arXiv subjects

N. La Palombara

Publications and source records attributed to N. La Palombara.

72 records · Page 4Linked to original sources

arXiv2010

An ultra-massive fast-spinning white dwarf in a peculiar binary system

White dwarfs typically have masses in a narrow range centered at about 0.6 solar masses (Msun). Only a few ultra-massive white dwarfs (M>1.2 Msun) are known. Those in binary systems are of particular interest because a small amount of accreted mass could drive them above the Chandrasekhar limit, beyond which they become gravitationally unstable. Using data from the XMM-Newton satellite, we show that the X-ray pulsator RX J0648.0-4418 is a white dwarf with mass > 1.2 Msun, based only on dynamical measurements. This ultra-massive white dwarf in a post-common envelope binary with a hot subdwarf can reach the Chandrasekhar limit, and possibly explode as a Type Ia supernova, when its helium-rich companion will transfer mass at an increased rate through Roche lobe overflow.

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arXiv2009

The discovery of a massive white dwarf in the peculiar binary system HD 49798/RX J0648.0-4418

An XMM-Mewton observation performed in May 2008 has confirmed that the 13 seconds pulsations in the X-ray binary HD 49798/RX J0648.0-4418 are due to a rapidly rotating white dwarf. From the pulse time delays induced by the 1.55 days orbital motion, and the system's inclination, constrained by the duration of the X-ray eclipse discovered in this observation, we could derive a mass of 1.28+/-0.05 M_sun for the white dwarf. The future evolution of this post common envelope binary system will likely involve a new phase of mass accretion through Roche-lobe overflow that could drive the already massive white dwarf above the Chandrasekhar limit and produce a Type Ia supernova.

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arXiv2009

XMM-Newton observation of the persistent Be/NS X-ray binary pulsar RX J1037.5-5647 in a low luminosity state

The spectra of several X-ray binary pulsars display a clear soft excess, which in most cases can be described with a blackbody model, above the main power-law component. While in the high-luminosity sources it is usually characterized by low temperature (kT < 0.5 keV) and large emission radius (R > 100 km), in the two persistent and low-luminosity pulsars 4U 0352+309 and RX J0146.9+6121 this component has a high temperature (kT > 1 keV) and a smaller radius (R < 0.5 km), consistent with the estimated size of the neutron-star polar cap. Here we report on the timing and spectral analysis of RX J1037.5-5647, another low-luminosity persistent Be binary pulsar, based on the first XMM-Newton observation of this source. We have found a best-fit period P = 853.4(+/-0.2) s, that implies an average pulsar spin-up dP/dt ~ -2E-8 s/s in the latest decade. The estimated source luminosity is Lx ~ 10^34 erg/s, a value comparable to that of the other persistent Be binary pulsars and about one order of magnitude lower than in most of the previous measurements. The source spectrum can be described with a power law plus blackbody model, with kTbb = 1.26(+0.16/-0.09) keV and Rbb = 128(+13/-21) m, suggesting a polar-cap origin of this component. These results strengthen the hypothesis that, in addition to low luminosities and long periods, this class of sources is characterized also by common spectral properties

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arXiv2009

A Deep XMM-Newton Serendipitous Survey of a middle-latitude area. II. New deeper X-ray and optical observations

The radio-quiet neutron star 1E1207.4-5209 has been the target of several XMM-Newton observations, with a total exposure of ~350 ks. The source is located at intermediate galactic latitude (b~10 degrees), i.e. in a sky region with an extremely interesting mix of both galactic and extra-galactic X-ray sources. The aim of our work is to investigate the properties of both the intermediate-latitude galactic and extra-galactic X-ray source populations in the 1E1207.4-5209 field. We performed a coherent analysis of the whole XMM-Newton observation data set to build a catalogue of serendipitous X-ray sources detected with high confidence and to derive information on the source flux, spectra, and time variability. In addition, we performed a complete multi-band (UBVRI) optical coverage of the field with the Wide Field Imager (WFI) of the ESO/MPG 2.2m telescope (La Silla) to search for candidate optical counterparts to the X-ray sources, down to a V-band limiting magnitude of ~24.5. We detected a total of 144 serendipitous X-ray sources. Thanks to the refined X-ray positions and to the WFI observations, we found candidate optical counterparts for most of the X-ray sources in our compilation. For most of the brightest ones we proposed a likely classification based on both the X-ray spectra and the optical colours. Our results indicate that at intermediate galactic latitude the X-ray source population is dominated by the extra-galactic component, but with a significant contribution from the galactic component in the soft energy band, below 2 keV.

