arXiv subjects

R. Romani

Publications and source records attributed to R. Romani.

10 recordsLinked to original sources

arXiv2024

X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei : NGC 1068

We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2-8 keV polarization of NGC 1068. We pointed IXPE for a net exposure time of 1.15 Ms on the target, in addition to two ~ 10 ks each Chandra snapshots in order to account for the potential impact of several ultraluminous X-ray source (ULXs) within IXPE's field-of-view. We measured a 2 - 8 keV polarization degree of 12.4% +/- 3.6% and an electric vector polarization angle of 101{\deg} +/- 8{\deg} at 68% confidence level. If we exclude the spectral region containing the bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises up to 21.3% +/- 6.7% in the 3.5 - 6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec scale radio jet. Using a combined Chandra and IXPE analysis plus multi-wavelength constraints, we estimated that the circumnuclear "torus" may sustain a half-opening angle of 50{\deg} - 55{\deg} (from the vertical axis of the system). Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of polarization angle orientation with respect to the radio-jet and torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN : the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic object is now feasible thanks to X-ray polarimetry.

astro-ph.HE↗

arXiv2023

From Stellar Death to Cosmic Revelations: Zooming in on Compact Objects, Relativistic Outflows and Supernova Remnants with AXIS

Compact objects and supernova remnants provide nearby laboratories to probe the fate of stars after they die, and the way they impact, and are impacted by, their surrounding medium. The past five decades have significantly advanced our understanding of these objects, and showed that they are most relevant to our understanding of some of the most mysterious energetic events in the distant Universe, including Fast Radio Bursts and Gravitational Wave sources. However, many questions remain to be answered. These include: What powers the diversity of explosive phenomena across the electromagnetic spectrum? What are the mass and spin distributions of neutron stars and stellar mass black holes? How do interacting compact binaries with white dwarfs - the electromagnetic counterparts to gravitational wave LISA sources - form and behave? Which objects inhabit the faint end of the X-ray luminosity function? How do relativistic winds impact their surroundings? What do neutron star kicks reveal about fundamental physics and supernova explosions? How do supernova remnant shocks impact cosmic magnetism? This plethora of questions will be addressed with AXIS - the Advanced X-ray Imaging Satellite - a NASA Probe Mission Concept designed to be the premier high-angular resolution X-ray mission for the next decade. AXIS, thanks to its combined (a) unprecedented imaging resolution over its full field of view, (b) unprecedented sensitivity to faint objects due to its large effective area and low background, and (c) rapid response capability, will provide a giant leap in discovering and identifying populations of compact objects (isolated and binaries), particularly in crowded regions such as globular clusters and the Galactic Center, while addressing science questions and priorities of the US Decadal Survey for Astronomy and Astrophysics (Astro2020).

astro-ph.HE↗

arXiv2016

Geminga's puzzling pulsar wind nebula

We report on six new Chandra observations of the Geminga pulsar wind nebula (PWN). The PWN consists of three distinct elongated structures - two $\approx 0.2 d_{250}$ pc long lateral tails and a segmented axial tail of $\approx 0.05 d_{250}$ pc length, where $d_{250}=d/(250 {\rm pc})$. The photon indices of the power law spectra of the lateral tails, $Γ\approx 1$, are significantly harder than those of the pulsar ($Γ\approx 1.5$) and the axial tail ($Γ\approx 1.6$). There is no significant diffuse X-ray emission between the lateral tails -- the ratio of the X-ray surface brightness between the south tail and this sky area is at least 12. The lateral tails apparently connect directly to the pulsar and show indication of moving footpoints. The axial tail comprises time-variable emission blobs. However, there is no evidence for constant or decelerated outward motion of these blobs. Different physical models are consistent with the observed morphology and spectra of the Geminga PWN. In one scenario, the lateral tails could represent an azimuthally asymmetric shell whose hard emission is caused by the Fermi acceleration mechanism of colliding winds. In another scenario, the lateral tails could be luminous, bent polar outflows, while the blobs in the axial tail could represent a crushed torus. In a resemblance to planetary magnetotails, the blobs of the axial tail might also represent short-lived plasmoids which are formed by magnetic field reconnection in the relativistic plasma of the pulsar wind tail.

astro-ph.HE↗

arXiv2015

Characterization of the Inner Knot of the Crab: The Site of the Gamma-ray Flares?

