ponti et al. 2010), the researcher has been invited to give review presentations in international conferences and to give seminars in several institutes. Even though the GC work was not a major goal of this Marie Curie project, the researcher (in agreement with the advisor) maintained their expertise in this subject area. Together with the group in Paris two papers have been published in this subject area; we have published one paper on Sgr A (Trap et al. 2011) and one paper on the Vela pulsar (Mattana et al. 2011). A further two papers are in progress. Moreover, we have been awarded two long Chandra (approximately 200 ks) observations, one joint VLT and XMM-Newton observation (100 ks), and a joint XMM-Newton (800 ks) + Hershel large project to continue to monitor the variations observed by Ponti et al. (2010) and Terrier, Ponti et al. (2010). We have also been awarded pre-approval for long (300 ks) Chandra ToO observation to study an outburst from Sgr A, should it occur. If such an event happens we will be very well prepared to extract the maximum scientific impact from it. Recently the researcher has been invited to write a review of the X-ray emission in the Galactic Centre.

As a result of the work performed during this fellowship, 24 papers have already been published in high profile refereed international journals, which correspond to an astonishingly high average publication rate of 1 paper per month of the fellowship. Further papers are in the process of being completed and will be published soon. Many additional non-refereed papers have been published in e.g. conference proceedings etc. Even though most of our results from this fellowship have been published in the last few months, our papers have already been cited more than one hundred times, demonstrating the high impact of the work and its value to the scientific community.

The researcher has been invited to or accepted to participate to several international conferences, where he has presented the project at its different stages. The importance of our project is now largely recognised by the international community. Our high profile work is yielding an increasing number of new collaborations and connected scientific projects. Several press releases (ESA, national and institute ones) have been published during the fellowship.

The project has proceeded as planned. The choice of the University of Southampton as the host institute has resulted in a very prolific scientific environment, boosting the success of the project. No problems have occurred. The money available for travels and consumables has been spent in a judicious and efficient fashion, in a similar manner to the first year of the project.