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arXiv2008

XIAO: a soft X-ray telescope for the SVOM Mission

The study of Gamma-ray bursts (GRBs) is a key field to expand our understanding of several astrophysical and cosmological phenomena. SVOM is a Chinese-French Mission which will permit to detect and rapidly locate GRBs, in particular those at high redshift, and to study their multiwavelength emission. The SVOM satellite, to be launched in 2013, will carry wide field instruments operating in the X/gamma-ray band and narrow field optical and soft X-ray telescopes. Here we describe a small soft X-ray telescope (XIAO) proposed as an Italian contribution to the SVOM mission. Thanks to a grazing incidence X-ray telescope with effective area of ~120 cm^2 and a short focal length, coupled to a very compact, low noise, fast read out CCD camera, XIAO can substantially contribute to the overall SVOM capabilities for both GRB and non-GRB science.

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arXiv2008

XMM-Newton observations of the low-mass X-ray binary XB1832-330 in the galactic globular cluster NGC 6652

We report on two XMM-Newton observations performed in 2006 of the luminous low-mass X-ray binary XB1832-330 which is located in the galactic globular cluster NGC 6652 and is probably an ultracompact binary (orbital period less than 1 hour). The aim of these observations is to investigate the low-energy absorption towards NGC6652 and in particular to search for Ne-rich material local to the binary, which has been suggested as a possible spectral signature of Neon rich degenerate companions. XMM-Newton observed the source twice, in 2006 September and October. High resolution spectroscopy with the RGS was used to estimate the ratio of the neutral neon to oxygen column densities to search for an anomalous Ne abundance in this X-ray binary. We find no evidence for anomalous Ne/O ratios, finding Ne/O=0.18+/-0.06 and Ne/O=0.17+/-0.03 (1 sigma uncertainties), in the two observations, respectively. These values are consistent with that in the interstellar medium. Timing analysis of EPIC data suggests possible periodicities at 9170+/-235 s and 18616 +/- 531 s, which need confirmation. A by-product of these observations consists of the spatial analysis of the source field, which resulted in the detection of 46 faint sources within the EPIC field of view, which are not present in the Second XMM-Newton Serendipitous Source Catalogue. All these faint sources are likely foreground objects. We performed the first high spectral resolution observations of XB1832-33, a probable ultracompact binary, without finding any evidence for an anomalous Ne abundance.

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arXiv2007

XMM-Newton observation of the persistent Be/neutron-star system X Persei at a high-luminosity level

We report on the XMM-Newton observation of the HMXRB X Persei, the prototype of the persistent and low-luminosity Be/neutron star pulsars, which was performed on February 2003. The source was detected at a luminosity level of ~ 1.4x10^35 erg/s, which is the highest level of the latest three decades. The pulsation period has increased up to 839.3 s, thus confirming the overall spin-down of the NS detected in the previous observations. The folded light-curve has a complex structure, with features not observed at lower luminosities, and shows a significant energy dependence. The spectral analysis reveals the presence of a significant excess at low energies over the main power-law spectral component, which can be described by a black-body spectrum of high temperature (kT_BB ~ 1.5 keV) and small emitting region (R_BB ~ 340 m); its properties are consistent with a polar-cap origin. Phase-resolved spectroscopy shows that the emission spectrum varies along the pulse period, but it is not possible to prove whether the thermal component is pulsed or not.

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arXiv2006

XMM-Newton and ESO observations of the two unidentified gamma-ray sources 3EG J0616-3310 and 3EG J1249-8330

The limited angular resolution of gamma-ray telescopes prevents a direct identification of the majority of sources detected so far. This is particularly true for the low latitude, probably galactic, ones only 10 % of which has been identified. Most counterparts of the identified low-latitude gamma-ray sources are Isolated Neutron Stars (INS), both radio-loud and radio-quiet (Geminga-like) objects, which are characterised by an extremely high value of the X-ray-to-optical flux ratio f_X/f_opt. Therefore, the systematic X-ray and optical coverage of low-latitude unidentified gamma-ray sources aiming at high f_X/f_opt sources seems one of the most promising ways to spot INS candidate counterparts. Since low latitude sources are heavily affected by the interstellar absorption at both X-ray and optical wavelengths, we have focussed on two middle-latitude, probably galactic, GRO/EGRET sources: 3EG J0616-3310 and 3EG J1249-8330. These two sources, which could belong to a local galactic population, have been selected owing to their relatively good positional accuracy, spectral shape and lack of candidate extragalactic radio counterparts. Here we report on X-ray observations of the two gamma-ray error boxes performed with XMM-Newton and on their optical follow-up carried on with the Wide Field Imager at the ESO/MPG 2.2m telescope. Less than half of the ~300 sources detected by the X-ray coverage have no optical counterparts. Among those, we have selected few interesting sources with f_X/f_opt > 100, which we consider promising INS candidates.