One of the most intriguing results from the gamma-ray instruments in orbit has been the detection of powerful flares from the Crab Nebula. These flares challenge our understanding of pulsar wind nebulae and models for particle acceleration. We report on the portion of a multiwavelength campaign using Keck, HST, and Chandra concentrating on a small emitting region, the Crab's inner knot, located a fraction of an arcsecond from the pulsar. We find that the knot's radial size, tangential size, peak flux, and the ratio of the flux to that of the pulsar are correlated with the projected distance of the knot from the pulsar. A new approach, using singular value decomposition for analyzing time series of images, was introduced yielding results consistent with the more traditional methods while some uncertainties were substantially reduced. We exploit the characterization of the knot to discuss constraints on standard shock-model parameters that may be inferred from our observations assuming the inner knot lies near to the shocked surface. These include inferences as to wind magnetization, shock shape parameters such as incident angle and poloidal radius of curvature, as well as the IR/optical emitting particle enthalpy fraction. We find that while the standard shock model gives good agreement with observation in many respects, there remain two puzzles: (a) The observed angular size of the knot relative to the pulsar--knot separation is much smaller than expected; (b) The variable, yet high degree of polarization reported is difficult to reconcile with a highly relativistic downstream flow.

astro-ph.HE↗

arXiv2010

Characteristics of Gamma-Ray Loud Blazars in the VLBA Imaging and Polarimetry Survey

The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed as part of the VLBA Imaging and Polarimetry Survey (VIPS). This large, flux-limited sample of active galactic nuclei (AGN) provides insights into the mechanism that produces strong gamma-ray emission. At lower flux levels, radio flux density does not directly correlate with gamma-ray flux. We find that the LAT-detected BL Lacs tend to be similar to the non-LAT BL Lacs, but that the LAT-detected FSRQs are often significantly different from the non-LAT FSRQs. The differences between the gamma-ray loud and quiet FSRQs can be explained by Doppler boosting; these objects appear to require larger Doppler factors than those of the BL Lacs. It is possible that the gamma-ray loud FSRQs are fundamentally different from the gamma-ray quiet FSRQs. Strong polarization at the base of the jet appears to be a signature for gamma-ray loud AGN.

astro-ph.CO↗

arXiv2008

A Measurement of the Rate of type-Ia Supernovae at Redshift $z\approx$ 0.1 from the First Season of the SDSS-II Supernova Survey

We present a measurement of the rate of type Ia supernovae (SNe Ia) from the first of three seasons of data from the SDSS-II Supernova Survey. For this measurement, we include 17 SNe Ia at redshift $z\le0.12$. Assuming a flat cosmology with $Ω_m = 0.3=1-Ω_Λ$, we find a volumetric SN Ia rate of $[2.93^{+0.17}_{-0.04}({\rm systematic})^{+0.90}_{-0.71}({\rm statistical})] \times 10^{-5} {\rm SNe} {\rm Mpc}^{-3} h_{70}^3 {\rm year}^{-1}$, at a volume-weighted mean redshift of 0.09. This result is consistent with previous measurements of the SN Ia rate in a similar redshift range. The systematic errors are well controlled, resulting in the most precise measurement of the SN Ia rate in this redshift range. We use a maximum likelihood method to fit SN rate models to the SDSS-II Supernova Survey data in combination with other rate measurements, thereby constraining models for the redshift-evolution of the SN Ia rate. Fitting the combined data to a simple power-law evolution of the volumetric SN Ia rate, $r_V \propto (1+z)^β$, we obtain a value of $β= 1.5 \pm 0.6$, i.e. the SN Ia rate is determined to be an increasing function of redshift at the $\sim 2.5 σ$ level. Fitting the results to a model in which the volumetric SN rate, $r_V=Aρ(t)+B\dot ρ(t)$, where $ρ(t)$ is the stellar mass density and $\dot ρ(t)$ is the star formation rate, we find $A = (2.8 \pm 1.2) \times 10^{-14} \mathrm{SNe} \mathrm{M}_{\sun}^{-1} \mathrm{year}^{-1}$, $B = (9.3^{+3.4}_{-3.1})\times 10^{-4} \mathrm{SNe} \mathrm{M}_{\sun}^{-1}$.

