PhD student: Elise Egron

• outreach page for the project, by Elise Egron

Start Date: Jul 2009
End Date: Jun 2012
Responsible Advisor: Luciano Burderi (Cagliari)
Co-Advisors: Jörn Wilms (FAU Erlangen-Nuremberg), Rudy Wijnands/Michiel van der Klis (Amsterdam), Tiziana di Salvo (Palermo)

Low Mass X-ray Binaries (LMXBs) containing an old, usually low magnetized, neutron star emit interesting and still debated spectra, often showing compelling similarities with the broad band spectra of binary systems containing black hole candidates. In particular two features, common to black hole and neutron star systems, need to be deeply studied, which are the so-called hard tail (a power-law shaped hard X-ray emission usually present in soft states) and the reflection features (a spectral component arising from the Compton reflection of the main Comptonization spectrum by a cold medium, like the optically thick geometrically thin accretion disk). The study of these features will give important insight on the analogies between accretion onto neutron stars and black holes (corresponding to common characteristics of the accretion flow onto low magnetized compact objects).  Moreover, this study may highlight differences (if any) between the spectral properties of these systems, which may give important information on characteristics that are unique for black holes and neutron stars (giving in turn the possibility, if any, to distinguish based on spectral characteristics between black holes and neutron stars).

An important spectral analogy between black hole and neutron star X-ray binaries should be found in the characteristics of the iron emission line. Black hole candidates show very strong and relativistically smeared lines at 6 - 7 keV, especially in their soft states (see, e.g., Fabian 2006). In the case of LMXBs containing neutron stars we expect a similar behavior of the iron discrete features, since (because of the weak magnetic field) the disk can arrive very close to the compact object. Indeed, iron emission lines at 6.4-6.7 keV are usually observed in these systems to be quite broad (up to 1 keV FWHM, see Figure and Di Salvo et al., 2005).  As in the case of black hole X-ray binaries, it has been proposed that the origin of the large width of these lines are Doppler and relativistic effects in the accretion disk. In this case, together with the line, an absorption edge and a reflection (Compton) bump should be produced as a consequence of the reflection of the main Comptonization continuum by the accretion disk.

The PhD student will perform a statistical study of reflection features in neutron star LMXBs, in order to demonstrate that these systems, in analogy with black holes, show relativistically smeared lines, although in these cases the relativistic distortions should never be as extreme as in the so-called microquasars. This would give a further indirect but important evidence for the presence of an event horizon in black holes.  To this aim, it is important to use adequate self-consistent models to fit the broad band reflection features, in order to check the hypothesis that the iron discrete features are produced by reflection of the main Comptonized component on an accretion disk and to investigate the presence of Doppler and relativistic distortions. This aim will be pursued using archival and dedicated observations from Chandra, XMM-Newton, BeppoSAX, RXTE, and possibly Swift and
Suzaku, which all have broad band (from soft to hard X-ray) capabilities and good energy resolution at soft X-rays.

An overview talk on the current state  of the project as presented at the Collaboration Meeting in September 2010 in Istanbul can be found here.

Related publications

6. Papitto, A., D'Aì, A., Di Salvo, T., Egron, E., Bozzo, E., Burderi, L., Iaria, R., Riggio, A., and Menna, M. T., 2013, The accretion flow to the intermittent accreting millisecond pulsar, HETE J1900.1-2455, as observed by XMM-Newton and RXTE, MNRAS, 429, 3411 (ADS-Entry)
5. Egron, E., Di Salvo, T., Motta, S., Burderi, L., Papitto, A., Duro, R., D'Aì, A., Riggio, A., Belloni, T. M., Iaria, R., Robba, N. R., Piraino, S., and Santangelo, A., 2013, Testing reflection features in 4U 1705-44 with XMM-Newton, BeppoSAX, and RXTE in the hard and soft states, A&A, 550, A5 (ADS-Entry)
4. Riggio, A., Burderi, L., Di Salvo, T., Papitto, A., Egron, E., Belloni, T. M., D'Aì, A., Iaria, R., Floris, M., Motta, S., Testa, V., Menna, M. T., and Robba, N. R., 2012, Subarcsecond Location of IGR J17480–2446 with Rossi XTE, ApJ, 754, L11 (ADS-Entry)
3. D'Aì, A., Bozzo, E., Papitto, A., Iaria, R., Di Salvo, T., Burderi, L., Riggio, A., Egron, E., and Robba, N. R., 2012, A complete X-ray spectral coverage of the 2010 May-June outbursts of Circinus X-1, A&A, 543, A20 (ADS-Entry)
2. Piraino, S., Santangelo, A., Kaaret, P., Mück, B., D'Aì, A., Di Salvo, T., Iaria, R., Robba, N., Burderi, L., and Egron, E., 2012, A relativistic iron emission line from the neutron star low-mass X-ray binary GX 3+1, A&A, 542, L27 (ADS-Entry)
1. Egron, E., di Salvo, T., Burderi, L., Papitto, A., Barragán, L., Dauser, T., Wilms, J., D'Aì, A., Riggio, A., Iaria, R., and Robba, N. R., 2011, X-ray spectroscopy of MXB 1728-34 with XMM-Newton, A&A, 530, A99 (ADS-Entry)

3. Papitto, A., Di Salvo, T., Ai, A. D., Burderi, L., Iaria, R., Egron, E., and Riggio, A., 2012, Broad iron lines from accreting millisecond pulsars, in: , 39th COSPAR Scientific Assembly, 39, 1443 (ADS-Entry)
2. Egron, E., di Salvo, T., Burderi, L., Papitto, A., Barragán, L., Dauser, T., Wilms, J., D'Aì, A., Riggio, A., Iaria, R., and Robba, N. R., 2011, Detection of a relativistic iron line in MXB 1728-34 with XMM-Newton, in: Alecian, G., Belkacem, K., Samadi, R., and Valls-Gabaud, D. (ed.), , Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics, 567-572 (ADS-Entry)
1. Egron, E., Di Salvo, T., Motta, S., and Burderi, L., 2011, Self-consistent study of the reflection component in 4U1705-44 with XMM-Newton, BeppoSAX and RXTE in the hard and soft state, in: Extreme and Variable High Energy Sky, held on September 19-23, 2011, Cagliari, Italy (ADS-Entry)

1. Riggio, A., Burderi, L., Di Salvo, T., Papitto, A., Egron, E., Belloni, T. M., D'Ai', A., Iaria, R., Floris, M., Motta, S., Testa, V., and Menna, M. T., 2012, Sub-arcsecond position of the the transient 11-Hz X-ray pulsar in the Globular Cluster Terzan 5 derived from a Moon occultation observed by RXTE, ATel 3892 (ATel-Link)