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arXiv2005

A deep XMM-Newton serendipitous survey of a middle-latitude area

The radio quiet neutron star 1E1207.4-5209 has been the target of a 260 ks XMM-Newton observation, which yielded, as a by product, an harvest of about 200 serendipitous X-ray sources above a limiting flux of 2E-15 erg/cm2/s, in the 0.3-8 keV energy range. In view of the intermediate latitude of our field (b~10 deg), it comes as no surprise that the logN-logS distribution of our serendipitous sources is different from those measured either in the Galactic Plane or at high galactic latitudes. Here we shall concentrate on the analysis of the brightest sources in our sample, which unveiled a previously unknown Seyfert-2 galaxy.

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arXiv2005

Low-energy absorption towards the ultra-compact binary 4U1850-087 located in the globular cluster NGC6712

We report the results of two XMM-Newton observations of the ultra-compact low-mass X-ray binary 4U1850-087 located in the galactic globular cluster NGC6712. A broad emission feature at 0.7keV was detected in an earlier ASCA observation and explained as the result of an unusual Ne/O abundance ratio in the absorbing material local to the source. We find no evidence for this feature and derive Ne/O ratios in the range 0.14-0.21, consistent with that of the interstellar medium. During the second observation, when the source was 10% more luminous, there is some evidence for a slightly higher Ne/O ratio and additional absorption. Changes in the Ne/O abundance ratio have been detected from another ultra-compact binary, 4U1543-624. We propose that these changes result from an X-ray induced wind which is evaporated from an O and Ne rich degenerate donor. As the source X-ray intensity increases so does the amount of evaporation and hence the column densities and abundance ratio of Ne and O.

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arXiv2004

X-ray and optical coverage of 3EG J0616-3310 and 3EG J1249-8330

The limited angular resolution of gamma-ray telescopes prevents the straight identification of the majority of the sources detected so far. This is particularly true for the low latitude, probably galactic ones, only 10 % of which has been identified. Most of the counterparts are Isolated Neutron Stars, both radio-loud and radio-quiet, characterised by an extremely high value of X-ray-to-optical flux ratio. The best way to search for INSs in the error boxes of unidentified EGRET sources is to perform the X-ray coverage of the gamma-ray field, followed by the optical characterization of each X-ray source. We applied this procedure to two EGRET sources, which could belong to a local galactic population: 3EG J0616-3310 and 3EG J1249-8330. Here we report on the analysis of about 300 X-ray objects, as well as on their optical study.

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arXiv2002

Timing analysis of the Seyfert-1 galaxy MCG-6-30-15 with XMM-Newton data

We present the main results of a deep timing analysis (in the 0.2-10 keV energy band) of the bright Seyfert-1 galaxy MCG-6-30-15 performed on XMM-Newton data. The light-curves and hardness ratios show that the source has large flux and spectral variations on time-scales of a few kiloseconds at most. From the analysis of both the power spectra and the structure function of the light curves we detected a relevant source variability between time-scales of ~0.3 and ~10 ks. Based on this result, we extracted a characteristic size of the X-ray emitting region of the order of 10^13 cm.

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arXiv2002

XMM-Newton and Wide Field Optical Imaging to crack the mystery of unidentified gamma-ray sources

The limited angular resolution of gamma-ray telescopes prevents the straight identification of the majority of the sources detected so far. While the only certified galactic gamma-ray emitters are associated with pulsars, more than 90% of the nearly 100 low latitude sources detected by EGRET on the GRO lack an identification. The best identification strategy devised over the years relies on the use of X-ray and optical data to single out possible counterparts, rating them on the basis of their extreme F_x/F_opt. Here we describe our multiwavelength programme based on the EPIC mapping of selected EGRET sources complemented by the optical coverage of the same fields obtained through the Wide Field Imager (WFI) of the ESO 2.2 m telescope. The field of view of the WFI, comparable to that of EPIC, will allow to directly cross-correlate X and optical data speeding up significantly the selection of interesting candidates worth follow-up optical (and X-ray) studies.

