Yoel Rephaeli

Observations of Extended Radio Emission in Clusters

We review observations of extended regions of radio emission in clusters; these include diffuse emission in ‘relics’, and the large central regions commonly referred to as ‘halos’. The spectral observations, as well as Faraday rotation measurements of background and cluster radio sources, provide the main evidence for large-scale intracluster magnetic fields and significant densities of relativistic electrons. Implications from these observations on acceleration mechanisms of these electrons are reviewed, including turbulent and shock acceleration, and also the origin of some of the electrons in collisions of relativistic protons by ambient protons in the (thermal) gas. Improved knowledge of non-thermal phenomena in clusters requires more extensive and detailed radio measurements; we briefly review prospects for future observations.

The discovery of rapid X-ray oscillations in the tail of the SGR 1806-20 hyperflare

We have discovered rapid quasi-periodic oscillations (QPOs) in RXTE/PCA measurements of the pulsating tail of the 2004 December 27 giant flare of SGR 1806-20. QPOs at ~92.5 Hz are detected in a 50 s interval starting 170 s after the onset of the giant flare. These QPOs appear to be associated with increased emission by a relatively hard unpulsed component and are seen only over phases of the 7.56 s spin period pulsations away from the main peak. QPOs at ~18 and ~30 Hz are also detected ~200-300 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected in the flux of a soft gamma-ray repeater or any other magnetar candidate. We interpret the highest QPOs in terms of the coupling of toroidal seismic modes with Alfven waves propagating along magnetospheric field lines. The lowest frequency QPO might instead provide indirect evidence on the strength of the internal magnetic field of the magnetar.

Strong-lensing analysis of a complete sample of 12 MACS clusters at z > 0.5: mass models and Einstein radii

We present the results of a strong-lensing analysis of a complete sample of 12 very luminous X-ray clusters at z > 0:5 using HST/ACS images. Our modelling technique has uncovered some of the largest known critical curves outlined by many accuratelypredicted sets of multiple images. The distribution of Einstein radii has a median value of’ 28 00 (for a source redshift of zs 2), twice as large as other lower-z samples, and extends to 55 00 for MACS J0717.5+3745, with an impressive enclosed Einstein mass of 7:4 10 14 M . We

New Multiply-Lensed Galaxies Identified in ACS/NIC3 Observations of Cl0024+1654, Using an Improved Mass Model

We present an improved strong-lensing analysis of Cl0024+1654 (z = 0.39) using deep Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS)/NIC3 images, based on 33 multiply-lensed images of 11 background galaxies. These are found with a model that assumes mass approximately traces light, with a low-order expansion to allow for flexibility on large scales. The model is constrained initially by the well-known five-image system (z = 1.675) and refined as new multiply-lensed systems are identified using the model. Photometric redshifts of these new systems are then used to constrain better the mass profile by adopting the standard cosmological relation between redshift and lensing distance. Our model requires only six free parameters to describe well all positional and redshift data. The resulting inner mass profile has a slope of d log M/ dl ogr �− 0.55, consistent with new weak-lensing measurements where the data overlap, at r � 200 kpc/h70. The combined profile is well fitted by a high-concentration Navarro, Frenk & White (NFW) mass profile, Cvir ∼ 8.6 ± 1.6,

Comparison of Cluster Lensing Profiles with ΛCDM Predictions

We derive lens distortion and magnification profiles of four well-known clusters observed with Subaru. Each cluster is very well fitted by the general form predicted for cold dark matter (CDM) dominated halos, with good consistency found between the independent distortion and magnification measurements. The inferred level of mass concentration is surprisingly high, 8 < cvir < 15 ( cvir = 10.39 ± 0.91), compared to the relatively shallow profiles predicted by the ΛCDM model, cvir = 5.06 ± 1.10 (for Mvir = 1.25 × 1015 M☉ h−1). This represents a 4 σ discrepancy, and includes the relatively modest effects of projection bias and profile evolution derived from N-body simulations, which oppose each other with little residual effect. In the context of CDM-based cosmologies, this discrepancy implies that clusters collapse earlier (z ≥ 1) than predicted (z < 0.5), when the universe was correspondingly denser.

