First light for SiFAP4XP

At the meeting for the 25th anniversary of the TNG (October 2021, ), the FGG staff announced that it was aiming to prepare a new fast photometer with polarimetric capabilities. The project was triggered by the challenging goal to observe in the optical domain the same targets of the IXPE satellite, the first to be specifically dedicated to X-ray polarimetric observations. The NASA/ASI satellite IXPE was successfully launched last December and is now undergoing its calibration phase. Our target was to offer a visible-wavelength fast-polarimeter ready to pair the observations of IXPE since the beginning.

In the tight time frame of just 4 months the FGG staff, in collaboration with the INAF-Roma Observatory and Catania University, decided to develop and build a new Silicon Fast Photometer for Polarimetry- finally named SiFAP4XP (read sifap-for-ix-pe).

Thanks to the know-how of the same team that built SiFAP and SiFAP2, and based on the same but improved technology, SiFAP4XP is a SiPM-based photometer exploiting MPPC detectors from Hamamatsu. SiFAP4XP is fed through the already available PAOLO polarimeter of DOLORES, a double Wollaston prism which splits the light from the target into its 4 polarimetric states. At the output of PAOLO, instead of the LRS CCD, a new optical relay with 4 photometers for the target and 4 for the nearby (~20 arcsec) sky has been installed (Fig. 1). The whole mechanics and electronics have been produced in the TNG workshops, then assembled, aligned and tested at the telescope (Fig. 2). The signal from the MPPCs is read by custom read-out electronics fed by a GPS disciplined clock capable to tag the time of arrival (ToA) of each photon with a time resolution of 8 ns. All the ToAs are referred to the UTC with an accuracy of ±25 ns thanks to a GPS system. Both the control and acquisition UI software has been developed by the Information Technology group of the FGG.

On the night of February 23rd SiFAP4XP finally reached its milestone, the first light on sky, observing the pulses of the Crab pulsar in polarimetric mode (Fig. 3). Unfortunately the weather allowed us to use only half of the 3 available test nights, and few things still need to be tuned in order to have the instrument fully optimized. Anyway, taking into account that SiFAP4XP has been built up in so short time, this is already an amazing result reached thanks to the effort of the well organized and focused staff of the TNG in synergy with INAF OA-Roma institute and Catania University. We are hardly working to be ready to offer the instrument to the scientific community as soon as possible to open up new collaborations and simultaneous observations with multi- wavelength facilities.

Power spectrum of contributions

INAF-FGG: A. Ghedina, M. Cecconi, M. Gonzalez, L. Riverol, H. Perez, M. Hernandez, J. San Juan and E. Poretti, with the rest of dTec, IT, Admin and TNG staff. INAF OA Roma: F. Ambrosino. Univ. Catania: F. Leone.

SiFAP4XP optical bench

Figure 1: A view of SiFAP4XP optical bench with the 4 detectors for the 4 polarized beams (starlight is coming from the right). At the center of the bench a flat mirror redirects the light from the nearby sky to the other 4 detectors, mounted on the other side of the bench. A red guide camera fed through a 92:8 BS is visible at the top left. By removing the Double Wollaston prism it is possible to observe in simple photometry on the main detector (red beam).


Figure 2: SiFAP4XP mounted at the focal plane of Dolores and ready for the first light.

The plot of pulsed profiles

Figure 3: The plot of pulsed profiles of the Crab pulsar from the 4 channels of SiFAP4XP with the corresponding Stokes parameters, folded with 64 phase bins, normalized (not background- subtracted) and barycentered to the SSB. An improvement of the centering of 2 detectors is probably needed.