arXiv subjects

Lorella Angelini

Publications and source records attributed to Lorella Angelini.

41 records · Page 3Linked to original sources

arXiv2003

Unveiling the multi-wavelength phenomenology of Anomalous X-ray Pulsars

During 2002-2003 the number of IR-identified counterparts to the Anomalous X-ray Pulsars (AXPs) has grown to four (4U0142+614, 2E2259+584, 1E 1048-59 and 1RXS J170849-400910) out of the six assessed objects of this class, plus two candidates. More importantly, some new common observational characteristics have been identified, such as the IR variability, the IR flattening in the broad band energy spectrum, the X-ray spectral variability as a function of pulse phase (which are not predicted by the magnetar model), and the SGR-like bursts (which can not be explained in terms of standard accretion models). We present the results obtained from an extensive multi-wavelength observational campaign carried out collecting data from the NTT, CFHT for the optical/IR bands, and XMM, Chandra (plus BeppoSAX archival data) in the X-rays. Based on these results and those reported in the literature, the IR-to-X-ray band emission of AXPs has been compared and studied.

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arXiv2003

Evidence of a cyclotron feature in the spectrum of the anomalous X-ray pulsar 1RXS J170849-400910

We report the results of a long observation of the anomalous X-ray pulsar 1RXS J170849-400910 obtained with the BeppoSAX satellite in August 2001. The best-fit phase-averaged spectrum was an absorbed power law plus blackbody model, with photon index Gamma~2.4 and a blackbody temperature of kT_bb~0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse we found an absorption-like feature at ~8.1 keV (a significance of 4 sigma), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9x10^11 G and 1.6x10^15 G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.

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arXiv2001

The Discovery of a Second Luminous Low Mass X-ray Binary in the Globular Cluster M15

We report an observation by the Chandra X-ray Observatory of 4U2127+119, the X-ray source identified with the globular cluster M15. The Chandra observation reveals that 4U2127+119 is in fact two bright sources, separated by 2.7". One source is associated with AC211, the previously identified optical counterpart to 4U2127+119, a low mass X-ray binary (LMXB). The second source, M15-X2, is coincident with a 19th U magnitude blue star that is 3.3" from the cluster core. The Chandra count rate of M15-X2 is 2.5 times higher than that of AC211. Prior to the 0.5" imaging capability of Chandra the presence of two so closely separated bright sources would not have been resolved. The optical counterpart, X-ray luminosity and spectrum of M15-X2 are consistent with it also being an LMXB system. This is the first time that two LMXBs have been seen to be simultaneously active in a globular cluster. The discovery of a second active LMXB in M15 solves a long standing puzzle where the properties of AC211 appear consistent with it being dominated by an extended accretion disk corona, and yet 4U2127+119 also shows luminous X-ray bursts requiring that the neutron star be directly visible. The resolution of 4U2127+119 into two sources suggests that the X-ray bursts did not come from AC211, but rather from M15-X2. We discuss the implications of this discovery for understanding the origin and evolution of LMXBs in GCs as well as X-ray observations of globular clusters in nearby galaxies.

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arXiv2001

The X-ray Globular Cluster Population in NGC 1399

We report on the {\it Chandra} observations of the elliptical galaxy NGC 1399, concentrating on the X-ray sources identified with globular clusters (GCs). A large fraction of the 2-10 keV X-ray emission in the $8' \times 8'$ {\it Chandra} image is resolved into point sources with luminosities $\ge 5 \times 10^{37}$ \ergsec. These sources are most likely Low Mass X-ray Binaries (LMXBs). In a region imaged by {\it HST} about 70% of the X-ray sources are located within GCs. This association suggests that in giant elliptical galaxies luminous X-ray binaries preferentially form in GCs. Many of the GC sources have super-Eddington luminosities (for an accreting neutron star) and their average luminosity is higher than the non-GC sources. The X-ray spectral properties of both GC and non-GC sources are similar to those of LMXBs in our Galaxy. Two of the brightest sources, one of which is in a GC, have an ultra-soft spectrum, similar to that seen in the high state of black hole candidates. The ``apparent'' super-Eddington luminosity in many cases may be due to multiple LMXB systems within individual GCs, but with some of the most extremely luminous systems containing massive black holes.

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arXiv1998

ASCA and Ginga Observations of GX 1+4

The X-ray binary pulsar GX 1+4 was observed with Ginga every year from 1987 to 1991, and with ASCA in 1994. During the Ginga observations, GX 1+4 was in the steady spindown phase, while the X-ray flux was not steady. The X-ray luminosity decreased down to L = 2.7\times10^{36} erg s^{-1} in 1991, after the peak activity of L = 1.2\times10^{37} erg s^{-1} in 1989. On the other hand, the absorption column density showed a drastic increase over the Ginga observation series. It reached a maximum of 1.4\times10^{24} cm^{-2} in 1991, indicating a rapid accumulation of matter in the vicinity of the source. The ASCA observation was performed on 1994/09/15, a month before the transition into a spinup phase. The source brightened again to L = 10^{37} erg s^{-1}. The absorption column density was observed to decrease for the first time to 2\times10^{23} cm^{-2}. The ionization degree of iron in the absorbing matter was determined to be Fe I--Fe IV using the ratio of the line-center energy to the absorption-edge energy. The low ionization degree is consistent with an absorbing matter distribution extending 10^{12} cm from the source. We compared the results with optical observations, and found the optical data also supports the picture. Based on the geometrical model, possible causes of the bimodal behavior of the source are discussed.

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