2-10 keV luminosity of high-mass binaries as a gauge of ongoing star-formation rate

Based on recent work on spectral decomposition of the emission of star-forming galaxies, we assess whether the inte- grated 2−10 keV emission from high-mass X-ray binaries (HMXBs), L HMXB−10 , can be used as a reliable estimator of ongoing star formation rate (SFR). Using a sample of 46 local (z < 0.1) star-forming galaxies, and spectral modeling of ASCA, BeppoSAX, and XMM-Newton data, we demonstrate the existence of a linear SFR -L HMXB−10 relation which holds over ∼5 decades in X-ray luminosity and SFR. The total 2−10 keV luminosity is not a precise SFR indicator because at low SFR (i.e., in normal and moderately-starbursting galaxies) it is substantially affected by the emission of low-mass X-ray binaries, which do not trace the current SFR due to their long evolution lifetimes, while at very high SFR (i.e., for very luminous FIR-selected galaxies) it is frequently affected by the presence of strongly obscured AGNs. The availability of purely SB-powered galaxies - whose 2−10 keV emission is mainly due to HMXBs - allows us to properly calibrate the SFR -L HMXB−10 relation. The SFR -L HMXB−10 relation holds also for distant (z ∼ 1) galaxies in the Hubble Deep Field North sample, for which we lack spectral information, but whose SFR can be estimated from deep radio data. If confirmed by more detailed observations, it may be possible to use the deduced relation to identify distant galaxies that are X-ray overluminous for their (independently estimated) SFR ,a nd are therefore likely to hide strongly absorbed AGNs.

Rossi X-Ray Timing Explorer Observations of the Coma Cluster

The Coma Cluster was observed in 1996 for 90 ks by the PCA and HEXTE instruments aboard the Rossi X-ray Timing Explorer (RXTE) satellite?the first simultaneous, pointing measurement of Coma in the broad, 2-250 keV energy band. The high sensitivity achieved during this long observation allows precise determination of the spectrum. Our analysis of the measurements clearly indicates that in addition to the main thermal emission from hot intracluster gas at kT 7.5 keV, a second spectral component is required to best fit the data. If thermal, this component has a temperature 4.7 keV, and it contributes 20% of the total flux. Alternatively, the second component can be a power law, likely due to Compton scattering of relativistic electrons by the cosmic microwave background. This interpretation is based on the measurements of diffuse radio emission and the similar values of the radio and X-ray spectral indices. A Compton origin of the nonthermal component would imply that the volume-averaged magnetic field in the central region of Coma is B 0.2 ?G, a value that is free of the usual assumption of energy equipartition. The energy density of the emitting electrons would then be ~8 ? 10-14 ergs cm-3. Barring the presence of unknown systematic errors in the RXTE source or background measurements, our spectral analysis yields considerable evidence for Compton X-ray emission in the Coma Cluster.

Detailed cluster mass and light profiles of A1703, A370 and RXJ1347−11 from deep Subaru imaging*

Weak lensing work can be badly compromised by unlensed foreground and cluster members which dilute the true lensing signal. We show how the lensing amplitude in multicolour space can be harnessed to securely separate cluster members from the foreground and background populations for three massive clusters, A1703 (z= 0.258), A370 (z= 0.375) and RXJ1347−11 (z= 0.451) imaged with Subaru. The luminosity functions of these clusters when corrected for dilution show similar faint-end slopes, α≃− 1.0, with no marked faint-end upturn to our limit of MR≃− 15.0, and only a mild radial gradient. In each case, the radial profile of the mass-to-light ratio (M/L) peaks at intermediate radius, ≃ 0.2rvir, at a level of 300–500(M/LR)⊙, and then falls steadily towards ∼ 100(M/LR)⊙ at the virial radius, similar to the mean field level. This behaviour is likely due to the relative paucity of central late-type galaxies, whereas for the E/S0 sequence only a mild radial decline in M/L is found for each cluster. We discuss this behaviour in the context of detailed simulations where predictions for tidal stripping may now be tested accurately with observations.