astro-ph↗

arXiv2007

The Sloan Digital Sky Survey-II Supernova Survey: Search Algorithm and Follow-up Observations

The Sloan Digital Sky Survey-II Supernova Survey has identified a large number of new transient sources in a 300 sq. deg. region along the celestial equator during its first two seasons of a three-season campaign. Multi-band (ugriz) light curves were measured for most of the sources, which include solar system objects, Galactic variable stars, active galactic nuclei, supernovae (SNe), and other astronomical transients. The imaging survey is augmented by an extensive spectroscopic follow-up program to identify SNe, measure their redshifts, and study the physical conditions of the explosions and their environment through spectroscopic diagnostics. During the survey, light curves are rapidly evaluated to provide an initial photometric type of the SNe, and a selected sample of sources are targeted for spectroscopic observations. In the first two seasons, 476 sources were selected for spectroscopic observations, of which 403 were identified as SNe. For the Type Ia SNe, the main driver for the Survey, our photometric typing and targeting efficiency is 90%. Only 6% of the photometric SN Ia candidates were spectroscopically classified as non-SN Ia instead, and the remaining 4% resulted in low signal-to-noise, unclassified spectra. This paper describes the search algorithm and the software, and the real-time processing of the SDSS imaging data. We also present the details of the supernova candidate selection procedures and strategies for follow-up spectroscopic and imaging observations of the discovered sources.

astro-ph↗

arXiv2007

The Sloan Digital Sky Survey-II Supernova Survey: Technical Summary

The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5 degrees wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for discovery of new objects. Supernova imaging observations are being acquired between 1 September and 30 November of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation.

astro-ph↗

arXiv2007

A Study of the Type Ia/IIn Supernova 2005gj from X-ray to the Infrared: Paper I

We present extensive ugrizYHJK photometry and optical spectroscopy of SN 2005gj obtained by the SDSS-II and CSP Supernova Projects, which give excellent coverage during the first 150 days after the time of explosion. These data show that SN 2005gj is the second clear case, after SN 2002ic, of a thermonuclear explosion in a dense circumstellar environment. Both the presence of singly and doubly ionized iron-peak elements (FeIII and weak SII, SiII) near maximum light as well as the spectral evolution show that SN 2002ic-like events are Type Ia explosions. Independent evidence comes from the exponential decay in luminosity of SN 2005gj, pointing to an exponential density distribution of the ejecta. The interaction of the supernova ejecta with the dense circumstellar medium is stronger than in SN 2002ic: (1) the supernova lines are weaker; (2) the Balmer emission lines are more luminous; and (3) the bolometric luminosity is higher close to maximum light. The velocity evolution of the Halpha components suggest that the CSM around SN 2005gj is clumpy and it has a flatter density distribution compared with the steady wind solution, in agreement with SN 2002ic. An early X-ray observation with Chandra gives an upper-limit on the mass loss rate from the companion of < 2x10^{-4} Msun/yr.

astro-ph↗

arXiv2005

A Search for Radio Emission from the Young 16-ms X-ray Pulsar PSR J0537-6910

PSR J0537-6910 is a young, energetic, rotation-powered X-ray pulsar with a spin period of 16 ms located in the Large Magellanic Cloud. We have searched for previously undetected radio pulsations (both giant and standard) from this pulsar in a 12-hour observation taken at 1400 MHz with the Parkes 64-m radio telescope. The very large value of the magnetic field at the light cylinder radius suggests that this pulsar might be emitting giant radio pulses like those seen in other pulsars with similar field strengths. No radio emission of either kind was detected from the pulsar, and we have established an upper limit of ~ 25 mJy kpc^2 for the average 1400-MHz radio luminosity of PSR J0537-6910. The 5-sigma single-pulse detection threshold was ~ 750 mJy for a single 80-us sample. These limits are likely to be the best obtainable until searches with greatly improved sensitivity can be made with next-generation radio instruments.

astro-ph↗