Handling the Background in IXPE Polarimetric Data

Marco, Alessandro Di; Soffitta, P.; Costa, E.; Ferrazzoli, Riccardo; Monaca, Fabio La; Rankin, John; Ratheesh, Ajay; Xie, Fei; Baldini, Luca; Monte, E. Del; Ehlert, Steven R.; Fabiani, Sergio; Kim, Dawoon E.; Muleri, Fabio; O’Dell, Stephen L.; Ramsey, Brian D.; Rubini, A.; Sgrò, C.; Silvestri, S.; Tennant, Allyn F.; Weisskopf, Martin C.

doi:10.3847/1538-3881/acba0f

Cited by 54 publications

(43 citation statements)

References 40 publications

(40 reference statements)

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“…To improve the polarimetric sensitivity, we applied a background rejection strategy (Xie et al 2021;Di Marco et al 2023). The method refines the sensitivity by applying energy-dependent cuts on photoelectron tracks from the level 1 data.…”

Section: Appendix a X-ray Observationsmentioning

confidence: 99%

X-Ray Polarization of BL Lacertae in Outburst

Peirson

Negro

Liodakis

et al. 2023

ApJL

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We report the first >99% confidence detection of X-ray polarization in BL Lacertae. During a recent X-ray/γ-ray outburst, a 287 ks observation (2022 November 27–30) was taken using the Imaging X-ray Polarimetry Explorer (IXPE), together with contemporaneous multiwavelength observations from the Neil Gehrels Swift observatory and XMM-Newton in soft X-rays (0.3–10 keV), NuSTAR in hard X-rays (3–70 keV), and optical polarization from the Calar Alto and Perkins Telescope observatories. Our contemporaneous X-ray data suggest that the IXPE energy band is at the crossover between the low- and high-frequency blazar emission humps. The source displays significant variability during the observation, and we measure polarization in three separate time bins. Contemporaneous X-ray spectra allow us to determine the relative contribution from each emission hump. We find >99% confidence X-ray polarization Π 2 – 4 keV = 21.7 − 7.9 + 5.6 % and electric vector polarization angle ψ 2–4keV = −28.°7 ± 8.°7 in the time bin with highest estimated synchrotron flux contribution. We discuss possible implications of our observations, including previous IXPE BL Lacertae pointings, tentatively concluding that synchrotron self-Compton emission dominates over hadronic emission processes during the observed epochs.

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Section: Appendix a X-ray Observationsmentioning

confidence: 99%

X-Ray Polarization of BL Lacertae in Outburst

Peirson

Negro

Liodakis

et al. 2023

ApJL

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“…We extracted the I, Q, and U spectra for each of the three DUs of IXPE using the software IXPEOBSSIM (v.30.0.0;Pesce-Rollins et al 2019;Baldini et al 2022) and following the background rejection prescriptions presented in Di Marco et al (2023). Moreover, spectra were computed in order to use the so-called weighted analysis method presented in Di Marco et al (2022; parameter STOKES = NEFF in XSELECT).…”

Section: X-ray Data Reductionmentioning

confidence: 99%

“…The spectra for the Stokes parameters were computed from a circular region with radius = 0 95 centered on the source, while an annulus with r in(out) = 1 2(3 5) was adopted to extract the background. This approach has been shown to enhance the sensitivity to polarization (Di Marco et al 2023). The resulting I Stokes spectra were grouped by requiring each energy bin to have a signal-to-noise ratio (S/N) greater than 7.…”

Section: X-ray Data Reductionmentioning

confidence: 99%

IXPE and Multiwavelength Observations of Blazar PG 1553+113 Reveal an Orphan Optical Polarization Swing

Middei

Perri

Puccetti

et al. 2023

ApJL

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The lower-energy peak of the spectral energy distribution of blazars has commonly been ascribed to synchrotron radiation from relativistic particles in the jets. Despite the consensus regarding jet emission processes, the particle acceleration mechanism is still debated. Here, we present the first X-ray polarization observations of PG 1553+113, a high-synchrotron-peak blazar observed by the Imaging X-ray Polarimetry Explorer (IXPE). We detect an X-ray polarization degree of (10 ± 2)% along an electric-vector position angle of ψ X = 86° ± 8°. At the same time, the radio and optical polarization degrees are lower by a factor of ∼3. During our IXPE pointing, we observed the first orphan optical polarization swing of the IXPE era, as the optical angle of PG 1553+113 underwent a smooth monotonic rotation by about 125°, with a rate of ∼17° day–1. We do not find evidence of a similar rotation in either radio or X-rays, which suggests that the X-ray and optically emitting regions are separate or, at most, partially cospatial. Our spectropolarimetric results provide further evidence that the steady-state X-ray emission in blazars originates in a shock-accelerated and energy-stratified electron population.

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“…Source I, Q, and U Stokes spectra were produced via the xpbin tools PHA1, PHA1Q, and PHA1U algorithms, producing a full data set comprised of nine spectra per observation, three for each DU. As the background region is dominated by source events, background subtraction is not applied (Di Marco et al 2023). Here we also used unweighted analysis.…”

Section: Tablementioning

confidence: 99%

IXPE Observations of the Quintessential Wind-accreting X-Ray Pulsar Vela X-1

Forsblom

Poutanen

Tsygankov

et al. 2023

ApJL

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The radiation from accreting X-ray pulsars was expected to be highly polarized, with some estimates for the polarization degree of up to 80%. However, phase-resolved and energy-resolved polarimetry of X-ray pulsars is required in order to test different models and to shed light on the emission processes and the geometry of the emission region. Here we present the first results of the observations of the accreting X-ray pulsar Vela X-1 performed with the Imaging X-ray Polarimetry Explorer. Vela X-1 is considered to be the archetypal example of a wind-accreting, high-mass X-ray binary system, consisting of a highly magnetized neutron star accreting matter from its supergiant stellar companion. The spectropolarimetric analysis of the phase-averaged data for Vela X-1 reveals a polarization degree (PD) of 2.3% ± 0.4% at the polarization angle (PA) of −47.°3 ± 5.°4. A low PD is consistent with the results obtained for other X-ray pulsars and is likely related to the inverse temperature structure of the neutron star atmosphere. The energy-resolved analysis shows the PD above 5 keV reaching 6%–10% and a ∼90° difference in the PA compared to the data in the 2–3 keV range. The phase-resolved spectropolarimetric analysis finds a PD in the range 0%–9% with the PA varying between −80° and 40°.

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Handling the Background in IXPE Polarimetric Data

Cited by 54 publications

References 40 publications

X-Ray Polarization of BL Lacertae in Outburst

X-Ray Polarization of BL Lacertae in Outburst

IXPE and Multiwavelength Observations of Blazar PG 1553+113 Reveal an Orphan Optical Polarization Swing

IXPE Observations of the Quintessential Wind-accreting X-Ray Pulsar Vela X-1

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