Galactic star formation rates gauged by stellar end-products

Young galactic X-ray point sources (XPs) closely trace the ongoing star formation in galaxies. From measured XP number counts we extract the collective 2-10 keV luminosity of young XPs, L y x XP , which we use to gauge the current star formation rate (SFR) in galaxies. We find that, for a sample of local star-forming galaxies (i.e., normal spirals and mild starbursts), L y x XP correlates linearly with the SFR over three decades in luminosity. A separate, high-SFR sample of starburst ULIRGs can be used to check the calibration of the relation. Using their (presumably SF-related) total 2-10 keV luminosities we find that these sources satisfy the SFR-L y x XP relation, as defined by the weaker sample, and extend it to span ∼5 decades in luminosity. The SFR-L y x XP relation is also likely to hold for distant (z ∼ 1) Hubble Deep Field North galaxies, especially so if these high-SFR objects are similar to the (more nearby) ULIRGs. It is argued that the SFR-L y x XP relation provides the most adequate X-ray estimator of instantaneous SFR by the phenomena characterizing massive stars from their birth (FIR emission from placental dust clouds) through their death as compact remnants (emitting X-rays by accreting from a close donor). For local, low/intermediate-SFR galaxies, the simultaneous existence of a correlation of the instantaneous SFR with the total 2-10 keV luminosity, L,, which traces the SFR integrated over the last - 10 9 yr, suggests that during such epoch the SF in these galaxies has been proceeding at a relatively constant rate.Young galactic X-ray point sources (XPs) closely trace the ongoing star formation in galaxies. From measured XP number counts we extract the collective 2-10 keV luminosity of young XPs, L_yXP, which we use to gauge the current star-formation rate (SFR) in galaxies. We find that, for a sample of local star-forming galaxies (i.e., normal spirals and mild starbursts), L_yXP correlates linearly with the SFR over three decades in luminosity. A separate, high-SFR sample of starburst ULIRGs can be used to check the calibration of the relation. Using their (presumably SF-related) total 2-10 keV luminosities we find that these sources satisfy the SFR-L_yXP relation, as defined by the weaker sample, and extend it to span about 5 decades in luminosity. The SFR-L_yXP relation is likely to hold also for distant Hubble Deep Field North galaxies, especially so if these high-SFR objects are similar to the (more nearby) ULIRGs. It is argued that the SFR-L_yXP relation provides the most adequate X-ray estimator of instantaneous SFR by the phenomena characterizing massive stars from their birth (FIR emission from placental dust clouds) through their death as compact remnants (emitting X-rays by accreting from a close donor). For local, low/intermediate-SFR galaxies, the simultaneous existence of a correlation of the instantaneous SFR with the total 2-10 keV luminosity, which traces the SFR integrated over (approximately) the last Gyr, suggests that during such epoch the SF in these galaxies has been proceeding at a relatively constant rate.

X-ray spectral components of starburst galaxies

X-ray emission processes in starburst galaxies (SBGs) are assessed, with the aim of identifying and characterizing the main spectral components. Our survey of spectral properties, complemented with a model for the evolution of galactic stellar populations, leads to the prediction of a complex spectrum. Comparing the predicted spectral properties with current X-ray measurements of the nearby SBGs M 82 and N253, we draw the following tentative conclusions: 1) X-ray binaries with accreting neutron stars are the main contributors in the 2-15 keV band, and could be responsible for the yet uninterpreted hard component required to fit the observed 0.5-10 keV spectra of SBGs; 2) diffuse thermal plasma contributes at energies ≤1 keV; 3) nonthermal emission, from Compton scattering of FIR and CMB radiation fields photons off supernova-accelerated relativistic electrons, and AGN-like emission, are likely be the dominant emission at energies ≥30 keV; 4) supernova remnants make a relatively minor contribution to the X-ray continuum but may contribute appreciably to the Fe-K emission at 6.7 keV.X-ray emission processes in starburst galaxies (SBGs) are assessed, with the aim of identifying and characterizing the main spectral components. Our survey of spectral properties, complemented with a model for the evolution of galactic stellar populations, leads to the prediction of a complex spectrum. Comparing the predicted spectral properties with current X-ray measurements of the nearby SBGs M82 and N253, we draw the following tentative conclusions: 1) X-ray binaries with accreting NS are the main contributors in the 2-15 keV band, and could be responsible for the yet uninterpreted hard component required to fit the observed 0.5-10 keV spectra of SBGs; 2) diffuse thermal plasma contributes at energies less than about 1 keV; 3) nonthermal emission, from Compton scattering of FIR and CMB radiation field photons off supernova-accelerated relativistic electrons, and AGN-like emission, are likely be the dominant emission at energies>30 keV; 4) supernova remnants make a relatively minor contribution to the X-ray continuum but may contribute appreciably to the Fe-K emission at 6.7 keV.