Advancements in High-Frequency Black Hole Observations

Observations of celestial objects through Very Long Baseline Interferometry (VLBI) at high frequencies, like 230 GHz, have given us the first images of supermassive black holes. This type of observation has unique advantages, but it also comes with challenges, especially at higher frequencies. One significant issue is the fast changes in the atmosphere that affect the signals we receive, making it hard to get clear images and measurements.

To improve these observations, we can apply new Calibration techniques that focus on using multiple frequencies at the same time. This paper discusses how these methods can enhance the capabilities of the next generation Event Horizon Telescope (ngEHT) by improving sensitivity and allowing for more reliable observations year-round.

#Challenges in High-Frequency Observations

The main problem with VLBI observations at high frequencies is the rapid changes in the atmosphere, which disrupt the signals. These fluctuations limit the time we can integrate the signal coherently. Coherent integration is important as it allows us to build a clearer picture by combining signals over time.

Moreover, because the signals from sources in space tend to be weaker at higher frequencies, it becomes harder to observe them compared to lower frequencies. This makes it necessary to focus on the brightest sources, limiting our observational capabilities.

#Promising Solutions

Next-generation calibration methods can help address these challenges. By using techniques that transfer calibration data from a lower frequency to a higher one, it is possible to mitigate the atmospheric effects. These techniques have shown promise up to frequencies of 130 GHz and can be extended to the ngEHT, which operates around 230 GHz and up to 340 GHz.

The proposed approach involves adding an 85 GHz band to the ngEHT system, which already includes 230 GHz and 340 GHz bands. This addition can improve performance significantly, enabling better sensitivity and the ability to conduct precise measurements throughout the year.

#Methods of Frequency Phase Transfer

Frequency Phase Transfer (FPT) is a method that uses data from lower frequency observations to adjust and correct the measurements taken at a higher frequency. This technique relies on the principle that atmospheric fluctuations vary in a predictable manner concerning frequency.

FPT allows scientists to derive solutions for issues like phase, delay, and rate from lower frequency observations and apply them to the higher frequency observations. This results in improved Coherence, or clarity, when analyzing data received from distant cosmic sources.

#Simulation Studies

To understand the benefits of using these techniques, simulation studies have been conducted. These simulations use realistic models of atmospheric conditions to study how well the proposed methods perform under different scenarios. The results have shown that using an 85 GHz reference frequency greatly enhances coherence at higher frequencies like 340 GHz.

The simulations reveal that when observing under optimal conditions, high coherence can be maintained for extended periods, allowing for the detection of weaker sources. This is particularly beneficial for targets that exhibit strong scattering, such as SgrA*, the supermassive black hole at the center of the Milky Way.

#Importance of Coherence

Coherence is a measure of how well the signal remains consistent over time. With the traditional setup, coherence was limited by the atmosphere's variability. However, the application of FPT and the introduction of the 85 GHz band expands the effective coherence time. This means that scientists can observe for longer durations without losing clarity in their images.

In practical terms, increasing coherence time allows for the connection of data collected over greater distances and through more challenging conditions, significantly increasing the number of targets that can be reliably observed.

#Enhanced Observations

The new calibration techniques allow for simultaneous multi-frequency observations. This capability is essential for overcoming difficulties caused by the atmosphere. With FPT, we can adjust the observations in real-time for the changes in atmospheric conditions, leading to more accurate and detailed images from the ngEHT.

The inclusion of the 85 GHz band not only improves the calibration process but also broadens the range of scientific questions that can be addressed. It opens up opportunities for further study of various astronomical phenomena while providing a richer dataset for analysis.

#Astrometric Measurement

Astrometry, or the measurement of the positions and movements of celestial bodies, benefits immensely from these new techniques. With FPT, we can achieve high-precision measurements that were previously impossible at such high frequencies.

Using the combined data from different frequency bands, scientists can determine the relative positions of sources with great accuracy. This capability is essential for studying objects like SgrA*, as understanding their movements can provide insight into the physics of black holes.

#Proposed Configuration for Observations

To maximize the benefits of these techniques, a tri-band receiver setup is recommended, incorporating 85 GHz, 230 GHz, and 340 GHz bands. This configuration will allow for flexible observations across varying atmospheric conditions and source brightness levels.

The observations should prioritize integer frequency ratios, which help prevent issues related to phase ambiguities. This ratio ensures that the data can be analyzed straightforwardly, without introducing additional errors.

#Hub-and-Spoke Array Configuration

An effective way to implement these observations is through a hub-and-spoke array configuration. In this setup, a few highly sensitive "hub" antennas are linked to smaller antennas spread out over a larger area, acting as "spokes." This arrangement allows for the combination of data from various locations, facilitating the calibration and observation of weak sources like SgrA*.

This configuration is particularly important for sources that experience strong scattering, as it enables connections between antennas that would otherwise be too distant for effective calibration.

#Final Thoughts

The application of frequency phase transfer techniques has the potential to significantly enhance the capabilities of the next generation Event Horizon Telescope. By improving sensitivity, coherence, and the overall quality of observations, scientists can tackle a broader set of research questions in astrophysics.

By focusing on the introduction of the 85 GHz band and employing multi-frequency techniques, the ngEHT can achieve reliable results under a wider range of conditions. This means that high-frequency observations will become routine, allowing for continuous monitoring and exploration of key astronomical targets.

With the right configuration and advanced calibration techniques, the ngEHT will truly revolutionize our understanding of the universe, providing a clear view into the complexities of black holes and other astronomical phenomena that lie beyond our current reach.