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arXiv2002

Status of the EPIC/MOS calibration

The XMM-Newton observatory has the largest collecting area flown so far for an X-ray imaging system, resulting in a very high sensitivity over a broad spectral range. In order to exploit fully these performances, a very accurate calibration of the XMM-Newton instruments is required, and has led to an extensive ground and flight calibration program. We report here on the current status of the EPIC/MOS cameras calibrations, highlighting areas for which a reasonably good accuracy has been achieved, and noting points where further work is needed.

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arXiv2001

Timing analysis of the core of the Crab-like SNR G21.5-0.9

The Crab-like SNR G21.5-0.9 was observed in the X-ray band (0.5-10 keV) by the XMM-Newton satellite for over 100 ks. The large effective area of the EPIC instrument has allowed us to to perform a deep search for pulsations from the central core of G21.5-0.9. No pulsations were found with upper limits on the pulsed fraction between 7.5 % and 40 % (depending on frequency and energy range).

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arXiv2001

The central region of M31 observed with XMM-Newton. II. Variability of the individual sources

We present the results of a study of the variability of X-ray sources in the central 30' of the nearby Andromeda Galaxy (M31) based on XMM Performance Verification observations. Two observations of this field, with a total exposure time of about 50 ks, were performed in June and December of 2000. We found 116 sources brighter than a limiting luminosity of 6 x 10(35) erg/s (0.3--12 keV, d=760 kpc). For the ~60 brightest sources, we searched for periodic and non-periodic variability; at least 15% of these sources appear to be variable on a time scale of several months. We discovered a new bright transient source ~2.9' from the nucleus in the June observation; this source faded significantly and was no longer detected in December. The behaviour of the object is similar to a handful of Galactic LMXB transients, most of which are supposed to harbor black holes. We detected pulsations with a period of \~865 s from a source with a supersoft spectrum. The flux of this source decreased significantly between the two XMM observations. The detected period is unusually short and points to a rapidly spinning magnetized white dwarf. The high luminosity and transient nature of the source suggest its possible identification with classical or symbiotic nova, some of which were observed earlier as supersoft sources.

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arXiv2000

The Extended X-ray Halo of the Crab-like SNR G21.5-0.9

Recent XMM-Newton observations reveal an extended (150") low-surface brightness X-ray halo in the supernova remnant G21.5-0.9. The near circular symmetry, the lack of any limb brightening and the non-thermal spectral form, all favour an interpretation of this outer halo as an extension of the central synchrotron nebula rather than as a shell formed by the supernova blast wave and ejecta. The X-ray spectrum of the nebula exhibits a marked spectral softening with radius, with the power-law spectral index varying from Gamma = 1.63 +/- 0.04 in the core to Gamma = 2.45 +/- 0.06 at the edge of the halo. Similar spectral trends are seen in other Crab-like remnants and reflect the impact of the synchrotron radiation losses on very high energy electrons as they diffuse out from the inner nebula. A preliminary timing analysis provides no evidence for any pulsed X-ray emission from the core of G21.5-0.9.

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arXiv2000

The central region of M31 observed with XMM-Newton (I. Group properties and diffuse emission)

We present the results of a study based on an XMM-Newton Performance Verification observation of the central 30 arcmin of the nearby spiral galaxy M31. In the 34-ks European Photon Imaging Camera (EPIC) exposure, we detect 116 sources down to a limiting luminosity of 6 x 10^35 erg/s (0.3--12 keV, d = 760 kpc). The luminosity distribution of the sources detected with XMM-Newton flattens at luminosities below ~ 2.5 x 10^37 erg/s. We make use of hardness ratios for the detected sources in order to distinguish between classes of objects such as super-soft sources and intrinsically hard or highly absorbed sources. We demonstrate that the spectrum of the unresolved emission in the bulge of M31 contains a soft excess which can be fitted with a ~ 0.35-keV optically-thin thermal-plasma component clearly distinct from the composite point-source spectrum. We suggest that this may represent diffuse gas in the centre of M31, and we illustrate its extent in a wavelet-deconvolved image.

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