Revisiting the sub-pulse drifting phenomenon in PSR J1822-2256: Drift Modes, Sparks, and Emission Heights (2111.07053v1)

Abstract: Sub-pulse drifting in pulsar radio emission is considered to be one of the most promising phenomenon for uncovering the underlying physical processes. Here we present a detailed study of such a phenomenon in observations of PSR J1822$-$2256, made using the upgraded Giant Meterwave Radio Telescope (uGMRT). Observations were made simultaneously using the Band 3 (300-500 MHz) and Band 4 (550-750 MHz) receivers of the uGMRT. The pulsar is known to exhibit subpulse drifting, mode changing, and nulling. Our observations reveal four distinct sub-pulse drifting modes of emission (A, B, C, and D) for this pulsar, with the drift periodicities of 17.9 $P_1$, 5.8 $P_1$, 8 $P_1$, 14.1 $P_1$, respectively (where $P_1$ is the pulsar rotation period), two of which exhibit some new features that were not reported in the previous studies. We also investigate the possible spark configuration, characterised by the number of sparks ($n$) in the carousel patterns of these four drift modes, and our analysis suggests two representative solutions for the number of sparks for a carousel rotation period, $P_4$, which lies in the range of $13$ to $16$. The large frequency coverage of our data (300-750 MHz) is also leveraged to explore the frequency dependence of single-pulse characteristics of the pulsar emission, particularly the frequency-dependent subpulse behaviour and the emission heights for the observed drift modes. Our analysis suggests a clear modal dependence of inferred emission heights. We discuss the implications for the pulsar emission mechanism and its relation to the proposed spark configuration.

• The paper identifies four distinct subpulse drift modes, with precise P₃ values revealing variations in spark stability and occurrence fractions.
• It employs dual-band uGMRT observations to estimate carousel spark configurations, suggesting 13–16 sparks based on rotation period and aliasing assumptions.
• The analysis links mode-dependent emission heights with drift periodicity, challenging conventional radius-to-frequency mapping in pulsar emission models.

An Analysis of Subpulse Drifting in PSR J1822$-$2256

The paper of subpulse drifting phenomena provides insight into the underlying mechanisms of pulsar emission, which remain incompletely understood. The paper "Revisiting the sub-pulse drifting phenomenon in PSR J1822$-$2256: Drift Modes, Sparks, and Emission Heights" by P. Janagal et al. provides a comprehensive analysis of PSR J1822$-$2256, leveraging observations from the upgraded Giant Metrewave Radio Telescope (uGMRT) across a frequency range of 300-750 MHz. This research contributes to our understanding of pulsar emission processes by examining drift modes, carousel configurations, and emission heights in one of astronomy’s intriguing subjects.

Summary of Observations and Results

The authors conducted dual-band observations using the uGMRT, obtaining high-sensitivity data that facilitated the scrutiny of single-pulse behaviors. Their analysis accurately identifies four distinct subpulse drifting modes, labeled A, B, C, and D, each characterized by unique $P_3$ values: 17.9 $P_1$, 5.8 $P_1$, 8 $P_1$, and 14.1 $P_1$ respectively. Mode A is found to possess the highest occurrence fraction and longest burst length, suggesting a relative stability in its spark configuration when compared to the other modes. The introduction of Mode D, with $P_3$ similar to a transitional mode alluded to in previous work, but occurring under different conditions, highlights the variability in subpulse patterns which may be dependent on specific observational conditions.

Carousel and Spark Analysis

The paper's exploration into potential carousel spark configurations yields two probable solutions for the number of sparks depending on the carousel rotation period, $\overline{P_4}$, which are conjectured to lie between 13 and 16. This analysis stems from the spark model, where coherence in subpulse drifting is attributed to the orderly rotation of discrete emission regions on the pulsar’s magnetosphere. These solutions, based on the observed $P_3$ values and assuming different aliasing orders, underline complexities in pulsar emission modeling and highlight the necessity for further observational constraints or theoretical developments to enhance the predictive capability of drifting models.

Implications of Emission Heights and Frequency Behavior

The authors leverage the extensive bandwidth coverage to analyze the frequency-dependent behavior of subpulse drifting and emission heights. Findings reveal that emission heights are mode-dependent and inversely related to $P_3$. Modes with lower $P_3$ appear to originate from higher altitudes. This anti-correlation presents a novel twist on the conventional radius-to-frequency mapping (RFM) expected for pulsars, suggesting that different drift modes might arise from distinct altitudinal regions within the pulsar magnetosphere. This could imply that the multipole configuration of sparks may influence emission properties, potentially pointing to a geometric dependency of carousel configurations with pulsar emission heights.

Polarimetric Insights and Pulsar Geometry

Utilizing polarimetric data from auxiliary observations, the authors further constrain the geometry of PSR J1822$-$2256. They provide measurements for the inclination angle and impact parameter, which play crucial roles in determining the observer's cut through the stellar magnetosphere. This geometric understanding is essential for correctly interpreting the observed subpulse patterns.

Concluding Thoughts

This research underscores the dynamic and complex nature of pulsar emission phenomena, providing valuable insights while also pointing to the limitations of current theoretical models. As new observational technologies become available, studies like this are pivotal for driving theoretical advancements in our understanding of neutron stars and their magnetospheric interactions. Further empirical attempts to distinguish and refine the carousel model with new datasets will be essential for unraveling the enigmatic nature of pulsar emission processes. Future research directions may aim to integrate these findings into broader theoretical frameworks, enhance model predictions, and explore potential applications in related astrophysical phenomena.